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chimney & dormer units part 2

In Part 1 we looked at the roof construction, up to the tiling or slating of the roof around the prefabricated chimney or dormer window units. In Part 2 we will look at the tiling and slating around the units.

Apron flashing detail
With the chimney or dormer unit fixed in position, with the battens gauged and fixed to the rafters, the lead apron flashing needs to be turned back carefully to expose the top batten, which should be as close to the chimney/dormer window unit as possible.
     The top course of tiles/slates should be laid and fixed. If the rafter pitch is 45º and above, all interlocking tiles should be nailed, and may also need clipping. If the rafter pitch is 50º and above, all plain tiles will need twice nailing, including any top tiles. If top tiles are not used, the apron flashing should lap the topmost tile by at least 165mm. If a cut course of tiles is used, the head of the tile should be cut and new nail holes drilled to fix each tile to the batten.
     If there is a gap greater than 25mm between the head of the tile and the unit framing, a timber support batten/board should be installed with the top surface flush with the trough of a profiled tile, or the top surface of a flat tile/slate. If the support board is any higher, the first tile on either side will kick up. The lead flashing should be dressed back onto the top surface of the tiles/slates.
     Where the tiles are profiled they should be dressed into each trough so that water is not trapped between the head of the tile and the unit framing. At each end, where the flashing passes under the first tile/slate on either side, the apron flashing should be supported. A welt should be formed by turning the flashing over on the diagonal to ensure water on the flashing does not drain off the ends.

Side abutment detail
The side flashings of the chimney/dormer window unit are generally an integrated secret gutter that sits above the rafter level within the batten depth. The edge tiles/slates will, therefore, lap onto the side of the side flashing by a minimum of 100mm.
      In most cases, the edge tiles/ slates should be mortar-bedded onto the GRP side flashing, leaving a drainage channel approx 50mm wide between the unit and the edge of the tiles/slates. These dimensions will vary depending upon the design of the unit. With tiles, the nib above the side flashing should be cut off to keep the tiles from kicking up.
     Depending upon the module of the tiles, and the overall width of the unit, will dictate if a cut or half tile is needed. Where possible, you should always try to finish with a full tile at the side abutment, as a half tile will be almost impossible to fix to the batten and may have no nib secured to a batten. With interlocking tiles, laid half bond, this will be a problem, as you can not avoid the use of a half tile. Some chimney/dormer units are designed to accept a separate cover flashing. In these instances it is possible to install soakers for use with double lap plain tiles and slates.
     The first tile/slate on either side of the unit should sit down tight onto the tile/slate below with the apron flashing dressed between them, and should lie in the same plane as all the adjacent tiles/slates. Any mortar bedding should not kick up the tile/slate.
     As the edge tiles are considered to be perimeter tiles, if the fixing specification for the roof requires the edge tiles to be clipped then the tiles at the side abutment should also be clipped using verge clips, including the first tile on either side.

The apron flashing laps the tiles below excessively – there is a trough between the framing and the head of the tiles collecting water. At the end of the apron flashing there is a big hole where the water drains under the tiles. The apron flashing does not go under the tiles far enough, and finally the distance between the roof windows has resulted in two half tiles, why not half bond the tiles and save the cutting?         
This can be achieved by nailing the verge clip to the apron flashing support board, provided it is at least 19mm thick, or by installing an additional length of batten against the head of the lower course of tiles.

Back gutter detail
It is essential that the perp lines up each side of the chimney/dormer window unit should be maintained to ensure that at the back gutter the tiles and slates line through correctly. This is more critical with interlocking tiles as there is a limited amount of shunt that can be used to correct the situation. The course of tiles/slates that run through above the back gutter should all lay in the same plane as the rest of the roof, otherwise the corner tiles/slates will not sit correctly.
     Where double lap slates/plain tiles are used, a course of under eaves slates/eaves tiles are used, care is needed to ensure that there is not a bond on bond at the corners. Therefore, if it can be achieved, the roof should be set out such that the first full tile/slate should lap across the corner to maintain the half bond. As with the side abutment, the first course of tiles above the back gutter are also considered to be perimeter tiles, and therefore if the perimeter interlocking tiles need to be clipped, the first row of tiles will also need to be clipped, using either eaves clips nailed into the tilt fillet, which must be adequately fixed back to the rafters to ensure that hurricane force wind uplift doesn’t rip the construction apart.
     If it is not possible to safely fix into the tilt fillet, an alternative would be to install a parallel batten 150mm away from the leading edge and to install verge clips in a vertical position. For this to work, the clip aperture will need to be reduced to the thickness of one tile.

Preformed chimney/dormer window units would appear to speed up the construction of a roof, but in so doing present problems for the roofer trying to incorporate the tiled/slated roof covering around the ‘one unit design fits all roof covering’ philosophy.
     The choice of unit and the position of the unit by the designer could save a lot of the problems. The detailing of the apron flashing and the back gutter is often left to the roofer to resolve, when it should be part of the chimney/ dormer window unit design.


  • The tiles/slates around the chimney/dormer window unit are considered as perimeters and should be nailed/clipped accordingly.
  • Avoid using broken bonded interlocking tiles with chimney/dormer window units that have only a preformed secret gutter side flashing.
  • Do not puncture through the GRP side and back gutter flashings unless it is a preformed tilt filet or where the design specifically allows it.  
Compiled by Chris Thomas, The Tiled Roofing Consultancy, 2 Ridlands Grove, Limpsfield Chart, Oxted, Surrey, RH8 0ST, tel 01883 724774
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