20 foot loft floor span

[Nicholas]

hi

what timber profile do i need for a twenty foot span for a loft conversion ? odpm tables stop at 5.4m is there a formula? do i need steel?

cheers and thanks neil

[Set Square]

Are these joists *really* unsupported for the whole 20 feet - or do they pass over any internal walls which could add support?

If it is truly a 20 foot span, you're going to need something pretty exotic - like 9" x 3".

An alternative is to put a 6" RSJ across the middle of the span, and to have two 10' joists cut into the RSJ. Then you'll only need 7" x 2" - which is far more manageable. The upstairs floors in my father-in-law's chalet bungalow (22' span) are done in this way.

[Peter Crosland]

A structural engineer perhaps? The BCO will want to see the calculations.

[G&M]

Nope. You need green oak and a lot of calculations to convince your BCO (who doesn't understand much beyond C16) that it worked in most grand buildings built since Roman times and will happily work in your house.

[neil buttle]

yeah i had thought it might be a bit nasty.

there is no internal wall the seven by two sounds cheaper and easier than the nine by three and a six inch steel will fit within the floor section

but i am a bit uncertain about my external walls as they bulge significantly so perhaps i will need to split the first floor internal space with a block wall if i chicken out of steel is thermalite ok or do i need to use concrete blocks?

is it sensible to use both with a set of concrete lintels bridging thrmalite 'panels' with concrete piers as support?

and as for the BCO, i have found them pretty ok in the past, but it makes sense to know how to put the figures together

cheers neil

[Set Square]

I would have thought that thermalite blocks topped with two courses of bricks (which are better able to take the point loads imposed by the joists) would be ok.

But what would support this internal wall? Would it be directly above a ground floor solid wall? If not, you'll need an RSJ or something at ground floor ceiling height on which to build it.

[John Rumm]

Sounds like getting the demo version of superbeam from here:-

be a good start - at least it will let you play with some figures and see what works.

Much will depend on what you are going to put on the beam. If it is just a standard floor you may get away with a "flitch" beam (i.e. a couple of wood beams with a steel plate sandwiched in the middle.

From a quick play with the above program I was able to get a beam design that worked on that span without being silly money or size. If you work on the principle of the maximum allowed deflection being 14mm (for beams that long it is specified as an absolute amount rather than a proportion of the beams length), you can meet this spec with two 50x200 C24 beams, and a 6mm thick (200mm high) flitch plate between them. Bolt the lot together with 12mm bolts every 600mm, alternate the bolts between top and bottom of the beam 50mm in from the edge.

[Grunff]

Use 7 Newton thermalites, not the bog standard (3N) ones. Also make sure that the surfaces which bear the timbers are well protected - a course of engineering bricks or similar.

[Michael McNeil]

Is it sensible to invest in a building with structural problems like this? I would have thought sorting your envelope more important than playing dolls houses.

[neil buttle]

[neil buttle]

michael no argument with this the envelope is part of the tasks builders and the like who have seen the structure say it will be fine (it isnt theirs to worry about)

i suffer from chicken likken syndrome so i am planning to use the internal structures to hold and hide steel tie bars amongst other tasks a new second floor will allow some gable ties to be nipped up reasonably tight

cheers neil

[Capitol]

Western Lumber tables show 12 x 2 SS grade is good for 19'11" on 12" centres with a dead load of 10lb/sq/ft and a live load of 40lb/sq/ft.

Regards Capitol

[Capitol]

ps, forgot to say 12 x 2 is perhaps a nominal size. IIRC 4 x 2 is 1.75" x

3.5"

Capitol