On Monday, August 22, 2016 at 6:50:30 PM UTC+1, Tim Lamb wrote:
In message , John
Rumm writes
On 22/08/2016 12:51, fred wrote:
I wish to span 3.0m or thereabouts with galvanised corrugated
sheeting. (Sheet size 3.0m x 0.6m) Three purlins, one top and bottom
and one mid span. Pitch of about 5 degrees. What size rafter would I
need and what spaceing ? (It's for a lean-to type shed Im
comtemplating)

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.




--
Tim Lamb

The 'shed' is really somewhere to keep a couple of small trailers out of the elements. It will back up to, but not join an existing shed. No gutters as 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