In his latest Consultant Case Study, Technical Roofing Consultant John Mercer discusses a project where foot traffic from roofers looking to address a problem on a pitched roof had caused real issues, plus he explains how he addressed the subsequent leaks through the roof tiles and outlines best practice for safe access over roof tiles…
In a recent site survey, I came across a roof that was clearly leaking through the roof tiling. The tiles were large format concrete interlocking flat tiles laid at a roof pitch of 17.5 degrees. Even though the tiles are designed to be laid at this pitch and were set at the correct headlap, rainwater was running freely through the tiles and into the building.
The roof had been originally installed in 2017 and the roofers had been back several times but could not find the reason for the leaks. In discussions with the building owner, it became clear that foot traffic over the tiling was a frequent event in an attempt to repair the roof, as well as for access to clean the windows. The owner told me that the roofers had replaced many tiles which had been damaged through foot traffic and they were suggesting that the tiles were faulty.
On close investigation of the roof, I found that many of the tiles were damaged, mostly with cracked or broken side interlocks – in some cases part of the lower end of the interlock was missing (see photo below) – and it should be noted that common indications of foot traffic over flat interlocking tiles are cracks or breaks to the lower right-hand corner of the tiles.
Safe access over tiling
In the past, roofers would leave a few tiles unfixed, then push them up so they can walk up the roof on the tiling battens. This course of action is no longer open to them as, since 2014, BS 5534 has recommended that, as a minimum, all tiles should be fixed, i.e. nailed and/or clipped.
Roof tiles must be regarded as a fragile roof covering, both in terms of safety of the operatives and the risk of damage to the materials. Therefore, it is important to plan tiling works so that battens can be used as footholds where they pass over the rafters, to avoid walking directly on laid tiles. Where access or working directly over tiling is unavoidable, this should be done from properly supported and anchored crawling boards or access ladders, suitably packed with foam or other compressible material to spread the load and avoid point contact loads on the tiles. Where regular access is required over laid tiles – for example to clean windows – some form of safe, permanent walkway or steps should be installed.
Flat interlocking tiles can be vulnerable to damage caused by foot traffic. Not because they are weak or otherwise faulty, but because they are laid cross-bonded and are therefore more susceptible to variations and unevenness in the roof structure than straight-bonded tiles are. Tiles are designed to have an even contact across each tile when laid, but unevenness in the roof structure can cause a point load contact between tiles, making them more at risk of damage under load.
Having established why the tiling was leaking, I then needed to find the cause of the water leakage through the underlay.
I found that the underlay was laid tightly across the rafters, with no drape. Water stains clearly showed that water was being trapped behind the tile battens and then finding its way through the batten nail holes. BS 5534 states that the underlay should provide a barrier to prevent wind-driven rain, snow, condensation, or dust from entering the roof space.
Modern underlays do not compress and ‘self-seal’ around battens and nails in the same way that bituminous underlays do. Therefore, it is important to lay the underlay with sufficient drape between each rafter to allow water to run into the drape, away from the nail holes, and to run safely under the tile battens to the gutters.
It can be difficult to lay underlay with a drape, particularly some of the lighter types. Therefore, a great alternative – particularly at low roof pitches – is to install counter battens over the underlay, fixed into the rafters, with bituminous nail tape between the counter battens and underlay. Counter battens can be standard 25mm deep tile battens, or, if there is a height restriction due to, for example, upstairs windows above a single storey extension, the counter battens need only be 10mm deep. The tile battens can then be installed over the counter battens.
The counter battens will ensure there is a gap between the tile battens and underlay to allow water to run freely towards the gutters without being blocked by the tile battens. The nail tape will form a watertight seal around the nail holes and between the counter battens and underlay.
It is also important to maintain watertightness in the underlay layer at junctions, such as abutments and around roof penetrations such as pipes and roof windows. The underlay should be turned up around junctions and penetrations and, ideally, taped and sealed to the upstand using a good quality adhesive roofing tape.
- Do not walk over laid tiles either during installation or for access afterwards. Try to plan work to avoid walking on the tiles or use crawling boards and ladders with suitable packing to protect the tiling.
- Where regular access is required over tiling, design and install safe permanent walkways and steps.
- Remember the underlay layer should be installed to provide a barrier to water ingress. Either lay the underlay with a drape or install counter battens and nail tape to allow water to run freely to the gutters and to protect the batten nail holes through the underlay.
- Maintain watertightness in the underlay layer at junctions and penetrations by turning the underlay up the upstand and sealing with roofing tape.