ANSWERS TO SOME COMMON QUESTIONS POSED BY ROOFING CONTRACTORS
Posted on 19th July 2021
Edilians’ technical support receives many interesting and diverse questions. I have put together below some of the more common questions we receive and the answers we give.
Can I use tiles below the manufacturer’s minimum recommended roof pitch?
The short answer is usually no. Using tiles below their minimum recommended roof pitch increases the risk of water ingress through the tiling, which in turn can lead to long term damage of the roof structure. There is also the issue of aesthetics; roof tiles, and in particular plain tiles, simply do not look as good at low pitches.
A typical example is a single storey, low pitch extension to an existing two-storey house with plain tiles on the main roof. Rather than installing plain tiles on the extension below their minimum recommended pitch, it is better to choose an alternative covering such as the Edilians Beauvoise or Double HP 20 tiles; both look great at low pitches as the angle of view of the roof from the ground gives the perception of the tile tails being closer together, mimicking a plain tile appearance.
If the use of a tile below its minimum roof pitch cannot be avoided, then the sub-roof below the tiles must be designed and constructed to be totally waterproof and durable, to the satisfaction of the local Building Control Department.
How should I fix perimeter tiles?
BS 5534 recommends that all perimeter tiles on a roof be twice fixed. This is a prescriptive recommendation regardless of the predicted wind loads on the building.
Generally, roof tile mechanical fixings are nail or screw at the tile head and a clip at the tail. Although tiles should be set out to avoid cut tiles at verges where possible, BS 5534 allows the use of suitable adhesive as a second fix in conjunction with a nail, screw or clip where it is not possible to use two mechanical fixings.
Why do my tiles break when I walk on them?
Roof tiles and slates are not designed to be walked on. They must be strong enough to withstand normal transportation, handling and installation, but this does not necessarily account for supporting the weight of a person. In any case, for health and safety reasons, roofers should be working in such a way that avoids walking on the tiles or by providing adequate protection and safe walkways if access over laid tiling is unavoidable.
A further problem caused by foot traffic is that broken tiles may only become apparent sometime after the tiles were laid, leading to complaints that the tiles are breaking on the roof. I have yet to come across correctly installed roof tiles that simply break on a roof without a load being applied to them. The usual reason why the tiles appear to break after being installed for a period is that they were cracked or broken before or during installation. Subsequent weather, such as frost and wind, will then open the cracks and cause sections of the tiles to dislodge. As this can happen months, even years after the initial damage was done, people incorrectly assume that the tiles are breaking by themselves.
Why do I have to mechanically fix ridge and hip tiles?
BS 5534 recommends that all ridge and hip tiles be mechanically fixed and there are many benefits in doing so, particularly for structural and safety reasons and to reduce future maintenance. In the latest edition of BS 5534: the British Standard for slating and tiling, the Scope has been amended to make clear that the Standard applies to the design, performance and installation of not only new build pitched roofs and vertical cladding; it also applies to re-roofing work, including repairs to existing slate or tiled roofs.
With regard to historically or architecturally important buildings, the Scope states that some elements of BS 5534 may not be appropriate where traditional and/or reclaimed materials are used; for example, a dry ridge system or visible mortar-bedded ridge tile mechanical fixings. In these cases, advice should be sought from the local planning authority and appropriate conservation organisation and all parties should agree on the final specification.
Do I need to ventilate my roof when using a ‘breathable’ underlay?
Firstly, even the word ‘breathable’ when used in relation to underlay is misleading; ‘breathable’ gives the impression of breathing, or air passing through, which, for many underlays, is not the case. Generally, there are two groups of modern underlays; vapour-permeable and air-permeable.
Vapour-permeable and air-permeable roofing underlays are beneficial in helping to prevent harmful levels of condensation from building up in the roof space. However, it is important to use these products correctly, in accordance with the guidance given in BS 5250 and with the information contained in the underlay manufacturer’s accreditation certificate.
In simple terms, there are two ways to control the risk of condensation build up in the roof space; ie either prevent water vapour from reaching the loft space in the first place, or remove it once it gets there and before it has chance to build up to harmful levels.
In new buildings, ceilings can be designed and constructed to minimise air and vapour leakage, which means that less water vapour passes into the roof space, so less ventilation is required to remove it. In older buildings with existing ceilings, this is not always practical and ceilings will be more air and vapour ‘open’.
So, decisions on how to control condensation in the roof space depends on a number of factors, including the air and vapour-tightness of the ceiling and the type of underlay. Therefore, keep these in mind and always follow the underlay manufacturer’s installation recommendations.
John Mercer: April 2019
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