New floor structure in large loft.

26 Sep 2010
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United Kingdom
I'll try to explain the problem.
300 Year old house which has a 7m x 20m loft space.
The beams and loft floor are all oak (all beams span the 7m and are not supported except in the walls) and have been fully treated. I want to keep the exposed beams in in the rooms below the loft and have the under side of the loft floor (hand cut oak planks) exposed. There is no insulation at present!

So,, my plan is to build 'LEVEL FLOOR' just above the existing floor in the loft. I will then insulate the space between the old and new floors and lay a new floor.

I will be developing the loft as living space later (few years) so floor needs to be strong enough for that but must not be supported off the oak. I would like the structure to be worm proof so I'm trying to avoid soft wood and although more oak is an option,,,, the beams would weigh and cost far too much,,, the existing beams are 18 or so inches sq. I'm thinking steel!! but what type and size? or are there any modern suspend floor systems?

The walls are 1m thick rock!! so supporting walls are not a problem

Anyone got any ideas? advice?
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450mm sq. oak beams? I don't think you need to do anything at all!

Your issues if converting the room at a later date will be fire regs (assuming this loft will become a 3rd floor) Which could entail laying plasterboard beneath the new floor, the beams will be alright for fire but the floor deck at present won't be.

Why not think about insulating the loft roof, you will have to if converting in the future anyway. Then you can leave all as is.

Got any pics of this 'castle'? (go on admit it, you are a Beefeater and live in the Tower) :LOL:

Thanks for the reply.

The problem is the beams are so uneven and the old floor in the loft is very bad. It looks ok from below as a ceiling but from above is not really useable. Too uneven and of course when in the loft you are walking on the boards of the ceiling below and there are gaps, it moves,,, and it's not insulated. I hear what you say about insulating the roof but i may not do anything in the loft for years but still want the house to be insulated as i am going with under floor heating down stairs,,, no insulation between ground and first floor and no heating upstairs (letting the heat rise).

I have found a place called Hadley Group that does rolled steel beams!! anyone used them?
I guess my problem is the 7m single span. Could I sit "C" beams on both walls (resting on the oak where it enters the walls) and then span the 7m with something else?? above the existing foor?
Yes doable, but 7m will need an engineered solution in steel, or timber which will be even deeper beams. Hope you've got plenty of head room in there!
Hence my point about using the oak beams as the floor. get an engineers advice as to if they're up to it, if so you can lay a new floor over existing, with insulation sandwiched between. Any uneven-ness can be sorted using plywood packing.

You shouldn't be concerned about using softwood, just get pre-treated stuff. No issues then.
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Hi i have not a castle but a 200 year old farmhouse, had structural engineer survey it and he reccommended a new sub floor of about 7 x 3 timber, biggest span was about 4m
7m is pretty far in domestic terms.. might be worth looking into lightweight mezannine flooring systems.. probably with cold rolled joists with floor boards over..

It is very interesting to hear the options you propose and I really appreciate the effort to answer. I'm an engineer by trade but not civil.

I am really skeptical of a soft wood solution as the place is in France and the oak is already wormed,,, it's been empty for years and they don't really care about old houses like we do!! I know treated timber has a good life but I'm a bit of a perfectionist and WORM DON'T EAT STEEL!!! but RSJ's across 7m is going to be hard work. Rolled is looking good or a combination of rolled hangers and softwood joists!!!

Hard to know what to do,,, but I have a few ideas I need to mull over.

Thanks guys,,


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