It is a common miss-conception that truss rafter roofs are not suitable for loft conversions.
In fact, there is no reason whatsoever why houses built this way cannot be converted into that new loft conversion you have always wanted.
So what is a trussed roof?
Most modern houses built between the mid 1960s and the turn of the century will feature a truss rafter roof.
The main reason for this is that modern houses tend to have a sallower pitched roof. The houses were built in this manner to save money on materials.
So, What's The Problem?
The shallow pitch of the roof immediately makes the loft harder to convert because of the lack of headroom.
However, the main reason why it is harder to convert a trussed roof is because everyone of the structural timbers is absolutely essential to hold up the roof.
Therefore it is no easy task to remove any of the timbers without major restructuring work having to be carried out yet they must be taken out in order to free up room for the conversion.
The usual way in which trussed loft conversions are carried out is to install steel joists across the width of the roof and running the full length of the roof.
The beams are supported by the gable ends of the roof and replace the original wooden struts which are removed once the steel beams are in place.
If there isn't a gable end for support than new struts have to be installed to support the purlins.
The part of the wooden structure that remains is then supported on the new steel beams.
At the same time the joint at the top of each pair of rafters has to be reinforced. Usually with a timber 'collar' or sometimes with another steel beam.
Now all that doesn't really seem very complicated. Unfortunately it is.
Complex and very precise structural calculations have to be made by an engineer to ensure that the roof is supported correctly and it is no easy matter to get the beams inserted and the truss removed.
Take a peek in your loft and if you see that the roof timbers form into a ‘W’ shape, then your property has a trussed roof.
Other things to look for are a very shallow pitch to the sides of the roof and metal plates connecting the timbers. The illustration below should give you a further idea of what to look for.
OK. I’ve got a trussed roof. Now what?
Well. Don’t let it worry you. It simply means that your loft conversion will have to take into account the lower pitch of the roof.
In most cases this kind of loft conversion will require planning permission because the conversion will need to raise the original roofline of the property.
Because of the design of the truss frames the loft conversion may require a structural modification so, if you have this kind of roof, it is essential that professional structural engineers are consulted.
The lack of headroom in the existing roof space will be a further reason to alter the structure of the roof.
Raising the roofline to ensure adequate headroom in order to comply with the building regulations will be one of the first tasks undertaken during the loft conversion.
And, because of the load bearing demands the roof trusses will need to be replaced with steel girders.
This sounds highly complicated and, to a degree, converting a trussed roofed property is technically more demanding that working on an older property with a steeply pitched roof.
But. A specialist building company will be fully experienced in this kind of work and, a truss rafter loft conversion, is probably just as common as any other type of conversion.
So. Nothing to worry about then?
Absolutely. Having a home with a trussed roof design should in no way stop you from planning and building a successful loft conversion.
But do use specialist builders and an architect to draw-up your plans to ensure that the structural modifications are carried out correctly and safely.
For more info see the related pages below
Structural calculations for loft conversions are of course hugely important to ensure safety and that the proposed plans meet building regulations
What is the difference between planning permission and building regulations approval for loft conversions ?
Using And Fitting Loose Fill Insulation In Your Loft