With Technical Roofing Consultant John Mercer
Edilians’ technical support receives many interesting and diverse questions. John Mercer, Technical Roofing Consultant, has answered below some of the more common roofing questions.
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.
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. Where possible, tiles should avoid cut tiles at verges. 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 transportation, handling and installation. But this does not support the weight of a person. 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. This leads to complaints that the tiles are breaking on the roof. The usual reason why the tiles 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 are mechanically fixed. There are many benefits in doing so, particularly for structural and safety reasons and to reduce future maintenance. The Scope has been amended so that the Standard applies to the design, performance and installation of not only new build pitched roofs and vertical cladding, but 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, seek advice from the local planning authority and appropriate conservation organisation. All parties should agree on the final specification.
Do I need to ventilate my roof when using a ‘breathable’ underlay?
‘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 to prevent harmful levels of condensation from building up in the roof space. However, it is important to use these products in accordance with the guidance given in BS 5250 and with the information contained in the underlay manufacturer’s accreditation certificate.
There are two ways to control the risk of condensation build up in the roof space:
- Prevent water vapour from reaching the loft space in the first place,
- 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. This means that less water vapour passes into the roof space. Therefore requiring less ventilation to remove it. In older buildings with existing ceilings, this is not always practical and ceilings will be more air and vapour ‘open’.
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.
Technical Roofing Consultant