Floors & insulation
Get the key construction details right, and you’ll be on the straight and narrow road to a successful attic conversion
Discover what you need to know to get these important structural elements right
Whether your project needs full planning permission or not, if you’re creating habitable space then you’ll need to gain building control approval for the works. This will include areas such as structural safety, thermal performance, sound insulation and fire protection and emergency egress. Here’s what you need to know.
Strengthening floors
In most existing attics, the timbers you see at floor level are actually ceiling joists. They’re designed to take the ‘dead loads’ of the ceiling and any related fittings (lights, extract fans etc) below, and perhaps a little storage. Do not be fooled by the presence of a few loft boards installed by previous owners: unless the house was built with a future room-in-roof conversion in mind, the joists almost certainly won’t have been engineered to support the live loads imposed by habitable use.
The first step is to get someone who knows what they are talking about – either a structural engineer or a loft conversion specialist – to inspect the existing setup and calculate what’s required. Fundamentally, a beefier floor structure needs to be inserted without unduly disturbing the ceiling joists (which will remain in place). If there’s too much deflection in either of these, it could lead to expensive damage to the finishes below – or worse.
The design will need to account for potential issues such as locations where the original joists have to be cut
and braced to allow a route for the new ones, as well as weak points above window openings on the storey below.
If alterations are needed to the roof itself (such as removing truss webs or purlins and replacing them with dwarf walls lower down the rafters), then the floor specification will form part of this structural design.
The conventional solution is to combine suitably-sized timbers with rolled steel joists (RSJS) where extra support is required. The new floor will be raised slightly above the old on wall plate packers to protect the ceiling. An alternative is the easy-install Telebeam system (see right), which uses telescopic aluminium beams to carry the loads of both floor and roof.
Once this work is complete, alterations to the rafters and bracing can be made sequentially, according to the engineering design, to avoid leaving the roof in a weakened state.
Insulating the space
Whenever you’re creating new habitable zones, you need to consider thermal performance. In a loft, that means adding plenty of insulation overhead.
The simplest option is to pack the space between the rafters with suitable insulation (such as foam-based boards), leaving a 50mm ventilation gap between this and the roof felt above to prevent condensation. This is known as a ‘cold roof’. To meet Building Regs requirements for energy efficiency, you’ll typically also need to add a layer of insulation beneath the rafters. High-performance options such as multi-foils can help to minimise the amount of head height lost. Insulating plasterboard is also available.
If you can’t get enough thermal protection in while maintaining adequate headroom, or you’re planning to strip the roof covering anyway, then you could opt for a ‘warm roof’ setup. This involves insulating above the rafters, thereby creating an unbroken thermal envelope that should deliver improved performance. If you can achieve this without raising the roof by more than 150mm, your project may not require formal planning permission (unless you’re making other significant alterations).
You’ll also need to insulate the floor structure – primarily against sound transfer. A common approach is to fit a layer of Rockwool or a similar sound-deadening product. If you’re opting for a home cinema or teenage den in the loft, you might want to look into solutions such as British Gypsum’s Silent Floor system – but bear in mind these will encroach on clear headroom. Party walls, gables and internal partitions may all need insulating, too.