In certain circumstances it may be possible to convert the roof space into a room. This, of course, needs Local Authority consent, as it involves structural alterations as well as altering the appearance of the building. Normally, planning approval is nec- essary only for dormer windows inserted in the roof structure in the front elevation or above the ridge line. Windows now manufactured can be fixed between the existing rafters without projecting in front of the roof. These are commonly acceptable to the authorities.
In houses with low-pitched roofs, conversion of the attic is impracticable as adequate headroom cannot be achieved. However, purlin or strutted- purlin roofs are adaptable, as support members are normally 2 ft (or 6.0 cm) apart, allowing additional space to be deployed with minimum structural alteration.
Natural light, ventilation, heating and thermal insulation must be provided in a roof that is to serve as living space, be it guest room, playroom or study. In some cases the floor will need to be reinforces as well as adequately covered.
Daylight and ventilation can be achieved by means of dormer or roof windows, the opening area of the window being 1 /20th of the floor area of the roof space.
Roof rooms should be properly insulated to prevent heat wastage. The space between rafters may be filled with insulating board, glass-fibre matting, mineral wool, polystyrene or other insulant material. Glassfibre or other matting provides an insulant layer when fixed between the stud walls of a dormer.
The normal sequence adopted when constructing a room in the roof is as follows: 1 The roofing tiles and battens in the area of the new dormer windows are removed, together with the felt below, to allow access for the new floor timbers.
2 The existing ceiling joists should next be strengthened. This is most easily done by building in new floor joists of adequate size between the existing timbers supporting the ceiling below.
3 To reduce sound transmission to the rooms below, it is advisable to lay fibreglass or similar material between the joists.
4 The floor-covering should be laid next.
5 The final principal task in converting the roof space into living space is to construct dormer windows.
Dormers. The size of the dormer using a pitched roof is determined by the height of the top of the window frame and the height of the ridge to the roof.
The slope of the small roof over the dormer has to be the same angle as the main roof. If a wide window is required,
then a flat roof is formed over the dormer window and the basic construction remains the same.
It may be necessary to use a nonstandard window frame. This can be made by a specialist or by yourself.
To determine the actual window size you will have to work back from the rafter sizes and allow for the framing timber and boarding.
This assumes that the roof is tiled with battens and felt on common rafters.
1 Establish the exact position of the window. Working from the ridge outwards will determine the spread (or width) of the dormer. From here the common rafters affected are to be trimmed for the opening. The size of timber to be used should be at least -lin. (13 mm) thicker than the size of the common rafters.
2 To expose the battens supporting the tiles, cut the roofing felt with a sharp knife.
3 Next, remove the tiles from the outside over the opening; take care not to break any as you will be able to re-use the tiles on the dormer roof.
4 With tiles removed, the battens can now be trimmed to the opening size.
5 The vertical studding is now erected on the floor joists and fixed to the inside face of the common rafters. This studding will also form the support to the dormer roof as well as the side or cheek of the dormers.
6 On to the top of this studding are fixed the ceiling joists and rafters. The ridge of the dormer roof will be its own centre. You should now have the skeleton frame of the dormer.
7 Next, fit the stiffening members to the skeleton. In this case, these take the form of boarding, fixed to the gable cheeks and the roof of the dormer. Note: the ridge
should be a good lin. To 1 in. (1 .2-2.5 cm) above the boarding, so that the tiles may be hung correctly.
8 Before slotting in the window frame, dress No. 6 lead or other flashing material over the bottom trimming member.
9 The window frame can now be inserted and nailed into position.
10 The tile battens can next be nailed on. Work out the centres of the battens so that the tiles line up with the existing roof tiles.
11 Any soakers are next fixed.
12 The other apron is laid on the roof of the dormer and dressed over the top row of tiles.
13 All tiles are then fixed. The corners are clad with special angle tiles. As well as being fixed with nails they are pointed with mortar. The ridge tiles are also laid on mortar. Care must be taken to ensure that the gable tiles line up with the tiles on the cheeks of the dormer where they meet at the head of the window.
14 Finally, line the ceiling joists with plasterboard and apply a plaster skim.