Assessing the feasibility of a loft conversion
Assess the roof shape
The overall form and profile of the roof will have a major bearing on whether the roof is suitable for a conversion to usable space. Here are some of the common forms of roof shape.
A roof consisting of a ridge with gable end walls is generally the easiest type of roof to create living space . Party of gable end walls, together with any internal load bearing walls, can usually support any new beams or trussed purlins which may be required.
Roofs with Intersecting Pitches (Valleys)
Properties with intersecting pitches as shown in the picture can be difficult to convert unless existing internal cross walls are present to provide support for new timber beams or purlins. Insertion of new walls or columns may be required at the intersections to support new beams.
Converting a loft with a hipped roof can be complicated unless internal load bearing walls are available or extra beams are inserted at floor to support secondary beams or purlins. Hipped roofs mainly require a Hip-End dormer or Hip to Gable dormer.
The minimum ceiling height
The size and shape of the existing roof space will determine whether it can be used for living accommodation. A ceiling height of 2.4m is the preferred minimum for habitable rooms. (2.4m is required for a dormer conversion) This height need not be maintained over the whole floor however practical height is needed for standing/walking or accommodating furniture.
The maximum ceiling height
There is no upper limit on the height of rooms subject to structural limitations. It is often more convenient to fit a ceiling and side walls to limit the usable volume. It is common for the side walls to be around 1m high so that furniture can be placed in front of them and to allow the fitting of insulated access panels so that the eaves can be used as storage and to gain access.
Space for access and fire escape
The conversion of roof space into habitable loft conversion must include provision for safe access and exit. This requires a fixed staircase which must comply with Part K of the Building Regulations in respect of its pitch (Maximum pitch of 42° on a domestic flight), stair width and balustrades. A 2.0m minimum ceiling height of is required on the last tread of the staircase.
The finished internal room dimensions should allow for:
- The thickness and depth of the new floor joists
- The depth of the rafters and any thermal insulation below them
What legislation will be involved?
Most loft conversions do not require planning permission, as you can make certain types of changes to your house what is called "permitted development rights", 40m3 on terrace houses, and 50m3 on semi and detached houses. This derives from general planning permission in Parliament not by the local authorities, however bear it in mind that permitted development rights, which apply, to many common projects for houses do not apply to maisonettes/flats or other buildings. Permitted development rights are different if the property is a listed building.
Building Regulations must be adhered to on every loft conversion as Building Regulations set standards for the design and construction of buildings, Building Control approval of your project will ensure that your conversion is structurally safe, & in the unlikely event of a fire primarily to ensure the health and safety of occupants in or about the structure. If a conversion is started without Building Control approval, heavy fines can be imposed on both parties. Full compliance guarantees certification of the works upon completion; this is important to process the property sale in the future.
Order 2015: sets out classes of development for which planning permission is automatically given, this is only provided that no restrictive conditions are attached or the development is exempt from the permitted development rights.
Planning legislation allows accommodation to be extended into the roof space without planning permission, providing the extension is within the rules set out under the Town & Country Planning Legislation. Such alterations are made under what are known as permitted development rights. Alterations and inclusions are defined under Schedule 2 which is separated into two classes:
- Class A Part 1 Enlargements, improvements or alterations of dwellings generally.
- Class B Enlargement or conversion of a dwelling that involves addition or alteration to the roof.
The primary aims of planning control are to prevent excessive increases in the house volume and to prevent an increased height or substantial change of the shape of a roof that will affect the visual relationship of a building to its neighbours. However, planning permission is not required unless the roof volume is increased, in planning terms the ‘volume’ of a house includes the roof, whether occupied or not therefore merely converting the roof will not increase the existing volume. The advantage of using the space within the existing roof height and shape is that, in the majority of cases, planning approval will not be required for the conversion. However, if the roof conversion is part of a larger extension of the house, the total increased volume will have to comply with the statutory limits. Planning approval will always be required if there is a change of use of the building involved. Planning approval will also be required where rooflights/velux windows project above the roof plane on the slope facing the highway..
Permitted development rights are restricted in conservation areas, and areas of outstanding natural beauty and on any listed building. Any alteration to a building, which affects its appearance in these circumstances, will always require planning permission for any alteration. Any work to a listed building, even if it does not alter the appearance will required listed building consent.
When a loft conversion on a building is planned, all building work must confirm with the statutory requirements of the relevant national Building Regulations, when a property or building exceeds two floors there are more stringent requirements to be met. These cover the requirements, such as structural stability, thermal insulation, fitness of materials etc. They are supported by examples of construction that are accepted as capable of meeting the requirements. Other methods may be acceptable, but compliance must be demonstrated to the satisfaction of the Building Control Surveyor. Building Control Surveyors often adopt a flexible approach to conversion because of the individual nature of the work, but they should be consulted at an early stage in the design.
Applying for planning and building consent
If the proposed project requires planning consent then an application has to be made to the local authority using relevant application forms. The application has to include details of the design showing how it affects the appearance of buildings concerned. There is a fee payable to the local authority who are required to notify the applicant of the decision of the planning committee with an agreed time scale. The application can be made by anyone involved in the project, but is often made by the architect, surveyor or loft conversion specialist.
Building Regulation consent can be applied for in one of two ways, either a Full Plans Application or by giving a Building Notice. A Full Plans Application is the most common route and involves sending in sets of plans showing all details and calculations subject to the Building Regulations. The local authority is required to give their decision within an agreed time scale. An inspection fee is payable to the Building Control Surveyor who will visit the loft conversion at key stages during construction to ensure that work complies with the regulations.
The alternative of giving a Building Notice costs the same but the total fee must be paid when the Building Notice is given. Full plans are not required but it is probable that The Building Control Surveyor will request a layout plan of the loft conversion with details of beam sizes etc and may request support structural calculations. The Building Notice must be issued before work commences and the onus is on whoever is carrying out the work to comply with the Building Regulations. The Building Control Surveyor will visit the site frequently to ensure that work complies with the Regulations.
Complying with the building regulations
New structural members or alterations to existing structural members in order to convert an existing roof must be undertaken by a structural engineer, this is to ensure both the stability of the existing structure and the structural sufficiency of the new structure, formal calculations may be requested by Building Control in support of an application for approval.
Means of Escape
A house that is extended or altered to add a third storey has to comply with stringent emergency escape provisions. The conversion of a roof space that provides one or more new habitable rooms at second floor level should include the following provisions:
- The existing stairway/stairwell at ground and first floor levels should be enclosed and the stairway should open directly to a final exit, or to a space which provides two escape routes, each to a final exit, and separated from each other.
- The new stairway must comply with the requirements of the regulations. A new staircase within an existing stairway enclosure should be separated from any new rooms, and should be separated from that room and the rest of the house by fire resisting construction.
- Escape routes over balconies or flat roofs (of fire resisting construction) will require to be protected by guardrails in accordance with the Regulations.
- The installation of hard wired and battery back up smoke alarms in all habitable rooms off the escape route is mandatory, unless you have a open plan staircase then a sprinkler system may be introduced.
All loft conversions must conform with fire requirements in the National Regulations. This largely concerns the conversion of an upper floor ceiling to a new floor construction, which must then achieve the fire resistance of an intermediate floor – normally 30 minutes.
The requirement for fire resistance includes any member supporting a floor, even if this extends beyond the actual floor area of the accommodation, I.e. between the outer wall of an loft space and the external wall at the eaves. (Note, if the roof accommodation will be of a different occupancy, such as a separate flat, to that on the floor below, it may be necessary to achieve full separating floor fire resistance, usually 60 minutes.)
If an existing floor supports a new staircase to habitable space in the roof, If an attic wall forms part of a separating wall, between properties, it must achieve the full fire resistance of a separating wall.
If the roof space incorporates a chimney or a flue any new construction must comply with the space separation requirements for combustible material. No combustible material other than a floor, skirting, picture rail, mantle-shelf or architrave should be closer than 200mm to a flue in a chimney or 40mm from outer surface of a masonry chimney. Nails, screws or other metal fixing timber to a masonry chimney should be at least 50mm from the flue always remember to consider the position of the chimneys and ventilation stacks in relation to the position of windows.
Surface spread of flame
Walls and ceilings to any new accommodation within the roof must comply with appropriate spread of flame requirements. Plasterboard will meet the spread of flame requirements. Any surface that slopes at an angle of 70 degrees or more to the horizontal is deemed to be a wall.
All walls separating dwellings must have adequate airborne sound insulation. Existing separating walls in roof spaces may not provide adequate resistance to airborne sound if the roof is converted into occupied space on one side of the separating wall. If an attic room abuts a separating wall, the wall must therefore be lined to provide equivalent sound insulation to the separating wall elsewhere in the building.
Gaps left in existing separating /party walls should be filled with bricks or blocks with a sand and cement mix. If a timber frame is used it should only be fixed to the floor and roof structure, any gaps between the framing and the floors and roofs sealed with acoustic sealant. Separate timber frames will reduce the roof space dimension between separating walls by approximately 90mm.
Plasterboard lining to a timber framed separating wall will normally be adequate for fire resistance but additional plasterboard may be required to improve sound insulation. Mineral fibre/plasterboard laminates can be used to line timber frame and masonry separating walls to improve both sound insulation and fire resistance.
In residential dwelling houses if a loft conversion is going to be constructed, construction should be designed in such a way that it provides reasonable resistance to sound from other parts of the same building and from adjoining neighbours.
Ventilation of Rooms
Windows to habitable rooms should be proportional to the floor area with background ventilation as required in all rooms. Bathrooms should be provided with an opening window, and a mechanical ventilation fan should also be provided which may be operated intermittently.
Ventilation of Roof Space
The introduction of a room into a previously open roof space will restrict the flow of natural ventilation through the roof intended to avoid condensation. The existing ventilation provision, if any, at the eaves may have to be increased to maintain a continuous airflow, a 50mm gap should be maintained on a flat roof dormer to provide a lateral flow of air.
Where accommodation occurs within a roof space. It is important to incorporate a vapour control layer on the warm side of the insulation for both walls and sloping ceilings.
Adequate room ventilation is required to prevent the build of water vapour within the rooms. Extract ventilation to bathrooms and kitchens and background ventilation to habitable rooms should provide sufficient ventilation for this purpose
The pitch of any stair leading to a room within the loft must confirm with Building Requirements, they must also conform to requirements of escape, and require some form of fire protection.
Adding a loft conversion to a two storey building effectively increases it to three storeys with consequent increased requirements, there is no restriction on the type of staircase, which may be a straight flight, a winding staircase, a quarter landing or a half landing. Loft ladders are NOT to be used to provide access to any new room in the roof other than for storage.
In instances where there is insufficient headroom at the top of a staircase a VELUX roof window can provide additional height as well as light and ventilation. An alternative means of achieving the required head height is to create a dropped landing. This also creates an interesting design feature.
Any habitable accommodation inserted into the roof space will be above this insulation and has to be insulated in line with the requirements of the current National Regulations. This can be done by either insulating the whole roof pitch from ridge to eaves, or by insulation around the new accommodation only.
The first method is more complicated to do, and will involve more insulation material, particularly in the apex of the roof.
The second method saves on the amount of insulation required, the walls and any part of the sloping ceiling and any horizontal ceiling over the habitable space will in this case need to be insulated.
Any gable wall in the roof accommodation will have to be insulated to conform with thermal requirements for external walls. This can be done by using plasterboard with insulation laminated to the back face or in timber-frame construction by inserting mineral wool insulation between the studs before lining them with a vapour control layer and plasterboard.
Daylight and View Out
Regulations state there is a specific requirement that glazed areas must be equivalent to a proportion of the floor, any habitable room should have a source of daylight and a view out. Good practice would suggest a minimum glazed area equivalent to 1/10 of the floor area for this purpose.
Roof windows can provide these facilities and dormer windows general appearance of a roof should not be affected by the conversion of the roof space, even if the height and shape have not been affected.
If a roof overlooks a ‘public highway’, or any projection above an existing roof plane it will require planning approval. Roof windows parallel to the slope of roof do not require planning approval as they do not project significantly and will therefore have a limited effect on the appearance of the roof. In a particularly critical environment, such as a conservation area, a recessed type of roof window can be used instead of a standard type.
Where a new bathroom is located in a loft conversion, provisions are required for hot and cold water supplies and drainage. Access must be made available to for maintenance to any water tanks re-located into the void areas.
If pipe-runs from the hot water cylinder are long, and the appliances will only have occasional use, for example a visitors bathroom, the use of electric instantaneous water heaters to basins and showers might be considered as an alternative.
If you choose to have a bathroom in your loft conversion the position of the existing drainage will influence the location of sanitary fittings in that there is a maximum permitted length for unvented branch connections from baths and basins. If the lengths of branch pipes exceed these requirements they must be directly ventilated to the external air.
Alternatively a ventilating pipe may terminate inside the building if fitted with an air admittance valve, a AAV installed internally are usually fitted within rooms or in the roof space, the great advantage of using a AAV is that they remove the need for holes through the roof or external walls, and reduce the amount of pipework required.
Loft conversions usually have low heat losses due to the high elemental insulation requirements required, as long as the existing boiler has sufficient capacity there should be no problem adding additional radiators. If the conversion forms part of a larger refurbishment then a new central heating system may be viable.
Any re-roofing necessary in converting the roof must conform with the performance of the original roof in terms of fire resistance, durability and general fitness for purpose. Preservative treatment to roof timbers is not mandatory, but if evidence of previous infestation of woodworm or previous fungal decay is visible treatment should be undertaken prior to sealing the roof structure.
The structural design of the conversion
Your existing ceiling joists are very unlikely to be adequate to act as floor joists.
Fit new deeper joists between existing ceiling joists, if existing binders are to be removed the existing ceiling joists will require support from the new floor joists and their weight should be added to the floor joist dead load. Position new steel beams above existing ceiling. Insert new floor joists into beam web.
New partition loads in the roof space should not be supported by the floor decking only, additional joists will be required below internal non-load-bearing partitions running parallel with the floor joists.
Floor openings formed for stair access can impose heavy loads on trimmers. Trimming joists onto existing load-bearing walls, or onto new walls or floor beams either transfers these concentrated loads. Double joists may be required as trimmers to frame stair openings.
Beams to support floor joists can be of timber or steel. Depending on the structural integrity, generally timber based beams are doubled or trebled up to conform to stress grades.
The structural design of timber beams will need to be undertaken for each case individually. Loads on beams should be assessed and beams designed in accordance with standard engineering principles.
Exposed beams supporting floors must provide 30 minutes fire resistance. This can be achieved by oversizing timber beams ,timber burns at a known ‘charring rate’ of 20mm per half hour.
It is recommended that any steel beam leaves a minimum headroom of 2.0m, which is the mandatory minimum height for landings and stairways. When steel beams are used and timber joists are fitted between, it is important to allow for the shrinkage, which will occur, when the joists dry to their equilibrium moisture content.
The floor deck or ceiling should not bear on the steel beam and cause distortion of the floor or ceiling. A movement gap of approximately 12mm between the steel and the floor deck and 5mm between the steel and ceiling will generally be adequate for moisture movement.
If new floor joists are to be installed over an existing ceiling it is advisable to allow 10mm clearance between the underside of the joist and the ceiling to allow for possible deflection of the joists under floor loading.
Floorboards should be cramped before screwing, and all screws should be below the board surface, screws should be 55mm in length 3.5mm in width. Three screws should be used at each support. The ends of boards should be butted over joists with both boards adequately supported. End joists should be staggered so that they are at least two board widths apart. Any board should span at least three joists.
Flooring to habitable roof space should be laid in accordance to reduce vibration and damage to existing plaster ceilings to be retained, fixing floor decking to any floors by screwing is preferably to nailing.
Loft floor decking is usually fitted to a house that already has heating therefore moisture content isn’t really a issue and this prevents excessive shrinkage and possible distortion, tongued and grooved flooring is usually used in loft conversions 2400mm x 600mm
When purlins and supports are to be removed to form usable space, alternative support must be provided to the rafters. It is important to establish the total function of the existing purlin since in some cases purlins and ties may provide resistance to outward thrust as well as vertical support.
A number of alternative solutions are possible depending upon the size and construction of the roof, such as the following (either individually or in combination)
- If existing purlin support has to be removed alternative support off existing load-bearing walls below or onto a new supporting structure in the roof space is required.
- Insert new purlins at new levels, this may necessitate additional rafter support because of the longer rafter span between the new purlin and the eaves. This can be provided by an additional purlin or by stiffening the rafters with new timber alongside the existing.
- Insert a ridge beam, the simplest way of installing a ridge beam is to go from gable/party to gable/party walls. Inserting a ridge beam should be considered when it is necessary to remove existing ceiling joists. Adding a ridge beam minimises outward thrust in the rafters.
- It is uneconomic to use solid timber purlins for unsupported spans in excess of 4.0m. Internal load bearing walls or posts should be used where the span exceeds 4.0m. For longer spans, or where no intermediate support is possible, fabricated timber or steel beams should be considered
- Deep beams may be used to support both roof and floor load, using either metal hangers nailed to the primary beams to carry the floor joists.
Alternative timber beams include:
- Plywood web I-beam
- Plywood box-beam
- Steel flitch beam
Beams can be built into existing cavity walls but to allow for end supports the beams must be approximately 200mm longer than the clear span of the room or space. Timber beams should not be built into external solid masonry walls. Heavy duty galvanised steel hangers are an alternative means of supporting timber beams. Steel hangers should be appropriate for the load to be supported. Hangers should be supported directly by bricks or concrete blocks with the face of the hanger tight against the wall face.
Only timber posts or studs must support timber beams in timber frame walls. This may require walls to be opened up to insert new timbers. Alternatively it may be possible to position the post against the wall face, f it is necessary to insert new studs on posts into an external timber frame wall, the vapour control layer must be reinstated and the effect on the thermal insulation of the wall considered.
If new rafters are to be inserted between existing ones or along side, they should be selected from span tables set out by building control, if purlins are repositioned, it is essential to check the effective rafter span, the overall rafter length required should be checked against the availability of suitable timber and provision for lapped or spliced joints made when necessary.
Calculation of Roof Loads
The roof imposed load and dead load from roof tiles or slates, timber battens, underlay or boards, rafters, insulation and linings is measured on the slope, the total weight is resolved to produce a vertical load on plan.
The specification of the windows is particularly important in loft conversions, not only are they crucial in providing in providing sufficient ventilation suitable for installation. Both centre-pivot and top-hung windows can be rotated to provide access for cleaning. Repainting of the pre-finished external aluminium cladding is not necessary.
Incorporating a roof window will usually reduce the total roof load since the weight of the window may be less than that of the roofing materials, which it replaces. The imposed load on the roof is unaffected. Narrow roof windows can be fitted between rafters of 600mm spacing, otherwise trimmed openings will be formed to fit the required size of window.