Home Contractor's Corner Consultant case study: Why a house roof should not be put on a commercial building

Consultant case study: Why a house roof should not be put on a commercial building

by Jennie Ward

In the latest instalment of his excellent consultant case study series of articles, Pitched Roofing Consultant John Mercer explains why a house roof should not be put on a commercial building…

John Mercer, Pitched Roofing Consultant.

Recently, I have attended roof inspections on commercial buildings where the roofs are leaking badly. In each case, the buildings are several storeys high and are situated in severe driving rain exposure zones.

These buildings have single lapped flat concrete tiles on them, installed at the minimum headlap on relatively low rafter pitches. In each case, the rafters exceeded 9 metres long.

On inspection, removal of areas of tiling revealed clear evidence of water ingress through the tiling, through the side laps and over the headlaps. The underlay, vapour permeable membrane, is laid taut with the tile battens installed directly over the underlay and fixed into the rafters. Therefore, there are no gaps between the underlay and tile battens to allow water to drain down the underlay to the gutters.

There are two separate problems with these roofs, which both combine to cause water ingress into the building. Firstly, water is routinely passing through the roof tiles. Secondly, the underlay layer is not waterproof. Inevitably, there will be occasional extreme weather conditions when water may be driven through roof tiling or slating – though I emphasise ‘occasional’.

BS 5534: the British Standard Code of practice for slating and tiling recommends that the underlay be installed as a waterproof layer. If installed correctly, the underlay can deal with occasional extreme rain events and drain the water safely away to the gutters.

On the buildings I surveyed, the roof tiles are installed at the manufacturer’s minimum recommended headlap, i.e. maximum batten gauge. Something often overlooked by designers is that the average roof tile manufacturer’s technical data for its roof tiles should be regarded as the minimum requirement, i.e. suitable for the average domestic dwelling. Users need to be aware of caveats, such as restrictions on rafter lengths or rafter pitch in some circumstances.

Guidance for product use on commercial buildings
Roof tile manufacturers can provide guidance on the use of their products on commercial buildings. Any designer working on a large roof design should always contact the manufacturer for its recommendations. Equally, roofers should contact the tile manufacturer if specific guidance in the form of a comprehensive specification is not available from the designer.

Single lapped interlocking tiles drain water from each tile course onto the course below, and so on, until the water reaches the gutter. Any water entering the side joints is drained away by water channels and bars. Water can partially creep up the headlap through a mixture of capillary and wind action. The steeper the roof pitch, the harder it is for the water to penetrate the head and side laps. Conversely, on low pitch roofs, water can penetrate further up the headlap and, in extreme cases, reach the nail holes or the top of the tiles and leak into the structure. This problem is intensified on longer rafter lengths due to the greater volumes of water when water can flood and overwhelm the side joints of the tiles as well as the headlaps.

These risks can be partially mitigated by increasing the tile headlaps, though it is not possible to reduce the risk of water penetration through the side joints on long rafter lengths, other than by increasing the rafter pitch.

BS 5534 provides excellent and comprehensive information on the design of natural slate roofs, with guidance on rafter length and roof pitch in relation to slate sizes and headlaps. Unfortunately, the Standard cannot do the same for manufactured roofing products, where the design of the product heavily influences its performance. Whilst some elements of a roof tile’s performance are known through long experience in use, manufacturers rigorously test their products to determine suitable technical data.

In the absence of recommendations on rafter lengths in BS 5534 for single lapped slates and tiles, commonly adopted practice is to increase a roof pitch by 1 degree for every half metre over the recommended maximum rafter length. For example, take a tile with a minimum recommended rafter pitch of 22.5 degrees and a maximum recommended rafter length of 6 metres. For a roof with a rafter length of 9 metres, the roof pitch should be at least 22.5 + 6 = 28.5 degrees. Though, I again stress the importance of consulting with the tile manufacturer at the design stage.

Underlay drape
In roofs where the underlay is laid unsupported, directly over rafters, it is important to lay the underlay with a drape to allow water to run in the troughs to the gutters. If the underlay is laid taut, with no gaps between the underlay and tile battens, water entering the batten cavity will then become trapped behind the battens. Because modern underlays do not compress around the tile battens or self-seal around nail holes, this water can find its way through the nail penetrations and into the roof structure.

An alternative method to having a drape is to install counterbattens over the underlay, fixed directly into the rafters. The counterbattens lift the tile battens clear of the underlay, providing a path for water to run under the battens and towards the gutters. In this application, the counterbattens need only be 10 to 15mm deep to satisfy the BS 5534 requirement for a space between the underlay and tile battens to enable water drainage. As an extra precaution, bituminous nail tape can be installed between the counterbattens and underlay to create a seal around the nail holes. A further option is to use a proprietary under-tile system that is designed to be 100% watertight.

• Always regard roof tile manufacturer’s technical data as a minimum requirement. Consult the manufacturer for any building larger or taller than the average domestic house.
• Consider the location of larger buildings, in particular the driving rain category of the area.
• Consider increasing tile headlaps and roof pitch for roofs with long rafter lengths, or design in features to interrupt the rafter length.
• Ensure the underlay layer is designed and installed to be completely watertight, including all junctions and penetrations.


Tweet John: @Johnmercer3

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