I'm building a lean-to that is 7.5' wide with rafters on 16" spacing. I messed up when placing the rafters by not allowing enough room for a 2x4 purlin plus the tin on top of the rafters, and it still fit under the existing roof for drainage purposes. Can I place the purlin on the bottom side of the rafters and screw the tin directly to the rafters on the top side?
I've got a question about purlin placement.
- Thread starter kz.1
- Start date
Won't do any good underneath the rafters. The tin panels need support that runs the opposite direction of the ribs.
I suppose you could could cut short board and install the purlins between the rafters flush with the top but that would be a ton of work.
With rafters only 16" apart you could use 1x4 if that 3/4" would make the difference
I suppose you could could cut short board and install the purlins between the rafters flush with the top but that would be a ton of work.
With rafters only 16" apart you could use 1x4 if that 3/4" would make the difference
Thanks. I was trying to avoid installing between the rafters but that's looking like the best option at this point. Live and learn I suppose. LOL I appreciate the information.
TurnipTruck
Well-known member
Do you have room for 1/2” plywood?
nadogail
Well-known member
If you can find long enough screws they would fill the gap, 3&5/8, plus screw into the purlins that were placed on the underside of the joists. Short blocks of 2X4 on top of the wrongly installed purlins would fill the gaps and allow the sheet metal to be held against something solid.
Firebrick43
Well-known member
I would do ply. Just make sure to use a metal roof compatible underlayment on top of it.
theoldwizard1
Well-known member
If your roof is 7.5' and the rafters are 16" O.C., why do you need purlins at all ?
I won't argue, but I don't think it is really necessary. It's a shed !
Firebrick43
Well-known member
Because R panels are not to be screwed to the flats but on the ridges. It highly unlikely that the rafters will line up with the ridges and it will look wavy even if they do.
cheap carport sheds get around this by running the panels across the roof instead of up and down but then the ridges dam water and the seams leak. Ok for a metal carport, maybe if your expectations are low but not if you have a wood framed shed.
OSB or ply fixes all the support/attachment/clearance problems the OP is facing but also it cant tolerate the condensation that forms on the bottom of the R panels hence the need for an underlayment. Many underlayment's can not tolerate the heat plus expansion/contraction that metal roofs experience therefore the old way was to place tar paper with a slip layer of rosin paper on top. There are synthetic underlayment's now that are rated for metal roofs.
Thanks for the reply. Everything I could come up with searching for information says that purlins are a must. If it ever stops raining around here, I'll remeasure the clearance to see if I can squeeze a 1x4 in there or use a flashing to transfer the water from the shop roof to the lean-to. That could be the easiest solution and I could still use the 2x4's.
That is some helpful info. Thanks. The lean-to is just the roof with no enclosed sides, so I don't know if condensation would be a problem. Heat is an issue to consider here in FL. The wind is another, so I thought purlins would strengthen the structure.
theoldwizard1
Well-known member
Pain in the ****, but I would toe nail/screw "purlins" between the rafters.
KnurledNut
Well-known member
Can you drop the rafters where it ties in?
mike93lx
ALLIANCE MEMBER
If you want the project, you could let in the purlins (cut slots for them). I probably wouldnt do it for more than 1x4, though. Probably comparable work to installing them between
Metal roofing isn't my thing, so sheathing would be my default
Metal roofing isn't my thing, so sheathing would be my default
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Firebrick43
Well-known member
Why when you can easily sheath the roof and strengthen the structure significantly at the same time with a diaphragm?
theoldwizard1
Well-known member
Strength not required, except maybe against wind uplift.
Never heard of either of these, but I don't live in the south.
2x4 purlins toe nailed/screwed between rafters are cheap.
Firebrick43
Well-known member
Its never a bad thing, but the ease of install it compared to toenailing between rafters is worth it. I have done that once and will never do it again. Its a terrible job especially if there is only 16"