Expert Opinion: Party Wall Roof Systems
Over the last 12 years in my role as a consultant engineer or as Assistant Chief Fire Officer, I have seen hundreds of houses and compartment walls (party walls) where there has been non-compliant construction around compartment walls and the roof.
I feel there is a lot of ambiguity with regards the junction of party walls and a roof. Historically the regulations called for the compartment (separating) wall to extend over roof level for a distance.
The regulations have been revised over the years to allow other methods of construction to be provided to maintain the integrity of a compartment either side of the wall.
I am of the opinion there is insufficient design details for contractors and installers to get this the most important of life details right.
During numerous inspections I have seen various methods of construction which have tried to achieve the requirements of Diagram 13 (b) of Technical Guidance Document B.
From my observations there is very few pitched roof buildings i.e. terraced houses or buildings with pitched roofs that comply with the requirements of Diagram 13 (b) for pitched roofs.
Traditional construction methods looked for any timber battens that traverse roofs to be embedded in mortar and also to be separated for a distance of about 1-2 inches along the compartment (party wall).
The theory behind this was that if the timber batten traversed the party wall without a break then if a fire in the roof space of one building started, the heat and fire would warp and burn the timber batten breaking the attached roof tiles and allowing the fire breathe further with more oxygen from the open air. If the timber batten failed it would then fall back into the premises where the fire occurred and would not spread into the neighbouring premises.
Currently the timber battens are allowed traverse the party walls without a break. This gives rise to the timber battens on the non fire compartment being susceptible to heat transfer and possible charring.
In addition to this if the timber members fail on the fire compartment side, due to the fulcrum effect (overturning moment) this causes the roof tiles or slates on the other side to lift making an open pathway for the fire to spread to the adjoining building or compartment.
The current detail has given rise to fire spread in terraced houses and apartments throughout Ireland and the UK. A detailed specification needs to be provided to both designers and industry to ensure fire spread over roofs is limited to the compartment of origin.
The majority of fatalities due to fire are in residential homes or other residential buildings i.e. hotels, nursing homes. Fire Spread by way of the roof in semi detached and terraced homes or apartments can greatly add to a potentially dangerous issue. Therefore it is essential to provide a detail which limits fire spread over this detail.
Building Regulations State:
Internal fire spread B3
(1) A building shall be so designed and (structure) constructed that, in the event of fire, its stability will be maintained for a reasonable period.
(2) (a) A wall common to two or more buildings shall be so designed and constructed that it offers adequate resistance to the spread of fire between those buildings.
(b) A building shall be sub-divided with fire resisting construction where this is necessary to inhibit the spread of fire within the building.
(3) A building shall be so designed and constructed that the unseen spread of fire and smoke within concealed spaces in its structure or fabric is inhibited where necessary.
(4) For the purposes of sub-paragraph 2 (a), a house in a terrace and a semi-detached house are each to be treated as being a separate building.
Technical Guidance Document B (Fire Safety) Currently the regulations permit timber battens to fully transverse compartment walls, see below:
Extract Technical Guidance Document B 2006 Section 22.214.171.124 (b)‘Timber tiling battens or combustible boarding not exceeding 25 mm in thickness used as a substrate to the roof covering may be carried over the compartment wall, provided that they are fully bedded in mortar or other suitable non-combustible fire stopping material for the full width of the wall’
The guidance provided in Approved Document B (UK) is clearer however this detail still permits timber battens to traverse party walls however the guidance doesn’t restrict the size of the boarding or timber battens to 25mm see below:
Extract Section 5.12 Approved Document B for Dwellings ‘Any combustible boarding used as a substrate to the roof covering, wood wool slabs, or timber tiling battens that are carried over the compartment wall should be fully embedded in mortar or other suitable material over the width of the wall.’
Diagram 2: Extract Section 5.12 Approved Document B for Dwellings
Again there is insufficient detail on the diagram for designers and builders to work off to ensure that this critical construction detail is constructed properly. The issue with the issue with the fulcrum effect(overturning moment) with the use of timber battens , wood slabs being over the party wall is not fully addressed by this detail.
Extract Clause 32.5 A compartment wall should be taken up to meet the underside of the roof covering or deck, with fire-stopping where necessary at the wall/roof junction to maintain the continuity of fire resistance.
If a fire penetrates a roof near a compartment wall, there is a risk that it could spread over the roof to the adjoining compartment. To reduce this risk, a zone of roof 1.5 m wide on either side of the wall should have a covering of designation AA, AB or AC (see 36.5.2) on a substrate or deck of a material of limited combustibility, as set out in Figure 25.
In buildings not more than 11 m high, in occupancy characteristic A, B, Ci and Cii buildings, combustible boarding used as a substrate to the roof covering, wood wool slabs, or timber tiling battens, may be carried over the compartment wall provided that they are fully bedded in mortar or other suitable material over the width of the wall.
There is insufficient detail on the diagram for designers and builders to work off to ensure that this critical construction detail is constructed properly. Again the issue with the issue with the fulcrum effect(overturning moment) with the use of timber battens , wood slabs being over the party wall is not fully addressed by this detail.
In addition to the above B4 of the Building regulations for External Fire Spread states that the ‘external walls of a building and roof of a building shall be so designed and constructed that they afford adequate resistance to the spread of fire to and from neighbouring buildings’. I am of the opinion that current roof details do not do enough to restrict the spread of fire over the roof.
The three regulatory guidance documents currently don’t provide a detailed solution on how to overcome this problem of timber battens and boarding passing over a party/compartment wall. This leads to non-compliant design and a scenario in which fire can pass over compartment walls.
There is insufficient detail on the diagram for designers and builders to work off to ensure that this critical construction detail is constructed properly.
While trying to research a suitable detail I came across FireBar. This is a party wall solution which addresses the above issues and restricts the spread of fire over a party wall.
FireBar Hot Dogs are a cavity barrier which meet the minimum requirements of the Building Regulations and have been tested has been independently tested by BRE Global and has met the requirements of BS EN1366-4: 2006 ‘Fire resistance tests for service installations. Linear joint seals’.
The test above has proven that the product is unlikely to fail in its intended use like the ones that have obviously failed in a number of fires at timber framed houses.
If installed properly cavity barriers prevent the spread of harmful gasses that cause the deaths of people in adjoining houses or apartments.
The system is a very detailed system providing detailed diagrams at each stage of the roof construction. The timber battens are attached using steel links which means they do not pass over the wall.There is also a Firebar centre brush/cockscomb which is placed along the party wall.
This system has been tested independently by BRE and has been proven to work in both a design scenario and a real working system
The system consists of the following:
- Timber battens tied to the party wall by way of a steel link batten which ties two independent battens over the party wall. This ensures in the event of fire only one batten will fail. This restricts the over turning moment and ensures that the likelihood of fire spread over a compartment wall is very limited. Firestopping material called a hotdog which consists of rock fibre or glass fibre in a polythene sleeve, which is in itself encased in a galvanised wire mesh. This provides the required firestopping.
- The FireBar system has been tested under the supervision of the Institute for Industrial Research and Standards, IIRS, to a method approved by the UK Fire Research Station head to head against the system shown in Diagram 13 (b) of TGD B. The findings of the tests showed that no spread of fire took place over the compartment wall that maintained it’s structural integrity using the FireBar system whereas the compartment wall using the TGD B detail failed.
The FireBar system has also been successfully tested on a timber framed party wall.
In the event of a fire the timber battens in the fire compartment fail without causing the timber battens in the adjacent compartment to fail due to the over turning moment of the failed members.
Therefore the integrity of the neighbouring roof is maintained and reduces significantly the likelihood of fire spreading over the party wall.
It is difficult to understand how the approved detailed system that has failed repeatedly in tests and has been shown to fail in house fires that have occurred over the years is still the approved construction detail of Part B of the Building Regulations yet the system which has passed tests is not supported by a similar approval.
I believe this system or similar type of system should be made the standard construction detail for pitched roofs of this type and could be used on all roof types.