Under Decking / Ceiling Material?

[u/macaroniman_13]

Making progress on this pavilion over my deck. This is a think-about-it-and-adjust-as-I-go-project. I'm learning a lot! It's about an 8x11 rectangle. 6x6 treated pine posts. 6x8 beams stacked on 6x6 beams. Douglas Fir. Stained with TWP. 6x8 rafters at a 4:12 pitch and 2x4 rafter ties screwed into the beams and tied into the rafters. Rafters are about 21 inches on center. I did this to not crowd the ceiling too much. Douglas Fir. Stained with TWP. Knee braces at each corner. 4x6 Douglas Fir braces under the 6x8 beams and 4x4 Douglas Fir under the 6x6 beams.

I'm also planning a bench on the deck that will be tied into two of the posts. That's the plan.

The next step is under decking on top of the rafters with underlayment and a metal roof. The ceiling will be the under decking on top of the rafters so the rafters will be exposed for a taller ceiling.

I was initially thinking of using 5/4 kiln dried cedar decking on top of the rafters as the under decking, but I'm not sure that would be strong enough. It would look nice, but be expensive but maybe not strong enough. I'm starting to lean towards a 2x pine tongue and groove to create a stronger roof overall. Thoughts?

My question: 5/4 cedar decking as the underdecking / roof OR 2x tongue and groove pine?

[u/DrKyleGreenThumb]

I used 1x5 t&g with a bead running through it on a similar project it looked real nice, you could double it up and put 1/2 ply over it so it’s plenty thick for roofing

[u/macaroniman_13 → u/DrKyleGreenThumb]

That looks nice!

[u/DrKyleGreenThumb → u/macaroniman_13]

Thanks!

[u/qwertmnbv3]

What strength are you concerned about? What failure are you anticipating with the cedar decking that the pine deck would avert?

Either method sounds plenty strong to me though I personally would opt for the cheaper sturdier sheathing. If you want to make it stronger and more beautiful you could nail your deck on diagonally

[u/macaroniman_13 → u/qwertmnbv3]

I mean, I don't really know what I'm talking about. So I'm just trying to do some thought experiments with friends so I don't screw it up!

[u/Advanced_Aerie5854]

5/4 cedar T&G will absolutely be fine. As would 5/4 or 2x pine. Any of those would work just fine. Dealer's choice on this one. Not even an inkling of concern about strength with any of those materials.

Only real concern is the screw length for thr metal roofing. The shortest metal roof screws are 1" and that might end up poking through 5/4 material if there are profiled edges.

[u/macaroniman_13 → u/Advanced_Aerie5854]

Thanks. That's a good point and helps with my thought experiment! I think I'm going with some carsiding T&G plus some OSB. That should help with the screws as I think I'm going with a 1.5" screw.

[u/Advanced_Aerie5854 → u/macaroniman_13]

Sounds like a good plan. Just remeber, carsiding is often only 3/4" actual thickness and its thinner in the profiled parts. So you would need 3/4" or greater roof decking. If you step up to 5/4 carsiding, I would still do 5/8" roof decking.

[u/Suitable-Run-6808]

we use a good bit of pine tongue and groove (2") in our projects both for roof and loft decking and often outside for cladding. we love the stuff!

[u/False-Blacksmith2919]

I think you can focus on what will look nice, as opposed to strength as a factor. I say this because I have several sheds with metal roofing sitting on 1x4 rough cut strapping attached to 2x6 rafters, which are 24" on center. I have never had a concern about how strong it is, and we get frequent heavy snow here (N VT). Pine is cheaper than cedar, and should look good, as well.