Thanks for the previous help with cement bag sizes, that bit is in place now.
Q1. The beams are to support a light weight corrugated flat roof of internal size 8' x 15'. Beam spacing will be a little under every 2 foot with the 5 beams running the length of the 15 foot with two sets of bracing noggins at 5' in from each ends. Unsupported span is 15 feet.
Ends will be firmly fixed down to structure and protruding through walls.
Will 5" x 2.5" (actual sawn size) be adequate?
Q2. What is a good cost effective alternative to asbestos cement corrugated roof panels?
15ft unsupported span is far too long, the beams will sag, is there no possibility of going the opposite way and using 8ft beams?....failing that, I fear it will require a purlin in the centre, possibly steel.
The message from Harry Bloomfield contains these words:
Probably. If it was a floor you would want 9" x 2" joists to span 15' but you don't want to load it with anything more than wind and your own weight as you fix it down.
I think you will find that modern corrugated sheet does not contain asbestos.
Plastic covered steel is an alternative. I can't remember what it is called but it has a section something like (IIRC) _____ ______ _/ \_____/ \_
and is a doddle to fit.
You don't say which way the roof slopes but the beam layout suggests it is down the short dimension. However should you want it to slope down the long dimension the steel sheets can be ordered in in quite long lengths, I think in excess of 15'.
Two layers of corrugated iron with fertiliser bags laid in between. The bags (overlapping like tiles) make the holes in the corrugated iron waterproof.
(If your corrugated iron doesn't have any holes in it, you've paid too much for it)
I've just built a garage with 16ft span, using 8x2 at 4ft centres. My gut feeling is that 5" is a bit light, and the joists will sag visibly under their own weight. Bearing in mind that deflection is inversely proportional to the cube of the depth, even a modest increase in depth makes a big difference (e.g. 5" deflects 4 times as much as 8" for the same total load). If you decide to do proper calcs, the deflection and maximum stress are the easy bits - deciding on the wind load is the tricky bit.
Box section steel sheet is about 6 quid a square metre, and is available in long lengths, so you could get away with no end laps at all. Minimum suggested pitch is 5 degrees iirc, or about 1 in 12. Fibre cement sheets are only available in shorter lengths (< 3m ?) and really need a steeper pitch, particularly if they're laid such that there are end laps, despite the practice of makers of cheap concrete garages.
The problem with steel sheets is that the insulation value is minimal, and you'll get condensation dripping off the underside in some conditions. The design of insulated roofs with vapour barriers etc is quite a complicated business: my compromise was to deck the roof with secondhand 18mm ply then cover it in polythene, then clad in steel.
and services > Roofs > Site assembled > Standard roof profiles > View Profiles > R32-1000
takes you (I hope) to some basic data on one of the most common profiles. I was wrong: it's 4 degrees (1 in 15). Note that it's also often found in 0.5mm thickness, which imho is a bit thin unless you support in continuously, as I did.
If you want to do it by the book, 15' is 4.57m. For a flat roof rafter,
600mm rafter spacing and 50mm wide timber, you need 220mm deep joists, which are good for 4.74m. Officially, your intended 63x122mm joist would only be good for 2.79m.
If you wish to use 63mm wide joists, then 195mm is very fractionally undersized, being good for 4.54m.
Depending on the time and trouble you want to take, you might want to use a structure like designed by the mathematician John Wallis
"Wallis tackled a practical problem - how to provide a flat structure such as a roof to span a large square open space, using only wooden beams much shorter than the required span and only supported around the edges. He devised a repeating pattern of short but interlocking beams, which could be extended to span any sized space."
See
As it turned out, this design wasn't used for the internal roof of the Sheldonian Theatre, Oxford - Wren used a different technique - a composite roof truss.
Actually - you might not - further investigation here shows:
"However, if the roof structure had been used, it may not have stood the test of time. Every wooden beam in the structure was vital because it supported the weight of other wooden beams. If one beam failed, the whole structure would collapse. So if woodworm nibbled away, or a lightning strike or earthquake damaged one of the beams, the roof would fail. Most other roof structures can withstand a bit of a battering, but in John Wallis's design every beam was doing a vital job and none of them could afford to be damaged."
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