Can any of the structural wizards on here, tell me the SWL for a 17ft long piece of crappy builders yard 4"x2" timber, mounted on edge (ie; the 4" side vertical), for a centrally suspended load?

Or to put it another way, what maximum weight can I safely hang from one of the garage roof timbers?

Any figures you will get will include the load of the roof deck, membranes etc. The bit left after all of that is the load you can put on it.

Any load which can safely be carried at mid span of a 5.18m of 100x50 unclassified timber, isn't going to be hellish much.

Away for a think.

Is there anyway you can strenghten it?
I helped a mate do this so he could use the space in garage roof trusses for storage.
For instance adding width by clamping extra 4x2 to each sidem glueing and screwing them thoroughly to make them as one?
Or by using really good quality ply wood each side as above.
If you do consider this, make sure the strenthener also sit into the supporting wall at each end as the original joist do.
Then re-post with new dimensions of your beam and enquire again.


Also see earlier posts this week with regards to weight of snow on roofs!

[Edited on 4/12/10 by slingshot2000]

You shouldn't really be using 4x2 to support anything at 17' long unless there are lots of them close together or they are held up from above by something. At 17' span you should be using 6x2, I wouldn't risk hanging anything else from them.

Pete's right.

I can't honestly think of an application where I would use a 100x50mm as a joist anyway.
That's without hanging a load off it.

If it's a masonry wall, you're probably best to put a proper load carrying timber or steel member in there.

Let me add, my garage roof is done in 4x2 (or 100x50mm), but mine is made into trusses with the clear span being exactly divided into three by the internal members which then carry on up to the rafter members. Standard basic truss pattern.

Had I been doing a flat roof, I would have done it in at least 150 if not 200 x 50mm.

Thanks for the comments guys.
Should have given more info, sorry.
Firstly it is not a flat roof. The 17' long 4"x2" is the horizontal member of a triangular roof truss, supporting a pitched roof.
The trusses are 15 in number at 16" centres. 3 or 4 have verticals in tension from the rafters.
I am wondering if it is up to supporting a central rail to carry the ubiquitous Aldi winch, mounted on a carriage.
I can hang off one (102Kg) OK, but it won't hold this for long without serious deflection and resultant issues.
I hung my bare chassis off two and had to put supports under them as they started to sag significantly.
If one will support say 30Kg indefinately then I think job will be a good 'un.

I suppose I could test it by hanging a 30Kg weight off one and checking deflection over time, say a couple of
weeks. I was just hoping someone could give me a definative answer to save a load of hassle with rope and scrap.

[Edited on 4/12/10 by Confused but excited.]

That is quite a long span for a timber flat roof. I don't believe your roof complies with building regs. 2" x 4" timber might do for ceiling joists, but nothing else.

The relevant code would be BS 8103-3. Using Table 60 for a "non-trafficable" roof, for a 5.2 m span you need 3" x 8" (75 x 195) joists at 18" (450) centres, Grade C16 timber.

Cheers R

Dear Confused-B-E,

Based on the simple logic that you are likely to try this regardless of advice to the contrary (yes, we all do that).

Why not hang from a point away from the centre, i.e. 4ft from the wall rather than 8.5ft from the wall. This way it will be that much less likely to fail. Perhaps not strong enough (technically) but you are going to do this anyway, so ....


Matt

I used to run two short lengths of scaffold pole across two roof beams with a cross pole making an H pattern.
This spreads the load across two beams.
Then used short diagonals from beam end to wall.
You could also use a couple of acrojacks to bring the down force to a narrower point on the beam.

Thanks for all the input guys.
Ah, Matt you know us all too well.