The question for fire resistance in cavity closers is how to achieve tested ‘safer alignment and confinement’ onsite, says John Shillabeer from Cavity Trays
Buildings should be constructed to eliminate or minimise as much as possible, all potential fire and fire safety risks likely to arise. In theory this should be a fairly straightforward undertaking, but in practice the building envelope can present both designer and builder with considerations that are not always immediately obvious.
Cavity wall construction relies on the external leaf (that can become saturated with rain) from not coming into contact with the inner leaf that is required to be kept dry. Wall ties are shaped to structurally link the skins and prevent rain tracking across to the inner leaf. The cavity successfully separates and isolates the two. But in the event of a fire, both air and fire can be drawn through the cavity within all the external walls of the building, fuelling combustion.
The question arises of whether there is an optimum way of protecting the external envelope at one of the most frequently occurring features – where it is punctuated by window and door openings? Such openings present the designer and contractor with the usual heat-loss and damp-transference considerations, but with multi-occupancy and numerous other structure categories, also the additional important requirement to protect against fire and fire transference.
Fire-resistant cavity closers
Fire-resistant cavity closers are an accepted way of addressing window and door reveals. A typical closer contains a non-combustible thermal and fire-resistant core that straddles the cavity, effectively linking both skins. Such closers provide passive fire protection. Applied to all four sides of an opening, the completed installation provides a level of fire resistance, commonly 30 or 60 minutes.
To establish the number of minutes, closers are subjected to testing at a fire research facility. These do a good job – one can accept they want to get it right, and accordingly best practice construction is witnessed. Products interface squarely, correctly and consistently. But if one looks at actual onsite building tolerances, they are different, prompting the question of whether currently-accepted site tolerances adversely affect the performance of fire-resistant cavity closing?
Are such tolerances compatible with those required with a fire-resistant closer, bearing in mind closer and masonry must interface consistently and without gaps, to perform? Construction tolerances accepted by various authoritative bodies vary, but in general building, the following are commonly witnessed;
(External skin)
- ±8 mm maximum deviation in any length of wall up to 5 metres
- ±8 mm storey height (up to 3 metres)
- 8 mm max, plumb in any storey up to
3 metres - 4 mm max, external reveals over 1 metre
(Internal skin)
- 3 mm max, out of level over 1.5 metres – 5 mm beyond 1.5 metres
- 3 mm max, reveal is permitted to be 3 mm out of vertical over 1.5 m – 5 mm beyond 1.5 metres.
- 5 mm out of square is permitted for reveals up to 250 mm deep.
Optimum performance of any fire-rated closer is dependent on correct incorporation. Put simply, the relationship between a fire resistant closer and masonry should match those under which the test results were achieved. Cavity wall openings constructed to the varied tolerances shown above differ from those used by testing stations and are unlikely to replicate the
test results.
Testing stations qualify their test results relate only to the behaviour of the specimens of the elements of construction under particular conditions of test and do not reflect actual behaviour in fires. All openings in cavity walls should be formed squarely and uniformly, observing best practice to fulfil the objectives and requirements of Building Regulations Approved Document Part B.
Benefits of metal ties
While core functionality relies on continuous interfacing, core longevity can be aided with the use of stainless steel metal ties as opposed to plastic ties securing the closer to the masonry. Metal ties that project through the closer body and continue into the actual protective core itself, can extend the period the resistant core is anchored and held in position when fire is attacking. Core continuous interfacing and core retention being the objectives. The building fabric should be constructed so that there are ‘no reasonably avoidable gaps within various elements, at the joints between elements, and at the edges of elements such as those around window and door openings’, as stipulated within L1A 5.9.
A site agent of many years standing perceptively summed up a parallel situation; “When you buy a new car, the manufacturers often claim a number of miles per litre, and I do not doubt such a figure is honestly produced, albeit dependent on a featherlight foot on the accelerator, well inflated tyres etc. Real everyday driving is different.”
Fire testing establishments and bodies such as the BBA quite understandably do not provide warranties. It is down to those constructing the building to aspire to the highest standards to get it right. While one continues to witness variable masonry tolerances, one questions whether building envelope fire resistance will actually provide the intended level of protection in the event of a serious fire? Compliant construction should leave nothing to chance.
John Shillabeer is director at Cavity Trays