Is My Loft Suitable for Conversion?
Conversion Assessment: The features that will decide the suitability of the roof space for conversion are the available head height, the pitch and the type of structure, as well as any obstacles such as water tanks or chimney stacks. An inspection of the roof space will reveal its structure and physical dimensions.
Head Height: Take a measurement from the bottom of the ridge timber to the top of the ceiling joist; the useable part of the roof should be greater than 2.2m.
Pitch Angle: The higher the pitch angle, the higher the central head height is likely to be, and if dormers are used or the roof is redesigned, then the floor area can be increased.
Type of Structure: Two main structures are used for roof construction — namely traditional framed type and truss section type. The traditional framed type is typically found in pre-1960s houses where the rafters and ceiling joists, together with supporting timbers, are cut to size on site and assembled. This type of structure has more structural input, so is often the most suitable type for conversion. The space can be easily, and relatively inexpensively, opened up by strengthening the rafters and adding supports as specified by a structural engineer.
Post 1960s, the most popular form of construction used factory-made truss roof sections. These utilise thinner – and therefore cheaper timbers – but have structural integrity by the addition of braced diagonal timbers. They allow a house roof to be erected and felted in a day, which is a big advantage to a builder. However, this type of truss suggests that there are no loadbearing structures beneath, and so opening up the space requires a greater added structural input. This will normally involve the insertion of steel beams between loadbearing walls for the new floor joists to hang on and the rafter section to be supported on — together with a steel beam at the ridge. This added structural input requires skill, knowledge and equipment that would limit scope as far as DIY is concerned — and a far greater cash outlay. It is advisable to seek advice from specialist firms in this instance.
Low Head Height?
If the initial roof space inspection reveals a head height of less than 2.2m, there are two available – but costly – solutions that will require professional input.
Solution 1: Raise the Roof: This would involve removing part or whole of the existing roof, and rebuilding it to give the required height and structure. This is structurally feasible, but the major problems are the high cost and getting planning approval. If the whole roof area needs removing, a covered scaffold structure, to protect the house from the weather during the works, would also be required.
Solution 2: Lower the Ceiling in the Room Below: The ceiling height in some rooms in older properties may be 3m or more, so if the roof space height is limited there is the option of lowering the ceilings below, providing it still allows at least 2.4m. This will require all the existing ceilings in question to be removed, causing much mess. With this method a plate will need to be bolted to the wall using shield anchors or rawlbolts, for the new floor joists to hang from. There is also a need for a suitable tie between the roof structure and the dwarf wall formed, to prevent the roof spreading. Any DIY involvement will be limited to supervised demolition and clearing up.
Your Building Control inspector will specify exactly what you require. The roof structure can be insulated in one of two main ways. The most straightforward is to use a ‘cold roof’ method. This involves filling the space between the rafters with 70mm-thick slab foam insulation such as Celotex, ensuring that there is 50mm spacing between the roofing felt and the insulation (for ventilation via the roof and soffit vents). In addition, 30mm slab insulation is attached to the inside of the rafters, giving a total of 100mm of insulation. The rafter thickness is often less than 120mm, so a batten may be required along each rafter to allow the 50mm spacing and the 70mm insulation. The roof section requires 300mm of mineral wool insulation (e.g. Rockwool), or 150mm of slab foam insulation, such as Celotex. This method can be undertaken by the DIYer.
The other main method is ‘warm roof’. This method uses 100mm Celotex insulation or similar over the rafters, and a covering capping, followed by the tile battens and tiles. This is not really a practical option unless the roof coverings have been stripped off. It could be used with a dormer, especially if it has a flat roof. Continuity of insulation between walls and roof is required to avoid any cold bridging. The dormer walls can be insulated with 100mm Celotex between the studwork. The internal partition walls use a 100mm quilt that will provide sound insulation. Plaster – board is attached to one side of the wall then the quilt inserted, followed by plaster – board on the other side. Insulation is also placed between floor joists, and this is typically 100mm-thick Rockwool fibre or similar — mainly for its sound-reduction properties.