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)

On 22/08/16 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)


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.

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!)



--
Cheers,

John.

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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

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)

Does snow loading need to be taken into account?
--
Cheers,
Roger
____________
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checked.

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

On Monday, August 22, 2016 at 7:29:50 PM UTC+1, Roger Mills wrote:
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)

Does snow loading need to be taken into account?
--
Cheers,
Roger
____________
Please reply to Newsgroup. Whilst email address is valid, it is seldom
checked.

Don't frighten me :-) I'll depend on global warming :-) Its only a small structure, I wouldn't be very concerned about it collapsing, more worrried about the wind taking it away.

In message , fred
writes
On Monday, August 22, 2016 at 6:50:30 PM UTC+1, Tim Lamb wrote:
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.

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

You'll be fine:-)

Farm Yorkshire boarding is normally 4"x1" with 1" spacing. Keeps out
more driven rain than you might expect.

--
Tim Lamb

On Tuesday, August 23, 2016 at 8:43:43 AM UTC+1, fred wrote:
On Monday, August 22, 2016 at 7:29:50 PM UTC+1, Roger Mills wrote:
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)

Does snow loading need to be taken into account?
--
Cheers,
Roger
____________
Please reply to Newsgroup. Whilst email address is valid, it is seldom
checked.

Don't frighten me :-) I'll depend on global warming :-) Its only a small 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 pitched roofs for which I made small trusses but this single span low pitch slope had me a bit worried.