Depending on the roof structure and planning constraints, a loft conversion is one of the most straightforward ways of getting extra space. Almost all houses can benefit from this extension with a bit of basic planning.
IS MY LOFT SUITABLE FOR CONVERSION?
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.
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.
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. 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.
It’s a good idea to use the low space under the eaves for storage — but you could create a more dramatic effect by having a gallery overlooking the floor below.
The existing ceiling joists are unlikely to be adequate to take a conversion floor, so additional new joists will be required to comply with the Building Regulations. The size and grade would be indicated in Trada Span Tables or specified by a structural engineer, who will have taken into account the span and the separation distance for a given loading. The new joists span between load-bearing walls, and are normally raised slightly above the existing ceiling plasterwork. This spacing must be sufficient to prevent any new floor joist deflection from touching the ceiling plaster below. The new joists run alongside the existing joists. Above window and door openings, thicker timbers are used to bridge the opening, so that pressure is not put on the existing opening lintel. RSJs may also specified to distribute the load, and in some installations are used to carry the ends of the new joists.
The roof structure can be insulated in one of two main ways. The most straightforward is to use a ‘cold roof’ method. This involves partial filling the space between the rafters with phenolic foam insulation such as Kingspan or Celotex, ensuring that there is 50mm spacing between the roofing felt and the insulation (for ventilation via the roof and soffit vents). In addition, a suitable thermal laminate plasterboard is attached to the inside of the rafters. The rafter thickness is often less than 120mm, so a batten may be required along each rafter to allow the 50mm spacing and the insulation. The roof section requires 350mm of mineral wool insulation e.g. Rockwool.
The other main method is ‘warm roof’. This method uses 100mm Kingspan or 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 between the studwork.
The internal partition walls use a 100mm quilt or 25mm Isover APR 1200 that will provide sound insulation. Plaster - board is attached to one side of the wall then the insulation inserted, followed by plaster - board on the other side.
Insulation can also placed between floor joists, and this is typically 100mm-thick Rockwool fibre or similar — mainly for its sound-reduction properties.
The ideal location for a staircase to land is in line with the roof ridge: this will make best use of the available height above the staircase. The minimum height requirement above the pitch line is 2m, although this could be reduced to 1.9m in the centre, and 1.8m to the side of a stair. In practice, the actual position will depend upon the layout of the floor below, and where necessary the available height can be achieved using a dormer or adding a rooflight above the staircase or, if appropriate, converting a hip roof end to a gable.
Maximum Number of Steps:
The Regulations specify that the maximum number of steps in a straight line is 16. This is not normally a problem, as a typical installation usually only requires 13 steps.
The maximum step rise is 220mm, whereas the step depth or ‘going’ is a minimum of 220mm; these measurements are taken from the pitch point. The step normally has a nose that projects 16-20mm in front of the pitch line. However, the ratio of size must not exceed the maximum angle of pitch requirement of 42°. Any winders must have a minimum of 50mm at the narrowest point. The width of steps is unregulated, but in practice the winders are likely to limit the reduction in width.
The height minimum is 900mm above the pitch line, and any spindles must have a separation distance that a 100mm sphere cannot pass through.
WINDOWS & DORMERS
The loft conversion will require a means of getting natural light and ventilation, and the most straightforward method is to use rooflights that follow the pitch line of the roof. This type is fitted by removing the tiles and battens in the position that the rooflight will be fitted. The rafters are cut to make way for the rooflight after suitably reinforcing the remaining rafters. The rooflight frame is then fitted within the new opening, and flashings added before making good the surrounding tiling. This type of window is the most economic, and more likely to be allowed without planning permission, under your Permitted Development rights. Conservation rooflights, which are slightly more flush to the roofline and are made of metal, can also be specified.
Dormers not only give natural light but can add space to a loft conversion; they can be at the ends or sides. They are particularly effective where the pitch angle is high, as the useful floor area can be increased. The mansard type will give maximum conversion roof space because it projects the maximum available head height, thus giving a greater usable floor area. A hip to gable conversion has a similar effect.
Dormers and other similar conversions are normally installed by opening up the roof, and cutting the required specified timbers to size on site. They normally involve compound angle cuts. Care also needs to be taken with the roof and side coverings, to get a good weatherproof structure.
Some companies will make the dormers off site in their workshop and lift into place. This process allows quick installation, and quick weatherproofing.
Dormers can have gabled or hip roofs, and with careful design can enhance a roof line. In practice, a mixture of the available types can result in the maximum light and space, and provide a fire exit.
The plasterboard ceiling in the upper rooms will delay the spread of fire to the roof space in an unconverted house. However, when an opening is introduced for the staircase the risk is shared with the conversion — therefore, safeguards must be in place to reduce the risk.
All habitable rooms in the upper storeys served by a single staircase should have an escape window with an obstructed openable area of at least 0.33m², a minimum 450mm high x 450mm wide, and not more than 1.1m above the floor level. For loft conversions to existing two storey houses, more stringent provisions apply, due to the greater risk associated with escape via high-level windows. These require a new 30- minute fire-resistant floor to the loft conversion, and a protected 30- minute fire-resistant stair enclosure discharging to its own final exit, with fire doors to all rooms (except bathrooms and WC). The fire doors do not need to be self-closing.
At least one mains-operated smoke alarm with battery backup must be installed in the circulation space of each storey. All alarms are to be interconnected.
(Homebuilding & Renovating)