Rafter sizing

andyandy2 · 22 Jan 2009

I'm building a lean to outbuilding which will connect exsisting outbuildings to the house, one outbuilding is being converted into a utility room.
Progress on brickwork is slow due to weather either freezing or raining, so i'm working out plans for the roof
Rafters will be about 11' 6" from ridge to wall plate, the roof will be 20 x 10 natural slates and the roof will be 12' wide wall to wall, roof pitch will be about 25 degreses
Exsisting outbuildings have rfters 9' 6" from ridge to wall plate they are 3" x 2 1/4 " with a central purlin of about 6" x 6" which seems abit overkill.
I seem to remember reading that 4 x 2 is good for a 12' span, although this looks a bit flimsy, I don't mind putting purlins in although would prefer not central to the roof, (possible future rooflight, depending on natural light)
I do have some 4" x ! 1/2" timbers could these be of any use maybe bolted together

Any advice much appreciated

noseall · 22 Jan 2009

100x50mm is no where near enough for clear span.

you may get away with 150x50mm at 400centres.

Shytalkz · 22 Jan 2009

Yep, should just about be ok, theoretically; more than adequate in practice.

Shytalkz · 22 Jan 2009

I meant Nose's suggested sizing btw and C24 grade

andyandy2 · 22 Jan 2009

Thanks noseall and shytalkz.

What if i was to put 2 purlins in at approx 4' and 8' what size rafters could i use then and what size purlins would you recommend

Shytalkz · 22 Jan 2009

4x2 for the rafters; don't know re the purlin until you give me a clue for the span!

andyandy2 · 22 Jan 2009

thanks shytalkz, the span is 12 ft wall to wall

Shytalkz · 22 Jan 2009

ok hold up

Shytalkz · 22 Jan 2009

If I understand you, it's basically 12'x12'.

Use 2 pairs of 63x175C24 at 1/3 span of rafters, inclined to rafter slope; or 2 pairs of 50x200C24 at same positions placed vertically, with rafters birdsmouthed over. Rafters 50x100C16 at 400.

mikric · 22 Jan 2009

Oh slow tonight Shy

10mins and no reply
If it was nt rainin I would get my tables out of the car

Bugger your faster than I thought

Shytalkz · 22 Jan 2009

That's cos it was interspersed with a domestic: 18yo man-child throwing his toys out of his pram, through not getting his own way

.

Shytalkz · 22 Jan 2009

I do hope they're the TRADA tables and not your defunct Part A ones btw

mikric · 22 Jan 2009

Yeh ! and I wouldnt have come with timbers doubled up ,just tree trunk sized lumps . As I say ask a SE everytime

Yes we got new ones with trimmer details in now Woo hoo.

andyandy2 · 22 Jan 2009

Ok thanks very much, When you say 2 pairs is that 2 bolted together ?

Shytalkz · 22 Jan 2009

That's the beauty of sitting here with TEDDS: makes messing about with alternatives a p o p. If it was a hand calc, it would have been the first one into my head, oversized, but working!

diynot

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Rafter sizing

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