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Dormers, Roof Light and
Dormers and roof lights create openings within a roof structure. These should be constructed as per the design and provide adequate structural stability. Dummy chimneys apply additional load to the roof and are vulnerable to high wind. Fixing must be in accordance with the design.
Around all openings within a roof structure including dormers, 2 fixed rafters should be used to provide the necessary support.
Trimmers are required to the front and rear of the opening and should be of a suitable size to take the additional load of the dormer.
Traditional timber dormers must use external grade timber in its construction not less than 19mm thick for the cladded backing.
Underlay to be placed on the roof before the installation of the pre-formed dormer.
Pre-formed GRP dormers require an additional side flanges around the dormer to support flashings, secret gutter and box gutter.
A tilting fillet should be fitted at the bottom of the dormer to prevent the lead from sagging.
The tilting fillet and lead flashing should extend 400mm past both sides of the dormer.
****** link to Exallot installation guide
Dormer are fixed to the roof structure with skewed nails and fixing cramps at 600mm centres.
****** link to Exallot installation guide
Ensure 27mm Thermline Plus thermal board is placed at ceiling level.
Roof battens notched over the outer weather bar – cut edges to be treated.
****** link to Exallot installation guide
Tiles laid on a bed of mortar between 20mm – 80mm from the dormer cheek and pointed to a neat finish.
Tile clips and verge clips provided as with verge detail.
Dormer cleaned before scaffold dismantled.
Lead clad dormer cheeks must follow guidance set by the Lead Sheet Association.
Prior to installing lead cladding, a breathable membrane must be installed over cladding which laps over the cover flashing or secret gutter.
All lead work must be treated with an approved patination oil.
Fixing of lead must follow a staggered pattern between 25mm and 50mm when length of lead exceeds 500mm.
When less than 500mm, one row of fixing 50mm apart 25mm down from the top of the lead.
For welted joints, copper clips should be provided not more than 500mm apart.
For roof lights construction as with dormers.
Around the opening within a roof structure including dormers, 2 fixed rafters should be used to provide the necessary support.
Trimming members are required to the front and rear of the opening and should be of a suitable size to take the additional load of the roof light.
Adequate protection of the roof lights required due to follow on trades.
Neatly tile around the roof light ensuring the apron is supported.
Tiles should sit flat around the flashing with cut tiles no less than half the width used.
All dummy chimneys should be stored off the ground and in a safe place.
Check chimney for damage before installing.
Key areas to check are at joints and where the GRP interfaces with the pre-installed lead.
External grade timber, minimum 19mm thick placed between rafters supported on noggins for dummy chimneys.
3 layers of roofing membrane placed on top support boards below the dummy chimney.
This provides a vapour control layer to prevent condensation forming within the chimney.
Chimney fixed in accordance with the manufacturer’s installation guide.
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