Converting the loft

Will I need planning permission?

The answer is YES if:

• The property is in a conservation area, world heritage site, national park or area of outstanding natural beauty
• The loft will be larger than 50m3 for a semidetached or detached home (40m3 for a terraced property)
• You plan to use materials with a different appearance from the rest of the property
  Or if you plan to:
• Add a balcony, veranda or dormer window
• Change the height or pitch of the roof
• Or have the extension overhang the outer face of the wall of the original house.

This advice refers to houses. Check the Planning Portal for flats and other buildings. Contact your local planning authority if in any doubt.

Floor joists

Even if the loft was previously used for light storage, you will almost certainly have to strengthen the ceiling joists in the loft floor. You can do this by adding larger and stronger timber floor joists between the existing ceiling joists. Support them on the existing walls if they are strong enough, or add extra timber support beams if necessary. A professionally designed loft conversion plan that complies with building regulations will be needed. Check with a structural engineer.

Building regulations require 30-minute fire resistant floors for loft conversions in homes of two or more storeys.

Load-bearing walls and foundations

New floor joists will need support. This can be achieved either from an existing wall, which will need to continue all the way down through the house to a foundation, or by an adequate intermediate support, such as steel or timber beams. It is particularly important to assess the structural integrity of older properties and houses with rooms knocked through on lower storeys.

In some cases, the proposed increase in load could require underpinning of the foundations. Check with a structural engineer or your building control body.

Getting your services in

There are rules about where and how much a structural timber member can be notched or drilled for wiring and pipework, as even a small amount of drilling or notching can weaken the timber significantly.

Holes or notches should not be cut in any rafters, purlins or engineered timber joists unless justified by the structural engineer.

Take advice from a structural engineer and your local building inspector.

Engineered products, such as I-joists may have drilling positions marked. Follow manufacturer’s instructions.

Windows

Part L1B of the Building Regulations requires a target C rating (or U-value of 1.6) in England and Wales, though higher-rated windows are widely available. See Wood Windows.

Factory-finished timber window frames are a high quality solution. They may cost a little more than PVC-u, but will last up to twice as long, only need recoating every 8-10 years, and are much more environmentally-friendly.

When installing a roof window, achieve an airtight seal round the frame to maintain the thermal performance of the roof. Consider solar-control glass to minimize heat gain from the sun, or use a fitted blind. See Timber Trade Topic  Wood Windows for more information on frame types and installation.

Fire safety

Additional fire protection may be necessary in the existing parts of the house. For example, a typical loft conversion to a two-storey house will require new fire-resisting doors, and sometimes partitions, to protect the stairway, as it is too dangerous to escape via windows from floors above first floor level.

Fire doors are life-critical. So it makes sense to use certified products. Certification schemes, like the BWF-CERTIFIRE Fire Door and Doorset Scheme and BM TRADA’s Q-Mark Fire Door Scheme, ensure doorsets are fit for purpose. See Timber Trade Topic Fire doors & doorsets.

Mains powered, interlinked smoke alarms are also required within the stairway at each level; and you may need to upgrade the fire protection to some parts of the structure of the house, such as the floors, where building regulations require 30-minute fire resistant floors for loft conversions in homes of two or more storeys.
Consult Approved Document B (fire safety).

Insulation

Building regulations require a minimum 270mm of insulation in the void between the floor joists (see adding larger, stronger joists above).

You can achieve acceptable sound insulation by using a wood-based board, such as plywood or 22mm tongued and grooved chipboard, weighing more than 15kg/m2 for the floor surface. With a terraced or semi-detached house, the building control body may also ask for improvements to the sound insulation between the converted loft and the neighbour’s loft.

A timber floor makes sense, as it is durable, easy to keep clean and good-looking. If you are laying a decorative timber floor, use an acoustic underlay. See Timber Trade Topic Flooring for more information.

You should insulate your loft by insulating the roof itself rather than, or as well as, the loft floor. Use rigid insulation boards between the roof rafters, cut so they fit snugly between the rafters. They can then be covered by plasterboard, or timber panelling. Rafters aren’t usually very deep, so to get the best performance, you may have to insulate over them as well, using insulated plasterboard. If there isn’t room to do this, make sure you use the highest performance insulation board.

Wood panelling

As well as adding character to a room, wood panelling can improve thermal and acoustic insulation and mask defects.

Many species are suitable, but pine and spruce are the most popular and are also available in a variety of pre-finished stains and finishes. Boards come tongued and grooved in a range of sizes and decorative profiles with thicknesses from approx. 7mm to 25mm. Boards thicker than 12mm provide additional structural strength. For best results use timber manufactured from higher grades and kiln dried to 8-12%.

Fitting wood panelling

On stud walls and unplastered ceilings, the panelling can be nailed direct to the studs or joists.

Otherwise, 22 x 38mm sawn battens provide an easy surface on which to nail the panelling and also form a flat base (fig 1).

Pack out any substantial irregularities under the battens. Use ‘secret nailing’: drive the nails diagonally through the tongue of each board (fig 2). Only the first and last boards require nailing through the face. Punch these home and fill with wood filler.
Don’t forget to allow an expansion gap around the perimeter of the panelling, particularly across the width of the boards. Use skirting boards and moulded trims to hide these gaps (fig 3).

Sound insulation should also be considered between walls and floors in order to prevent noise transmission between floors or to an adjoining neighbour’s house.

Pre-finished panelling systems are becoming increasingly available. Designed for accurate and rapid installation, they also avoid the need for on-site finishing.