Home Contractor's Corner Go with the flow: low pitch problems on extension roofs

Go with the flow: low pitch problems on extension roofs

by Jennie Ward

Pitched roofing consultant John Mercer discusses a recent project he was called out to involving several leaks on an extension with a low pitch roof – an issue he says he comes across regularly in his role – and explains why this was happening and how to rectify the problems…

The following case study is based on an actual roof inspection I carried out on a single storey extension roof to the rear of a detached dwelling. However, I receive many requests to attend sites due to problems with extension roofs.

John Mercer, Pitched Roofing Consultant.

Despite the roof pitch being above the minimum recommended roof pitch for the tile being used, the homeowner was experiencing several water leaks through the roof.  Because the internal ceiling was not yet in place, it was possible to see exactly where the water ingress was originating from; namely around the two roof windows and at the eaves. 

An inspection firstly carried out from inside the building confirmed that water was penetrating the roof at eaves and around the roof windows.  Water staining indicated that water was penetrating almost the entire length of the eaves.  

An external inspection was carried out and roof tiles were removed in several areas, including at eaves, around the roof windows and in random, general areas of roof. At eaves, although underlay support trays were fitted, they sloped backwards rather than properly shedding water towards the gutter, and therefore water was pooling in a continuous channel behind the gutter. Water was then finding its way between the underlay and support trays and through the joints between the support trays.

The underlay was finished poorly around the two roof windows and did not properly extend up the window surrounds. This meant that any water penetrating the roof tiles above the windows and which ran down the underlay was able to run between the underlay and window surrounds. The tiling was finished poorly over the window flashings and against the window upstands, with large gaps between the tiling and the window flashing integral foam seals.  

On testing the tiling abutments against the windows by pouring water on the tiling above, water could clearly be seen running over the edge of the tiling and into the batten cavity. The water test mimicked a gutter downpipe which was shedding water from a higher roof onto the extension roof directly above one of the roof windows. 

Where tiles were removed in other, random areas of roof, water staining on the tile battens and underlay indicated that rainwater was penetrating the tiling, especially around the area where the downpipe was located. The underlay, a vapour permeable type, had been laid taut, with little or no drape, and water trapped behind tile battens had run sideways and penetrated the underlay through the nail holes where the battens were nailed into the rafters.

To remedy the various problems, I made the following recommendations:

Generally, the tiles and tile battens should be removed and re-installed over new underlay. The roof system should comprise of underlay laid relatively taut, no drape was required, with counter-battens fixed over each rafter, with bituminous nail tape fitted between each counter-batten and underlay. 

The tile battens can then be fixed over the counter-battens, providing a clear path for any water on the underlay to run towards the gutter.  The nail tape will seal and protect the nail holes through the underlay.

The underlay should be carefully turned up against the roof window upstands and taped to the upstands. In addition, underfelt collars should be installed around each roof window – these are available from the roof window manufacturer.

The fascia should be repositioned at eaves to ensure that the underlay support trays can be positioned to fully support the underlay and provide an adequate fall behind the fascia towards the gutter. Although not normally recommended by the manufacturer, I suggested that each underlay tray be lapped over the adjacent tray, with the lapped sealed and the underlay sealed to the trays using a suitable tape or adhesive.

When installing the roof tiles around the roof windows, any tile nibs that interfere with the window flashings should be carefully removed and the edge tiles secured using screws rather than nails to ensure the tiles sit as flat as possible over the window flashings.  The window flashing sealing foam should be trimmed as close as possible to the undersides of the tiles to close any gaps.

Ideally, the fall pipe draining water onto the extension roof from the above main house roof should be re-directed to drain directly into a gulley to avoid point loads on the extension roof that can overload the tiling during heavy rainfall.

In summary:

  • Use counter-battens and nail tape to protect the nail holes through the underlay.
  • Turn the underlay up against the roof window surround and seal. In addition, fit underfelt collars around each window.
  • Re-position the fascia at eaves and ensure the support trays provide an adequate fall and support for the underlay at eaves.
  • Take extra care when tiling around the roof windows, removing any tile nibs that interfere with the window flashings and fix the edge tiles with screws to encourage the tiles to sit flat over the window flashings.
  • Re-position the fall pipe to drain directly into a drainage gulley, rather than onto the extension roof.


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