ROOF

Roof Construction

Roof Framing

• After the roof design is selected, the next decision is the type of roof construction-- trusses or stick built.

• Trusses.– Less labor to install trusses than to stick build

roof.– Factory built

• Better quality control• Reduced construction cost• Hauled to site and lifted into place

– Computers allow complex designs.

• Stick built– Higher labor costs for complex roofs– More variability in quality– High level of skill required to produce

complex rafters– Requires more scaffolding and other

supports for construction

Roof--Truss

• Different types of trusses can be used for roofs.• The type used will be determined by the use of the building, size of

the building and/or the owners preferences.• Most trusses are custom built for the building.• Trusses can be wood or metal• Truss nomenclature:

Bottom Cord

Webs

Posts

GussetsRafters

Types of Roof Trusses

ScissorsHigher center clearanceSpans 20 to 40 ft

Mono (single slope)Sheds attached to buildingsSpans 20 to 30 ft

FinkPoplar efficient designSpans 20 to 50 ft

WoweHeavier ceiling loads than fink trussSpans 20 to 50 ft

Different types of roof trusses are available.Truss manufacturers custom build trusses for each building

Types of Roof Trusses--cont

PrattUsed with or without ceilingsSpans 20 to 60 ft

BelgianExtended fink trussSpans up to 80 ft

BowstringDifficult to constructSpans 40 to 120 ft

Roof Pitch

Roof Framing-cont.Pitch

• Whether using roof trusses or stick building the roof, the pitch of the roof must be selected.

• Pitch is the slope of the roof.• Roof pitch is indicated by a fraction(1/3, 1/4, Etc.) or slope

triangle.– When a fraction is used, it is the rise over the span.– When a slope triangle is used, it is rise over run.

What factors influence the best roof pitch for a building?

Roof Framing-cont.Pitch Problem

• Determine the rise for the rafter in the illustration for a 1/3 pitch.

Pitch =RiseSpan

Rise = Pitch x Span

=13 x 24 ft = 8 ft

Roof Framing-cont.Slope Triangle

• A triangle (slope triangle) is also used to indicate roof slope.• A slope triangle indicates the rise/run.

• The slope triangle in the illustration indicates that for every 12 inches of run there will be 3 inches of rise.

– Because a ratio is used, the 3 and 12 can have any units as long as they are both the same. Three (3) feet & twelve (12) feet would have the same rafter slope as 3 inches and 12 inches.

• The use of the slope triangle reinforces the concept the a rafter is the hypotenuse of a right triangle.

Roof Framing-cont.Slope Triangle Problem

• Determine the rise for the rafter in the illustration.

Rise = Run x Slope ratio

= 12 ft x3

12= 3 ft

Run =Span

2

=12 ft

2= 6 ft

Stick Build Rafters

Stick Built RafterTerms

• A common rafter is the hypotenuse of a right triangle.• The plumb cut is made so that the ends of two rafters will fit

together.– They are fitted flush when used in a rafter truss.– A ridge board is used when the rafters are “stick built”.

Stick Built Rafter Terms-cont.

• The tail cut can be left several ways.

• Common types are:

Stick Built Rafter Terms-cont.

• The birds mouth is used to increase the contact area between the rafter and the top plate.– Must not extend more than 1/2 way through the dimension of the

board.• Excessive depth can cause the rafter to split.

Stick Built Rafter Steps

• Four (4) steps in laying out a common rafter.

1. Mark the angle at the ridge board end (plumb cut) of the rafter.

2. Determine the rafter length.

3. Mark the location and size of the birds mouth.

4. Mark the angle of the rafter at the overhang.

Stick Built Rafter Step 1-Plumb cut

or a speed square can be used.

• The angle at the end of a rafter is determined by the pitch.• The angle can be calculated in degrees and laid out with a

protractor.• Because a rafter is the hypotenuse of a right triangle, a

framing square

Stick Built Rafter Steps.Step One - Plumb Cut cont.

• Mark the plumb cut on the rafter.– Place the 12 inch mark on

the body of the square on one edge of the board.

– Rotate the square until the inches of rise is on the tongue of the square and on the same edge of the board.

– Mark along the edge of the tongue.

Remember: the same edge of the square must be on the same edge of the board.

In this example the rise per foot of run is 8 inches.

Stick Built Rafter Steps.Square Use

Opposite sides of the square are used.

Yes because both edges of the square are on the same edge of the board.

The square marks are on two different edges of the board.

Why are these two illustrations examples of incorrect square use?

Does this illustration show correct use?

Stick Built Rafter Steps.Step Two - Determine the length

a2 = b2 + c2

a = b2 + c2

• The second step is determining the length of the rafter.

– The length is the distance from the peak of the roof to the outside edge of the top plate.

– One half the thickness of the ridge board must be deducted when it is used.

• The rafter length can be determined by calculation or by stepping.

• Determining rafter length by calculation:

– Pythagorean Theorem

Stick Built Rafter Steps.Example of Length by Calculation

Determine the total length of a common rafter for a building with a span of 12 ft. 6 in. and a 1/3 pitch. The building will use a 2 x 6 ridge board and a 6 in. overhang.

rise = span x pitch = 12.5 ft x 13= 4.17 ft

a = b2 + c2 = 6.252 + 4.172

= 39.0625 + 17.39

= 7.51... ft

7.51... ft - 0.0625 ft = 7.451 ft

a = b2 + c2 = 62 + 42 = 36 + 16

= 7.21 in

7.451 ft +7.2112

ft = 8.05 ft or 8 ft

Rafter length =

Subtracting 1/2 of ridge =

Length of overhang =

Total length =

Overhang rise =

rise = span x pitch = 12.0 in x 13

= 4.0 in

Rafter rise =

Answer: 8 ft

Stepping A Rafter

Rafter Length by Stepping

The stepping process uses the rise and run.Works best with a span that is an even foot.

A building has eight (8) inches of rise per foot of run and the run is five (5) feet. It will use a 12 inch overhang.

– The square is aligned like making a plumb cut. In this case 12 and 8 are used.

– A line is drawn for the plumb cut and the 12 inch mark on the square is marked on the board.

Step a rafter for a building with a 12:8 slope and a 10 foot span.

Rafter Length by Stepping - Second Step

The square is “stepped” along the board for each foot of run.

Rafter Length by Stepping - Third Step

Process is continued until the width of the run is “stepped” off.

Rafter Length by Stepping - Fourth Step

At this point the length of the rafter will be correct for a building with a run of five (5) feet.

If an overhang is used, the additional length must be “stepped off” for the overhang.

If a ridge board is used, the rafter length must be adjusted.

Rafter Length by Stepping - Fifth Step

Stick Built Rafter Steps.Step Six - Birds Mouth

Stick Built Rafter Steps.Tail Cut

If the rafter did not have an overhang, it would be cut off plumb with the back of the birds mouth.

Stick Built Rafter Step Seven - Overhang

• Laying out the overhang requires two (2) steps.

• In the first step the 12 inch distance of the overhang is marked.

Stick Built Rafter Step Seven - Overhang-cont.

• The second step is to mark the end cut.

• In this example it is a plumb cut.

• The same ratio is used.• If a square cut is desired, a

line square with the edge would be marked.

Questions