Attic Conversion Structural Questions

14 Nov 2013
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United Kingdom

I am thinking of getting my attic converted. It looks like an ideal space for conversion but the building regs seem to make building into the attic very difficult particularly in Scotland.

The house is a brick built construction with Scottish Slate roof. All internal walls are single skinned brick with plaster on top. The ceilings are all lath and plaster. I live in the upper flat with full rights to the attic bar access allowed to neighbours and the attic measure around 50x34 foot. . There is a Mansard roof and the flat area leaves a ceiling height of around 9 foot which stretches around 30x10 foot. All the rafters and joists are 164x64mm at 450mm centres. The flat below the attic has a central hall with two structural walls running the length of the house. The hall is 5.5 foot wide and there is a span of around 14 foot either side of the hall to the outside walls.

The attic is floored in the centre and feels very strong(short span). I have just removed a huge cold water tank which when full must have weighed 2.5 tonne. It was sat on an outside span close to the hall wall with no other support than wood spreading the load. It was probably there for 80 years and the joist below look like new, poker straight.

I want to do most of the work with friends in trades chipping in but if the structural work is going to be too expensive then I will probably leave it for another time.

I am hoping to create an open plan kitchen, diner living area where nearly all the traffic will be in the centre across the shortest span. Looking at span tables they all seem to deal with the new timbers. The wood in my attic looks so much better than the crap you can buy nowadays so I wondered if that counted in the structural calculations. It would be hugely helpful as the joists look under spec for flooring.

Looking for any inspiration really as the Mansard roof make insulating more difficult as I really don't want to break into the flat roof as it is in great condition.

Sorry about the sprawling posts but thought a fuller explanation would be best.

I will have to hire a structural engineer to do some calculations so does anyone know of any good ones in the Edinburgh area or is this something that can now be done online.

Also does anyone have any initial thoughts on the joists.

Many thanks
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If the timber is hardwood or higher grade then yes it counts in the calcs. However most loft spaces are not designed for the loading required for everyday use and will deflect causing cracks downstairs. It is pretty rare to be able to prove existing timbers when converting loft space..
Thanks Static. The wood looks like softwood but feels much harder when you are cutting it. The water tank is the same vintage so presumably similar wood and the grain is far tighter than newer woods. Is there a way old timbers can be tested without spending a fortune
Not really.. you would probably need to load test to failure one member which would be expensive and only really viable if you have 100s of members.

Just assume you will need sister joists and/or additional support beams.
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