a Practical Guide Corrosion of Steel Hardware Caused by ... · PDF fileCorrosion of Steel Hardware Caused by Pressure-Treated Lumber ... the wood cells, giving a “waterproof” treatment

Earthquake Retrofits for Wood-Framed Buildings—a Practical Guide

© 2014 Thor Matteson, Structural Engineer • www.shearwalls.com 181

Corrosion of Steel HardwareCaused by Pressure-Treated Lumber

This handout is part of an upcoming book on earthquake retrofits by Thor Matteson,Structural Engineer. Mr. Matteson has been concerned with fastener corrosion since2004, when the wood-treating industry began using new preservative formulationsthat aggressively attack steel hardware. Mr. Matteson is the author of “Wood-FramedShear Wall Construction—an Illustrated Guide” Rev 9/23/14


182 Thor Matteson, Structural Engineer • wwww.shearwalls.com © 2014

8.5 Pressure-treated lumber:a ticking time-bomb since… 2004?

Imagine constructing a house using building materialsthat will quietly destroy each other. Millions of homeshave been built using such materials; the hardwarepictured in Figure 8-45 shows severe corrosion of steelconnectors in contact with chemically treated lumber.Future failures of structural connections will surprise alot of building professionals and homeowners; if youhave this material in your home you still have time toavert disaster.

8.5.1 Background informationEver since the first building code there has been arequirement for mudsills or other wood members in contact with earth or footings to be decay-resistant. The codes allow foundation-grade redwood or cypress, or preservative-treated wood.“PT lumber”, or even just “PT” is the term used for wood that has been made artificiallydecay-resistant by the addition of preservatives. Technically the proper term is “preservativepressure-treated” lumber, as wood can also be pressure-treated with chemicals to make it fire-resistant (and possibly for other traits). I will stick with the term PT lumber here.

8.5.2 Alphabet soup—and unintended consequencesThe chemicals: Until 2004, the most common treatment chemical used in PT lumberintended for residential construction was chromated copper arsenate, or “CCA.” You may findPT lumber stamped with “CCA” or “CCA-C,” (or some other suffix letter; the trailing “C” isthe third formulation they came up with, after A and B). Arsenate is a chemical compoundthat includes arsenic, which has been a known human poison for centuries. The chromate isprobably not good for us either. Thankfully the CCA binds with the lumber and the toxinspretty much stay in the wood.

Since 2004, CCA has not been allowed as a treatment for wood used in residentialconstruction in the US. Some of the more common new chemicals used for preservativesinclude CA (Copper Azole), ACZA (Ammoniacal Copper Zinc Arsenate), CC (CopperCitrate), ACQ (Ammoniacal Copper Quaternary). All of the preceding chemicals bond withthe wood cells, giving a “waterproof” treatment suitable for fence-posts, decks, etc. Figure8-46 shows several labels from PT lumber.

Figure 8-45 Most wood nowsold to resist decay also attackssteel connectors. The red circleshows a rusted concrete anchor.At center is an H10 (compare toFigure 6-34). This installationwas about 10 years old.Photo: Matt Cantor, GGASHI

Alert: This section is long; there is a great deal ofmisinformation to address, and the subject involvessome technical information that needs explanation.



All of the preceding treatmentcompounds contain copper. Copper andsteel, in the presence of water andoxygen, create a “galvanic reaction”(galvanic and “galvanized” both givetribute to Italian scientist Luigi Galvani).This reaction occurs even with verysmall amounts of water, such as fromhumid air in a crawlspace. Oxygenmolecules will travel through wood, soembedding nails or other fasteners intowood does not protect them. The steelcorrodes as a result of this reaction.Corrosion is the chemical term for“rust.” Figure 8-47 shows some verydisturbing examples of rapid corrosion.

The “white rust” often seen ongalvanized steel connectors is the firstcorrosion by-product: zinc oxide. Afterthe zinc corrodes away, the underlyingsteel is no longer protected and it begins

Micron-izedCA

ACZA

CA-C

ACQ &instruct-tionsACQ-D

Figure 8-46 Tags from several varieties of pressure-treated lumber found onjobsites; all of them are rated “ground-contact,” which has higher chemical contentthan the minimum needed to protect lumber used as mudsills.

Why the “teeth-marks” in PT lumber?To speed up the process of getting thepreservatives into the wood, the wood isimmersed in chemicals inside huge vessels thatare then pressurized to force the mixture intothe wood cells.

Some species of wood will accept the chemicaltreatments more easily than others; wood thatis difficult to treat otherwise is usually“incised” by sending the lumber through rollersstudded with miniature knife-blades. Douglasfir is difficult to treat and needs to be incisedfor most chemicals. The “hem-fir” speciesgroup is also easier to treat, but woods in thisgroup are softer and do not provide the sameconnection capacity as Douglas fir. (Sincemudsills are not high-demand members, thereduced capacity can be addressed easily byusing more edge-nails when connecting bracingpanels.) Southern yellow pine acceptstreatment readily and does not require incising.



to rust. This process continues until all the steel has corroded away. ALL of it. The time ittakes depends on many variables—so: do you feel lucky? Note that hardware manufacturersadvise if you see any “red rust” on a connector then it should be replaced.

Compare the photos above with theanchor bolts shown in Figure 8-48,which were installed in either CCAor redwood mudsill material. Thethreads on the anchors are still well-defined, whereas the threads shownin Figure 8-47 are rusted badlyenough that removing the nut wouldbe impossible.

Figure 8-47 Photos show severe corrosion that occurred within five years of construction.Left: “White rust” turning to red rust. Photo: John McComas, GGASHICenter: Remains of nails used to fasten PT plywood. Photo: Keith Tarkington, GGASHIRight: Mudsill anchor photographed by author in May, 2014.

Figure 8-48 Bolts shown abovewere installed before PTchemical reformulation andshow minimal corrosion.Left: In a CCA mudsill, approx.35 years old; Right: In aredwood mudsill, approximately80 years old.

The Chemistry of Corrosion

The reaction between different metals is an electro-chemical process where electrons flow from one ofthe metals toward the other one. Chemists refer toone metal (the one that corrodes) as the “anode” andthe other as the “cathode.”

Any particular metal will act as either the anode orthe cathode depending on what other metal is present.Chemists rank metals on the “Galvanic scale.”Metals that corrode easily (magnesium, zinc,aluminum) are at one end of the scale. Metals towardthe other end of the scale include gold and platinum.Metals that do not corrode easily are called “noble”metals. You can find pure gold nuggets in naturebecause gold lasts millions of years withoutcorroding. Metals that corrode easily are almostimpossible to find in pure form in rock formations—they exist almost exclusively as oxides.

Iron (the main ingredient in steel) is near the middleof the Galvanic scale; if it is paired with zinc, itremains stable. When paired with copper, it corrodes.

(continued next page)



How much copper is in the wood?The American Wood Protection Association(AWPA) generates specifications for various“use categories” for lumber and minimumtreatment levels. Most PT lumber used inconstruction comes in one of two “retentionlevels” (the amount of chemical retained inthe wood after the treatment process). Thelowest level is “above ground,” followed by“ground contact.” Higher retention levels areavailable for marine use or wood pilings.

The amount of preservative remaining in thewood is measured in pounds of chemical percubic foot of lumber. The amount requireddepends on the type of chemical, since someare more effective at protecting againstdecay. For ACQ and CA to meet theminimum “above ground” use category, eachcubic foot of treated wood must contain 0.25pounds of chemical. For “ground contact”the minimum retention level is 0.40 poundsof chemical per cubic foot of wood.

8.5.3 Most suppliers stockonly the most corrosive wood

People use PT lumber for mudsills, fences,planters, retaining walls, play sets, decks andso forth. Some of these uses require the“ground contact” level of chemical treatment.Lumber suppliers have apparently gottenblamed when a contractor or homeownerinstalled “above-ground” rated wood in suchcases, and chose to stock only the “best”grade of lumber. Between the desire to avoidconfusion and save space that would beneeded to stock two different sorts of lumberin many sizes and lengths, most suppliersonly carry lumber treated with higherpreservative levels. Unfortunately thismeans the PT lumber you buy for a mudsill

The Chemistry of Corrosion(continued from previous page)

You may wonder why steel rusts when it isnot in contact with another metal; there isenough physical difference in the steel thatsmall regions will act as anodes and otherregions act as cathodes. In the photo below,simply bending the reinforcing barschanged the steel properties enough that thebent regions of the bars became anodicrelative to the straight sections, causingthem to rust first.

When the initial anodic regions rust away,other regions are next in line. Thus if wehave metal items that we want to keep fromcorroding, we need to protect themsomehow. One way to protect them is tocoat them with paint to keep the moistureand oxygen away. Another method is“cathodic protection,” i.e., protecting thecathode from corrosion.

Cathodic protection uses “sacrificialanodes.” The sacrificial anodes give theirlives to save the more noble metal you wantto protect. Boat owners may be aware ofzinc anodes used to protect their steel boathulls. (continued next page)



has 60% more copper in it than necessary.More copper means faster corrosion of steelfasteners.

8.5.4 Bolts versus nailsMost bolts shown in the accompanyingphotos probably still have almost their fullstrength. I am much more concerned aboutnails driven into PT lumber. The nails aremuch smaller than bolts and would corrodefaster. I would photograph nails if I could—in most cases they are covered with siding orstucco. You will never know the nails havefailed until sheets of plywood or stucco peelaway from the mudsill during an earthquake.

8.5.5 One solution:use Stainless Steelhardware and fasteners

Stainless steel contains chromium and nickel,which make it more corrosion resistant thancopper. You may be stuck with nasty PTlumber that someone else installed when theyreplaced your foundation. In this case theonly way to assure long-term performance ofconnections to the treated lumber is to usestainless steel fasteners and connectors.Every metal component that connects to, orpasses through the treated lumber should bestainless steel. Stainless steel nails should beavailable at most local lumber yards, but youmay have to special-order other components.The Maze Nail Company makes stainlesssteel nails. GRK Fasteners makes self-drilling structural screws. Note that SimpsonStrong-Tie and other companies offer someof their connectors in stainless steel, but thecost of stainless is about ten times that of theirregular connector line. Clearly you want toavoid the need for stainless steel fasteners.

The Chemistry of Corrosion(continued from previous page)

Aluminum or zinc anodes are commonlyused to protect steel. Magnesium anodesare used to protect aluminum (magnesiumcould also protect steel, or any other metalmore noble than it is; but magnesium ismore expensive than aluminum).

Galvanic corrosion occurs because ourmetal parts create a crude battery. Sincethe process involves very weak electricalcurrent, the anode (metal that corrodes)does not have to be in contact with thecathode—just joined to it by a wire orother metal part. The photo at left belowshows a concrete anchor with significantcorrosion only three months afterinstallation, and it was not even touchingthe ACQ-treated lumber; it was connectedto a UFP10, which was in turn attached tothe ACQ lumber. Most importantly, theprocess does not stop until all of the anodematerial has corroded. Sacrificial anodesmust be replaced periodically, orprotection is lost for the boat hull, oilpipeline, or structural framing connector.The photo below right shows a sidewalkaccess cover for Pacific Gas & ElectricCompany to replace an anode that isprotecting an underground pipeline fromcorroding; if they do not replace theanodes regularly, the pipelines wouldcorrode and eventually leak.



8.5.6 Addressing the problem,or ignoring it?

The wood-treating industry and the hardwaremanufacturers really don’t want to tellcontractors that durable fasteners and connectorswill cost ten times as much as what they wereaccustomed to paying before the switch to newPT formulations. The industries use a few waysto avoid telling you straight out that you shoulduse stainless steel connectors with treated wood.

Tactic #1: Misdirection & denial: One of theearly “answers” from industry was to deny that therewas a problem. The following is a quote from apreservative industry document dated 2004 (still atlarge on the internet as of September, 2014)regarding corrosion:

"It is generally recognized that the potentialfor fastener corrosion in forest productsbased building materials used in an interiorexposure environment is minimal becausethe equilibrium moisture content of the woodis maintained at a level that does not supportcorrosion reactions."

The above quote is generally recognized as the sortof statement intended to be as confusing, vague, andmisleading as possible. What they mean is, “it isgenerally recognized that lumber used indoors willprobably be too dry to allow steel to rust.” The mostmisleading term is “it is generally recognized.”Who recognizes this? And based on what? Testing?Hope? Unrelated studies? The document assertingthis “fact” quotes a study of moisture levels in wood,dated 1988. When we are talking about corrosioncaused by chemicals that were not even invented in1988, this reference by someone who wants to sell metheir product seems unhelpful at best. The second-mostmisleading term is “interior exposure environment.”Does this mean inside the heated or air-conditioned part

Figure 8-49 This installation wasfour years old when photographed.Note the red circle at the top of thepicture, which shows a rusted nailthat is just barely touching the PTledger.Also note that the PT ledger isseparated from a concretefoundation wall with tarpaper,which reduces the moisture thatwould wick into the wood from theconcrete.The manufacturer of the joisthanger shown recommendsreplacing hardware that has any“red rust” on it. Hint: Replace withstainless steel if you do not want torepeat this process every four years.Photo by John Fryer, GGASHI



of your home, or the damp crawlspace beneath it? One reason that PT lumber is used incontact with concrete is that the concrete wicks moisture up out of the ground, resulting inelevated moisture levels in the mudsill. Considering the severe corrosion of steel connectorsseen in the last few years, as shown in Figure 8-50 through Figure 8-53, I am not reassured bythis pitch from the wood-treating industry.

Figure 8-50 Six-year old installation in amechanically vented crawlspace, whereair circulation was provided by a fan toprovide reliable ventilation. It wouldtake a fairly long time—maybe decades—for this bolt to corrode to the point it lostsignificant strength. Nails are a differentmatter; see below.

Figure 8-51 Rusty edgenailing of a shear panel,approximately five yearsafter construction.Contractors havereported almost totalcorrosion of nails in justa few years. Photo bySkip Walker, SiliconValley ASHI/CREIA

Tactic #2: Minimize the problem: After thenew treatment chemicals had been corrodingfasteners in the real world for a few years,manufacturers started including warnings in thefine print on their labels. One suggestion is thatregular inspection and maintenance of thehardware will allow you to use the less-expensivefasteners they sell. “Regular inspection andmaintenance” is possible—but, let’s face it, highlyunlikely—on a deck or a fence. For mostconnections that are crucial to your house’sintegrity, inspection & maintenance is completelyimpossible: how do you inspect or maintain thenails driven into a PT mudsill to attach the bottomedge of the exterior sheathing on your house if thenails are covered by siding or stucco?

Figure 8-52 Accidental experiment—the builder left a couple of foundationanchors lying on the dirt under ahouse (photo above left). Photo aboveright compares one of them to ananchor installed in PT lumber nearby.The anchor that had been left on thebare ground still has shiny portions,whereas rust completely covers theanchor installed in the PT lumber.



Tactic #3: Pass the buck:Fine print on PT lumber labelsand hardware manufacturer’sbrochures tosses all theresponsibility for corrosion tothe consumer, just saying tomake sure you select theappropriate fasteners to usewith the chemicals inquestion. One hardwaremanufacturer states that theirproducts perform well undercontrolled tests, but also notesthat the test results may notapply to actual fieldconditions and suggest usingstainless steel if you areunsure about the specificinstallation. If they are notsure, how can you be sure?Unless you have a degree inmetallurgy and a laboratory todo your own tests, thehardware manufacturers areessentially recommendingstainless steel.

One hardware maker’s catalogindicates what type of fastenermaterial is recommended forconnecting to treated lumber,based on the treatment level,type of chemical, and whetherthe installation will be dry orwet. For any of the copper-containing wood treatments at“higher chemical content”levels they recommend usingstainless steel, copper, orsilicon bronze fasteners. Theydefine “higher chemicalcontent” as the levels required

All Galvanizing is not Equal

“Galvanizing” generally means to coat a metallic item withzinc. The zinc is applied in one of three ways: Electro-plating, mechanically, or by dipping the item into a bath ofmolten zinc. Each method has advantages anddisadvantages.

Electroplating uses an electric current to deposit a metal(in this case, zinc) onto the base metal being plated. Theplating is deposited one atom at a time from a bathcontaining a solution of a zinc salt. Electroplating depositsa fairly uniform layer of zinc, and gives parts a somewhatshiny, silvery appearance. This method is also called“electro-galvanizing.” For something as crucial as anearthquake retrofit system, electroplating does not depositenough zinc onto the base metal to offer adequateprotection from corrosion. That leaves mechanical or hot-dip galvanizing.

Mechanical galvanizing is done by tumbling a bunch ofmetal parts in a rotating drum with powdered zinc. Enoughzinc is pounded onto the surface of the metal to offerprotection. This method is used to coat threaded parts thatmight otherwise turn to blobs if they were dipped intomolten zinc. This method is sometimes referred to as“tumbling.” Mechanically galvanized parts have a dull-gray finish. Simpson “SDS” screws are currently suppliedwith mechanical galvanizing.

Hot-dip galvanized (“HDG” for short) parts are dippedinto a kettle of 900-degree molten zinc. Many galvanizershave kettles big enough to dip 60-foot street-light poles.The molten zinc heats the base steel to the point where fourdifferent iron-zinc alloy layers are formed. Two of thesealloy layers are actually harder than plain steel. HDGmaterial has a spangled appearance. Most sheet-metalframing connectors are hot-dip galvanized.

Hot-dip galvanizing can coat the entire surface of a partwith zinc, which offers the best protection fromcorrosion—however, see the sidebar on the following pagefor some bad news.



for wood to be rated as “ground-contact”—theonly PT lumber sold at most lumber yards. Sincethe building code only allows steel or stainlesssteel connectors for structural purposes, this rulesout copper and silicon bronze (although at twicethe cost of stainless steel, nobody is likely to usecopper or silicon bronze anyway).

8.5.7 Why is this still a secretafter 10 years?

Many reputable contractors and engineers are notaware of the problem with copper-containingwood treatments. Most likely this is because theyrarely get a chance to see connectors and fastenersafter nature has taken its toll.

Members of the American Society of HomeInspectors have shared many of the photosincluded here that show extremely severecorrosion due to copper-based lumber treatments.ASHI members are on the forefront of seeingearly failures of many different products. Leadpaint and asbestos were used on millions ofhouses, but that does make them any less bad nowthat we finally acknowledged they are harmful.I expect that copper-treated lumber will eventuallybe viewed in a similar way—and I hope it's soonerrather than later.

Figure 8-53 Anchor bolt on left is fromoriginal 90-year-old construction. Anchoron right was installed in PT lumber for aneight-year old remodel in the samecrawlspace. The anchor in the PTmaterial looks worse than the older one.Photos: Justin Brodowski

Not even all Hot-DipGalvanizing is Equal!

Most framing connectors are madefrom sheet steel that was galvanizedbefore the steel was shipped to thehardware manufacturer.

Sheet metal parts are cut from hugerolls of HDG steel sheet. The cutedges of the parts are bare steel. Sincethe steel will not corrode if there is ananode in contact with it, the bare edgeis protected—for a while. In thephotos of corroded connectors, though,you will notice the red rust almostalways begins to form near a cut edgeof the material.

Many galvanized nails are madesimilarly—the nail manufacturer buysspools of HDG wire from a steel mill,then cuts the wire into short pieces tomake the nails. The points and headsof the nails are not coated with zinc.Worse, the nail heads are formed bystamping the wire into a die so it flaresout. In the sidebar explaining thechemistry of corrosion we saw thatsteel begins to corrode where it hasbeen bent or deformed. Now we havea deformed area of the steel nail thatalso has no zinc coating on it—thus thenail head is usually the first place yousee rust.

Other than stainless steel, the bestreadily-available finish to resistcorrosion is hot-dip galvanizedmaterial. The best HDG protection isprovided by hot-dip galvanizingafter fabrication.

One contractor tells me he buysUFP10s and has them dipped.



8.5.8 Solutions—orexperiments?

Two methods to reduce corrosion without usingall stainless steel connectors are fairly common,but not proven over the long term.

8.5.8.1 Mixing stainless steeland galvanized connectorsdoes not solve the problem

For steel connectors and fasteners, copper is theenemy in PT lumber. But copper is not the onlymetal that will react with steel to cause fastenercorrosion. You do not want to introduce anothermetal that begins to attack your connectors—yetcombining stainless steel and galvanizedfasteners has this result. Stainless steel is morenoble than plain steel (see “The Chemistry ofCorrosion” sidebar). If you use stainless steeltogether with plain or galvanized steel, the plainsteel becomes the anode in the galvanic reaction.Now the plain steel corrodes because it is incontact with stainless steel; you could take thecopper-treated wood completely away and theplain steel would still corrode.

Sometimes fasteners are mixed intentionally,sometimes accidentally. Installers may think thatusing stainless steel nails will improve theconnection performance of galvanized framingconnectors or foundation plates. The cost ofmixing the two materials is certainly less thangoing all stainless steel.

Manufacturers clearly state that you should notmix different materials in the same connection.

8.5.8.2 Isolation membranesdo not isolate the nails

For corrosion to occur, the two metals in questionmust either be in contact or at least have a wayfor small electrical charges to flow between them

Relative size of different metalparts affects corrosion rates

We know that whenever two differentmetals are joined, one of them willcorrode—but how quickly? It dependson how much of which metal is present.

Say you have a two-pound steelfoundation plate joined to untreatedwood with stainless steel nails (addinganother PT metal is more complicationthan we need). The foundation plate isthe anode in this case, and will corrodeuntil it is gone. Compared to the nails,the plate has a large surface area.If iron atoms in the steel plate are lostfrom the whole surface of the plate,it would take a long time for the plateto lose significant strength.

If we switch the metals so we have astainless steel plate secured with steelnails, things get scary. Now we haverelatively small steel nails acting asanodes that need to protect the muchlarger stainless steel plate; the nailswill corrode very quickly. It could bethat the same amount of steel is lostover the same time-span in this case orthe one described in the precedingparagraph—but when the loss of a fewsmall nails means a failed connection,the second installation is much moredangerous.

In the first case above, we cannot counton losing a thin layer from the wholesurface of the steel plate. Likely someconcentration of corrosion will occurright around the stainless steel nail.



(water or moisture provides such a way). If the two metals are kept physically separated,corrosion will not occur. So why don’t we just wrap the evil PT lumber with a water-proofmembrane to separate it from the metal parts? There are products that attempt to do this—butI do not see how they can actually work. This is because you cannot isolate the nails from thePT lumber. If only the nails corrode, you still lose connection capacity—but the nails providea path for the electrical charges to flow, leading to possible corrosion of the “isolated” metal.

The membrane shown in Figure 8-54 ismarketed specifically for decks, which are oneof the main uses of PT lumber. At least youcan usually access connections under decks tomonitor the condition of the steel now andthen.

8.5.8.3 Proprietary coatingsMajor hardware manufacturers offer thickerzinc coatings on some of their products (forexample, Simpson’s “Z-Max” and USP’s“Triple Zinc”). These coatings just provide athicker layer of zinc (and since the parts are notgalvanized after fabrication, all the cut edgesare left uncoated). The thicker zinc buys sometime—but the corrosion reaction is patient andunrelenting, so I do not trust a mere delay inconnector death.

USP supplies some of their connectors with their “Gold Coat” finish. Gold Coat is “an organictop coat barrier layer” over galvanized steel. Coatings can get scratched during installation orhandling; I simply would not want to trust my house to a thin coating separating essentialconnectors from corrosion doom. But I may change my mind after 50 years if actual fieldinstallations show good performance…

8.5.9 How long a time is “long enough”?Many of the coatings and other protection methods and materials listed in the previous sectionwere developed for outdoor decks. Compared to a house, the expected life of a deck is muchless (as short as 10 or 15 years). What may be a satisfactory lifespan for a deck fastener wouldnot meet most homeowners’ expectations for the lifespan of their homes.

When a client with a 100-year-old house asks me to design an earthquake retrofit, I want todesign a system that will last another 100 years. Copper-containing PT lumber poses a severethreat to that goal; I don’t even want to rely on stainless steel, much less some unprovensystem—some of which have obvious flaws.

Figure 8-54 PT lumber separatedfrom a sheet metal connector with aself-adhered membrane. The nails stillpenetrate the PT lumber. This methodmay delay corrosion in the connector,but the membrane does not offer anyimproved protection for the nails.



8.5.10 A better alternative:Borate-treated lumber

Another class of preservative treatment chemicalsincludes borates. The two common formulationsare Disodium Octaborate Tetrahydrate (“DOT”) andSodium Octa-Borate (“SBx”). Borate treatments donot corrode fasteners even as much as CCA did, andtherefore do not require special fasteners. Boratesare not toxic to humans (this is the same class ofchemicals as borax that you use to wash your clothesor hands, and is a major ingredient in dishwasherdetergent). Borates are much less harmful to theenvironment than copper and arsenic-containingwood treatments.

Unlike copper-based treatments, borates arecolorless. Since carpenters are used to PT lumberhaving a green color to it, most borate-treatedlumber is dyed light green or light blue. Figure 8-55shows two samples of lumber treated with the samebrand of borate.

8.5.10.1 Limitations for boratesBorates will dissolve in water, so borate-treatedwood must be protected from liquid water—thisrules it out for building decks or fences. Borate-treated wood is perfectly acceptable for use infoundation sills.

Borate-treated lumber is also not allowed in areaswhere Formosan termites are present (Hawaii andFlorida, possibly other Gulf-coast areas).

Figure 8-55 Borate-treated lumberdoes not corrode fasteners; it is alsonot toxic to mammals. Borate-treated lumber cannot be exposed toliquid water, but is appropriate formost retrofit applications. It is alsousually less expensive than woodtreated with toxic chemicals.California Cascade Industries(phone 800-339-6480) suppliesborate-treated lumber throughoutthe state. A few lumber yards in theSF Bay Area [Osborne Lumber (inNewark), Sierra Point Lumber(Brisbane) and Golden StateLumber (several locations)] carryborate PT lumber as a regular stockitem in limited sizes and lengths.Special-ordering from other yards isvery worthwhile—but see thewarning in the sidebar, next page.

Borates do not kill termites directly

Termites do not actually digest wood—theyonly chew it up and swallow it. A variety offungus that inhabits termite stomachs breaksdown the wood. The termites get theirenergy from the fungus. Borates kill thefungus—thus the termites die of starvation.



8.5.10.2 Why don’t morelumber yards stockBorate-treated lumber?

As with the two treatment levels used forcopper-containing PT lumber, suppliers donot have space for yet another type ofmaterial. In most suppliers’ minds, thereis no advantage to borate treatments—borates are not water-proof, and thesuppliers assume the products they alreadystock will work in all situations. Somepeople in the construction industry are alsounder the false impression that borate-treated wood cannot be used for mudsillsbecause borates are not waterproof.

8.5.10.3 Other benefitsof borate-treated lumber

One advantage of borate-based treatmentsis that they will travel toward moisture inthe wood, which is exactly where you needthem. In log cabin construction, largeborate crystals are placed in holes bored inthe logs. If moisture reaches the crystal itbegins to dissolve. The borate movesthrough the moist wood, spreading outaway from the crystal. In this case the factthat borates will move through the wood ishelpful.

According to the borate-treatingcompanies, incising is not required for thetreatment to adequately penetrate thewood. If true, this means you do not needto spot-treat cut ends of boards.

For more information on boratetreatments, search for:“Timbor” by US Borax“Sillbor” by Arch Chemicals“Advance Guard” by Osmose

Warning: check the lumber yourself!

In 2005 I hired a contractor to build anaddition on my house. I told him to makesure he put in a special order well in advancefor borate-treated lumber to use for themudsills. The contractor and his crew arrivedto lay out walls the day before the lumberyard was to deliver all the framing lumber—he assured me that the yard had orderedborate-treated stock for the mudsills.A delivery truck arrived the next morning,including a dozen or so dark green boards formudsills. I checked the tags on the ends:ACQ. Those boards went back to the yard.I took the morning off, drove an hour to thenearest yard that had borate-treated lumber instock, and returned with what was needed sothe crew could begin building a structure thatwould not self-destruct.

Onward to 2014; I designed a foundationreplacement and earthquake retrofit for a 1915bungalow. I referred the owner to a premierfoundation contractor. The references on thedrawings to borate-treated lumber got theirattention and they special-ordered borate-treated mudsill stock. When I went to crawlunder the house to observe the work beforeconcrete was poured I immediately noticeddark green mudsills; no end-tags werevisible, so I went to the lumber yard the nextday. The only PT lumber they carried wasACQ, and they told me their supplier filledthe “special order” with ACQ.

Moral: If you’ve read this far, you knowmore about PT lumber than most lumbersuppliers, engineers, contractors and buildinginspectors. Don’t rely on them.



8.5.11 Another alternative:Isolate untreated wood withwater-proof membrane

The building codes generally call for foundationsills and posts resting on concrete to be of PTlumber or decay-resistant wood (IBC/CBCSection 2304.11). In some specific cases,untreated lumber is allowed if an “imperviousmoisture barrier” is used between the wood andconcrete (such as posts in metal post-baseconnectors). The spirit of the code seems toallow use of untreated wood if it is separatedfrom concrete by a moisture barrier. Ifseparating untreated mudsills or other membersfrom concrete was more common, the codewould probably include a specific provisionallowing this practice. The absence of such aprovision does not mean a membrane would notprotect the wood.

Figure 8-56 shows the beginning of a wall frameisolated from a concrete stemwall by self-adhered flashing. The flashing should blockmoisture from the concrete, thus protecting thewood from decay. Note that “tar-paper” doesnot block water vapor effectively and should notbe used.

I not like to give termites a path from the ground to untreated wood without crossing borate-treated lumber somewhere. However, I would take my chances with the system shown ratherthan use copper-containing treatments that I know will corrode fasteners

8.5.12 Recommended best practice for known damp areas:All of the above, short of stainless steel

For the amount of trouble it is to install most earthquake retrofits, you may as well spend moreon good materials to provide a superior system. Use borate-treated lumber, hot-dip galvanizedconnectors (if nothing else, at least the nails should be hot-dipped after fabrication), and anisolation membrane. The membrane will keep moisture from wicking into the wood structure.Even foundation grade redwood decays after decades of contact with damp concrete; theborate-treated material may not decay, but it could transmit moisture to other woodcomponents.

Figure 8-56 Untreated lumber shownhere is protected from moisture wickingthrough the concrete foundation wall by“peel-and-stick” membrane (also calledself-adhered flashing, commonly used towaterproof around window and dooropenings).

Documents

a Practical Guide Corrosion of Steel Hardware Caused by ... · PDF fileCorrosion of Steel Hardware Caused by Pressure-Treated Lumber ... the wood cells, giving a “waterproof” treatment