I wish to span 3.0m or thereabouts with galvanised corrugated sheeting. (Sh eet size 3.0m x 0.6m) Three purlins, one top and bottom and one mid span. P itch of about 5 degrees. What size rafter would I need and what spaceing ? (It's for a lean-to type shed Im comtemplating)
2x4" will be bombproof at 400mm spacing as that's a similar span to the flat roof dormer on my house and that's the wood in use.
You'll probably get away with 2x3" but there's no real gain so I'd go with 2x4, treated.
You would not need the centre purlin.
sounds like you have no more than 750mm between purlins, so the length of unsupported rafter is small. 3x2 would me more than adequate, and you could go for 600 centres. (in reality many a shed will have a larger span, wider spacing, and a heavier felt roof, or 2x2 or even thinner!)
If you are ever likely to stand on it, 4x2 would be ample.
Two other thoughts... at 5 deg. pitch rainwater is likely to wick up under the roof and may miss a gutter. Deforming the drip point slightly with strong pliers can solve this.
The other issue is condensation, particularly if moist air is trapped in your shed. It usually trickles down the underside and drips off at the purlins. Trapping a layer of dry air next to the steel can be a cure.
Does snow loading need to be taken into account?
The 'shed' is really somewhere to keep a couple of small trailers out of th e elements. It will back up to, but not join an existing shed. No gutters a s it is on the side of a ditch. THe sides will likely be left open or have
2 x 1s vertically at 1" spacing a la some farm sheds
(Sheet size 3.0m x 0.6m) Three purlins, one top and bottom and one mid spa n. Pitch of about 5 degrees. What size rafter would I need and what spacein g ? (It's for a lean-to type shed Im comtemplating)
Don't frighten me :-) I'll depend on global warming :-) Its only a small st ructure, I wouldn't be very concerned about it collapsing, more worrried ab out the wind taking it away.
Farm Yorkshire boarding is normally 4"x1" with 1" spacing. Keeps out more driven rain than you might expect.
g. (Sheet size 3.0m x 0.6m) Three purlins, one top and bottom and one mid s pan. Pitch of about 5 degrees. What size rafter would I need and what space ing ? (It's for a lean-to type shed Im comtemplating)
structure, I wouldn't be very concerned about it collapsing, more worrried about the wind taking it away.
Many thanks to all for all the suggestions. My two previous sheds had pitch ed roofs for which I made small trusses but this single span low pitch slop e had me a bit worried.
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