Hi all-
Another dumb question from Mr. Moltenmetal:
I’ve been required to put 2×2 purlins on top of my half-cathedral ceiling rafters to form a vent space under the sheathing. No biggie, I knew long enough in advance to set the ridge board at the correct height etc. I’m guessing I have to run the sheathing plywood with the long direction running across the purlins (i.e. the long way parallel to the rafters underneath). Is that correct?
The purlins are on 24″ centres, which means I have to use the H clips to support the seams. But the detail the city gave me doesn’t tell me what spacing to use the clips on. Given that I’m using 1/2″ ply (i.e. in excess of code here), what spacing should I use on the clips?
Thanks again-
Replies
Just like standard installation of H clips with rafters at 24" centers.
Place the H clips halfway between the purlins.
My personel preferance would be a minimum of 5/8" sheathing thicknes for 24" centers, either rafters or purlins. It may cost a little more and be above code minimums, but it is a " better building technique", that will give you a flatter roof plane.
Dave
Not a dumb Q. Dumb would be to find out later you're wrong.
A: I've seen 1/2" ply w/out H-clips...maybe that's dumb, but frequent. (One house we looked at had almost 1" deflection between trusses. Quite off-putting on a house 5 yrs old) But just put 'em mid-bay. What's code...3/8"? Barely acceptable, see above. What's the loading for snow? Set your mind at rest and go the best way u can.
I'm intrigued by the comment re "I knew long enough in advance to set the ridge board at the correct height etc. ". Over this side, we'd have the strapping stop shy of any ridge so's there's a gap we span with the ridge-vent. If the ridge-b'd is too high it'll negate the effectiveness of the vent.
cheers
***I'm a contractor - but I'm trying to go straight!***
Good question. You're right that the ridge board would interfere in the way you're talking about if it was a full cathedral roof with purlins on the rafters on both side, but in my case the last three feet of run of these rafters span a vented attic space between the addition and the old house. The mating rafters are in-fill from the existing house and have no purlins, and they run to the top edge of the ridge board. But there's more than enough venting area to make sure the roof and the attic space will vent properly.
Thanks for the quick answers. I had no idea whether you needed three or one of these clips between purlins to do the same job as if you had purlins on 16" centres. Now that I've got a couple sheets up, I'm not nearly as concerned- a single clip between purlins seems to work great. When it's nailed off, this sheathing feels quite comfortable underfoot.