07 October 2008What is a loft conversion
A loft conversion is any roof space used for any purpose other than light storage on loose boarding (light storage being household items e.g. suitcases). That is to say any loft space used as:
Also, a loft is considered to be a conversion if plastered and provided with heating and/or power.
If an altered loft could be used as a room it must be treated as being converted into a habitable room.
Building Control, what do they do?
Building Control enforce the Building Regulations on behalf of Sevenoaks District Council. The Building Regulations are there to ensure buildings are constructed or altered to national standards.
If you alter a roof space and don't get Building Regulation approval, it may put life at risk and jeopardise future property sale. Failure to get Building Regulation approval, have already been carried out, then a Regularisation Certificate could be issued.
Loft conversions - Structure
This guide is not intended to give guidance on the structural part of roof alterations. Roofs are too varied to be subject to general advice. For further information, you should refer back to your designer or structural engineer.
Is my loft suitable to be converted?
Loft conversions can be an effective way of providing additional space, but most cases will require the existing roof structure to be stiffened and upgraded. The extent of the structural work required will depend on:
Existing walls, foundations and lintels
The existing foundations will need to be exposed so it can be determined if they are suitable to accept the additional loading of the new loft room. If not suitable, they may require underpinning.
Generally underpinning is expensive and the feasibility of the project may depend on this.
There is likely to be additional loading on existing lintels and beams, so these also will need to be exposed.
Loft room floor construction
Typically, the existing ceiling below a roof is constructed of 100 x 50mm joists. Normally this is not suitable for the loft room floor. Usually, deeper joists will need to be laid alongside but independent of the existing ceiling joists.
The new floor joists must be laid from load bearing wall to load bearing wall (or beam). Joists need a minimum 50mm bearing each end, and should be lifted clear of the existing ceiling by 25mm. This allows for deflection in the new joists, and means the new timbers will not damage the existing ceiling when loaded.
Existing roof structure
To make the roof space usable, a trussed rafter may require more extensive alterations than a
traditional cut and pitch roof.
What is required to convert a roof structure?
Generally, ceiling joists are not designed to take the weight of a floor. New larger joists need to be positioned alongside the existing joists, to support the increased loading. Traditional cut and pitch roofs usually include diagonal struts to provide useable space these will need to be removed. The new floor joists need to be designed to support this load previously carried by purlins and struts, together with the new floor. Openings around stairwells need to be trimmed and ply sheathing used to brace dormer constructions. If a dormer is constructed then a new ridge beam to support the roof may be needed. The architect, designer or structural engineer
will be able to help with this.
Are there any internal load bearing walls?
Any internal wall, which is going to take additional load, must have its foundations checked. If the internal walls don't have adequate foundations, it may be necessary to underpin or install beams to support the new joists and roof structure.
Structural calculations
The roof structure should be designed and calculated by a designer or structural engineer.
These calculations and details must also be enclosed with your Building Regulations applications.
