Loft Room

[DOB]

Hello all I've just moved into a bungalow which I'm considering putting a loft room into.I've been trawling through all the loft room posts on this site and various other sites so I thought I'd throw it open to advice. The building is a 1972 detached, pitched roof with gables with the roof apex gable to gable as opposed to back to front.Concrete roof tiles held up by 10 roof trusses,constructed from 35mm x 150mm timber.The gap to be spanned is 18ft 6" wide and the joists run front to back and are approx 30 ft long.(no internal supporting walls) I plan on putting a floor in that is independent of the present one and supporting the roof (possibly) with purlins of some form,wether it be timber,steel or a composite of both. The problem part is the headroom,at present there is 2m to the bottom of the rafters which are 150mm,thus I should be able to fit the required insulation and get a 50mm ventilation gap,so hopefully the only extra space taken up at the top will be that which the plaster board occupies. What I am interested to know is,as I haven't got access to software to calculate necessary sizes,is it possible to span an 18 1/2 foot gap with timber ? - if so what would the dimensions need to be,I'd prefer to work in timber or possibly construct a flitch beam I only intend on putting one room up there with a dormer at the back and a velux at the front.The stairs will access directly under the apex,flush with the wall so will have 2m head room,if the present floor height remained unchanged.

Is there a minimum requirement for headroom within the room,it wouldn't be a problem for me as I'm 5'8",but obviously the world is generally full of taller people than me.

Also - do the fire regs regarding escape route from bottom of stairs to nearest door to outside apply to bungalows too,ie that any door opening onto said route is solid.

Thanks

[Tony Bryer]

Not easily, particularly since headroom is a factor. You would need to take the advice of a structural engineer.

The only headroom rules relate to staircases and landings, but substandard headroom is likely to be a major turn-off for most potential buyers. I suspect that any design is likely to involve a floor level that is higher than the top of the existing joists.

Is the property detached? If so, I'd be inclined to think about a total roof reconstruction to make proper rooms possible.

[DOB]

What I was considering was putting two beams across the narrowest span and running joists alongside the current ones.The size of the main supporting beams doesn't matter too much as they can be at the side of the room to be constructed and be the base of the dwarf wall rising to the rafters,so if I could use joist hangers on the main supporting beams that have the cup that holds the joist lower than top of the main beam I may be able to get enough headroom? if that makes sense

[John Rumm]

You may want some insulation under the rafter to combat the thermal bridging effect of the timber...

Gut feel says fairly substantial at that span (i.e. >250mm) (I would go do the sums, but I just killed the computer with the software on it!) - you can download the demo version of Superbeam from Tony's site:

You have the added complication that you may need the new floor to carry some roof load via your purlins (or more likely a dwarf wall).

Seems unlikely, unless you go for a steel floor - even then I would anticipate some gain in floor height.

Only over the stairs. However I would have though anything less than about 6'6" would make for a very "close" feeling room even if you could stand up in it.

Don't think so.

[DOB]

Thanks for quick response

Regarding thermal bridging - what depth of insulation is advisable under/over the rafters?

[John Rumm]

The usual recommendation is for 50 between and 35 under - but that assumes 100mm rafters. In your case I would expect you would get away with 20 or 25 below and say 70-80mm between.