Roof Ventilators and Terminals:

Fire Performance

CI/SfB (37.9)Uniclass L529May 2012

Roof Ventilators and Terminals: Fire Performance

There is a view held by some in the industry that roofventilators are an insignificant part of the roof and cantherefore be ignored with regard to fire. History hasshown us however that this is not the case and therehave been instances of roof ventilators contributing tothe spread of fire.

Below is a fire underway in a department store inBlackpool. The fire is well established and is beingattended by the fire brigade. Fire hoses are being directednot only at the burning building but also onto the roofacross the road.

Below, again in infra red, we see the roof opposite thefire source as red.

This shows the temperature of this roof slope due to theradiant heat from the fire.

Steam coming from this roof can clearly be seenshowing the extent of the temperature build up from theheat radiation from the fire on the opposite side of theroad.

Infra red imaging, shown below, allows us to seethrough the smoke. The snowy effect is hot airborne andburning embers. These show up as white and can beseen collecting in valleys and gutters. Such emberswould also collect behind roof protrusions such aschimneys and roof tile ventilators. If such products arenot fire rated there is a risk that they could catch alight,thereby propagating a fire on that roof.

Regulations RequirementsThe above shows why Building Regulations control roofmaterials to prevent fire spread (see page 3).

The National Federation of Roofing Contractors TechnicalBulletin 20 also clearly states ‘The materials used in themanufacture of roof ventilation terminals should notsignificantly increase the risk of firespread or penetration of theexternal roof envelope and complywith current Building Regulations.It will also be necessary toconsider the number of roofventilation terminals used in the roof.’

Fire - one of nature’s destructive forces

2

Roof Ventilators and Terminals: Fire Performance

3

Buildings RegulationsBuilding Regulations Approved Document BBuilding Regulations Approved Document B4 Externalfire spread Requirement states that: ‘The roof of thebuilding shall adequately resist the spread of fire overthe roof and from one building to another, havingregard to the use and position of the building.’

This applies to roofs, the roof covering material andparts of roofs, which would include accessories suchas tile and slate ventilators and terminals.

Note: Approved Document B Volume 1 coversdwellinghouses, Volume 2 covers buildings other thandwellinghouses.

Table 5 of Approved Document B4 2010 (reproducedbelow) sets minimum distances of roofs from siteboundaries or adjoining buildings, according to theirfire classification.

It shows that roofs and parts of roofs with a firedesignation of AA, AB or AC can be used withoutrestriction, ie with no minimum distances betweenbuildings or to boundaries on a particular site.

Fire ClassificationsSo what do the two letters of the fire classificationsmean and where do they come from?

The British Standard which describes the fire tests forroofs and defines the classifications is BS 476-3: 2004(amd 2007) ‘Fire tests on building materials andstructures - Part 3: Classification and method of testfor external fire exposure to roofs’.

The test consists of a preliminary ignition test,followed by two main tests for flame spread and firepenetration.

The results for the main tests give designations A-D,and the combination of two letters gives the overallclassification AA, AB etc.

Next we look at the tests in more detail.

Roof Ventilators and Terminals: Fire Performance

4

13.7m

BS 476-3 Fire TestBS 476-3 defines test conditions which simulateradiation from a fire, burning brands, and wind effect.These test conditions can be regarded for example asrepresenting the effects on a roof 7.6 metres aboveground level from a fire 13.7 metres away in a buildingwith a façade 15.2 x 15.2 metres and 50% windowopenings.

15.2m

7.6m

Fire penetration test The roof is exposed to radiated heat,with reduced pressure on theunderside to simulate wind. After 5 minutes the flame (burning brand)is applied for 60 seconds. The radiantheat is continued for up to 1 hour.

Spread of flame test The roof is exposed to radiant heatand to the test flame (burning brand)for 3 minutes.

Preliminary ignition test A test flame, representing burningbrands, is applied for 60 seconds.

The test is in three stages using different specimens for each stage:

The test results generate the performance designations as follows:

Fire penetration (first letter)A Those specimens that have not been penetrated within 1 hour.B Those specimens that are penetrated in not less than 30 minutes.C Those specimens that are penetrated in less than 30 minutes.D Those specimens that are penetrated in the preliminary ignition test.

Spread of flame (second letter)A Those specimens on which there is no spread of flame.B Those specimens on which the spread of flame is less than or equal to 533mm.C Those specimens on which the spread of flame is greater than 533mm.D Those specimens that continue to burn for five minutes after the withdrawal of the test flame or spread more than

381mm across the region of burning in the preliminary test.

For a full classification the prefix EXT.S. or EXT.F. is added to the two-letter classification to indicate whether sloping orflat specimens were tested, eg EXT.S.AA.

However, Approved Document B4 refers only to the two-letter classifications.

A suffix X is added if during the test there was dripping of molten droplets from the underside of the specimens, or thedevelopment of holes or mechanical failure

The test specimens are mounted at 45° for sloping roofsor horizontally for flat roofs.

Roof Ventilators and Terminals: Fire Performance

5

Passivent ventilator test

A Passivent roof ventilator is exposed to radiant heatand the test flame simulating burning brands.

Fire Tests on VentilatorsUnrated ventilator test

The specimen is subjected to radiant heat and the testflame (burning brand) is applied for 3 minutes.

At the end of the test the Passivent ventilator is intactand shows no ignition or flaming.Hence the Passivent ventilator achieves an AA rating.

Passivent ventilators are made from a speciallyformulated fire-retarded polymer and are self-extinguishing.

As the test progresses the material of the specimen isitself propagating fire, and molten material is drippingfrom the ventilator onto the roof underlay. This in turncatches alight, resulting in complete failure of thespecimen before the test flame is removed.

The roof is now well alight even though the adjacentroof tiles have not been affected.

Roof Ventilators and Terminals: Fire Performance

Passivent Limited, North Frith Oasts, Ashes Lane, Hadlow, Kent TN11 9QUTel: 01732 850770 Fax: 01732 850949

e-mail: projects@passivent.com www.passivent.com

ReferencesBS 476-3:2004 (amd 2007)

National Federation of Roofing Contractors TechnicalBulletin 20

Building Regulations Approved Document B4

The Passivent SolutionAll Passivent ventilators and terminals have beenindependently tested by the Loss Prevention Council(now BRE) to BS 476-3 and have been awarded an AAclassification - the highest possible. Thus they can beused without restriction on any roof.

For further advice or detailed information regarding aspecific project please contact our Technical ServicesTeam on 01732 850770.