Home Contractor's Corner Consultant Case Study: preventing leaks around dormer windows

Consultant Case Study: preventing leaks around dormer windows

by Jennie Ward

In the latest instalment of his excellent Consultant Case Study series of articles, John Mercer, pitched roofing consultant, tackles tricky issue of leaks around dormer windows and explains how he addressed problems on a recent residential project he was called out to assess...

John Mercer, pitched roofing consultant.

The importance of careful detailing around roof features such as dormer windows can sometimes be overlooked when it comes to weathering the junctions with the main roof tiling. Dormer windows with pitched roofs usually have valleys behind them, gutters to capture the water from the dormer roof, and side abutments.

I carried out a roof inspection on a dwelling recently where the roof was leaking badly around the dormer windows. The roof has a rafter pitch of 30 degrees and is tiled in large format flat interlocking concrete roof tiles. The dormers have pitched roofs and are positioned mid-slope in the main roof at the front and rear of the house. There are valleys where the dormer roof slopes intersect with the main roof and the side abutments are weathered with lead cover flashings which extend approximately 200mm over the tiling. There are gutters each side of the dormer roof slopes.

Rainwater from the main roof directly above the dormers is draining into the valleys behind the dormers. Water from the dormer roofs is draining into the valleys and into the gutters at each side. The valleys are draining water back onto the main roof slopes at each side of the dormers. The water from the gutters is also draining onto the main roof slopes at the same points as the valleys. This means that water is being drained in point loads onto the tiling directly above the dormer side abutments.

Although the gutter outlets face away from the side abutments, the water simply spreads out as it runs down the tiles and much of it passes under the flashings. Cover flashings over flat interlocking tiles are not an efficient way to weather side abutments as water can easily penetrate between the tiles and the flashing. In the case of this roof, the abutment flashings are wholly inadequate.

This roof is 30 years old and the leaks started about five years ago. An internal inspection revealed that the underlay, which was correctly turned up the wall under the abutment flashings, managed to prevent water from entering the roof structure until it eventually rotted and is now largely disintegrated.

Design conscious
Careful design could reduce the volume of water running into the abutments. For example, the gutter outlets could be positioned at the front end of the dormers rather at the rear, as they are here. The front face of each dormer is close to the main roof gutters; therefore the dormer gutters could drain directly into the main roof gutters.

Also, the lead at the lower end of each valley could be extended to shed further away from the dormer sides to avoid so much water from entering the abutment flashings.

Above: Flat tile flashing with secret gutter.

Most importantly, the side abutment flashings need to be installed correctly. Water can penetrate between the tiles and the flashing because there are no profiles in the tiles – for example pans or rolls – to stop the water. It is not possible to use lead soakers with single lapped tiles, therefore the standard detail is to use a continuous ‘secret’ gutter, as shown in the detail here. These can be either formed in lead on site or manufactured pre-formed in suitable materials such as fibreglass.

The gutter is installed under the tiles close to the wall to capture and drain away water that runs sideways off the tiles. The gutter lining extends up the dormer side by at least 75mm above the top surface of the tiles and is covered by a step flashing. The tiling is finished close to the dormer side, i.e. with a gap of no more than 15mm wide, to prevent access to birds and rodents. A cover flashing is usually installed in addition to the secret gutter to prevent the risk of debris such as leaves and pine needles etc. entering and blocking the secret gutter. A further option is to use pre-formed individual soakers, though advice on their use should be sought from the manufacturer.

Profiled tiles such as roman or pantile shapes do not suffer the same problems at side abutments. The flashing can be a simple cover flashing which extends over the tiling tiles by at least 150mm. It must also extend over a complete tile roll or upstand; therefore, if necessary, the flashing may need to be wider to achieve this. The roll or upstand in the tile profile prevents water from running sideways between the tile and flashing and leaking into the building.

Above: Profiled tile cover flashing.

A cover flashing is installed over the tiles and extends up the dormer side by at least 75mm. The flashing upstand can either turn into the wall or be covered by an additional separate step flashing that overlaps the upstand of the cover flashing by at least 65mm and turns into the wall. In the case of dormer sides, the upstand of the flashing will usually be covered by the dormer cladding.

Summary:
• Minimise the volume of water being shed in point loads directly above the dormer side abutments. Extend valleys to shed further away from the dormer sides.
• Do not drain water from the dormer gutters directly above the side abutments.
• Always use either secret gutters or preformed soakers at side abutments with single lapped flat interlocking tiles.
• Only use cover flashings without secret gutters with profiled tiles.

www.johnmercerconsultant.co.uk

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