Deck joist span for diagonal non-wood decking
I need to resurface my deck, which currently has 16″ OC joists. What I’m replacing are cedar 2×4 that are laid at a 45 degree angle, so the span is about 24″ for them. Of course they are fine and sturdy, but I have a lot of issues otherwise and they need to go.
I was really leaning toward composite and laying them the same way, but I’m having some concerns now. Trex, specifically, says to go at most 12″ OC if you go diagonal. But I’ve spoken to people who’ve laid standard 5/4 decking at 45 degrees with 16″ OC and had no issues at all.
I would think the diagonal sheer would offset the span width on composite just like it would for regular wood and bounciness wouldn’t be an issue. Am I wrong? I’ve also read about cupping even with Trex but again I would think the diagonal would help prevent that as well.
I’d like to avoid running new joists (I’d have to go with 8″ OC which makes for tight working room but also don’t want to make a big expensive mistake.
This isn’t a heavy traffic deck either – just my family and a few friends here and there. Any advice/input would be greatly appeciated!
Replies
Diagonally laid decking is always weaker than perpendicular laid decking.
I would keep the cedar myself, but you cannot lay composite decking diagonally on 16" spacing, it's too weak. So just add some joists to make 8" spacing.
There's composite and there's composite, but for most I would say that diagonal on 16" spaced joists is too much of a span. It won't fall down, but the bounce will bother you when you walk on it.
plan to use Trex transcend
DanH wrote:
There's composite and there's composite, but for most I would say that diagonal on 16" spaced joists is too much of a span. It won't fall down, but the bounce will bother you when you walk on it.
Can you expand on that statement? I'm assuming you're pointing out there will likely be a wide variety in quality available. Currently I was looking at Trex Transcend and standard 5/4 x 6 boards. Any red flags there besides the joist span mentioned?
MarkH wrote:
Diagonally laid decking is always weaker than perpendicular laid decking.
I would keep the cedar myself, but you cannot lay composite decking diagonally on 16" spacing, it's too weak. So just add some joists to make 8" spacing.
I would think that if the span is the same, diagonal would actually be stronger than perpendicular, wouldn't it?
As far as the cedar, just looking at the long term cost and effort and composite is getting more and more attractive.
Don
The diagonal bd will be longer and have more unsupported length, which means longer span.
if you go against manufacturers guidelines then you assume full responsibility and void any warranty.
Diagonal is weaker, since the actual span is 1.4 times the joist spacing. That makes the span across 16-inch-spaced joists about 22 inches. (The old squaw on the hippopotamus thing.)
Re the quality of different brands, I'm not familiar with current brands. Used to be there were a couple of brands advertising stiffer planks, but that was back when there was some really crummy sith being sold as well.
diagonal with same span
My question was about if the span is the same. Obviously if the joists are the same, then diagonal will create a longer board span. But the span on a diagonal board across 12" OC joists will be 16.9" vs 16". I would think that it would actually be a little stronger though because of the way the forces lay across the board.
The diagonal placement will provide a slight "boost" relative to stiffness, but I wouldn't expect it to be very much.
You can also get Trex in 1.5"
You can also get Trex in 1.5" (nomilal 2"). That is a lot stronger obviously. It is what we used to redeck a pier and it was fine.
This is a real lumberyard thing, not the Home Depot.
I'd stick with the manufacturer's recommendations. Otherwise if you put them down and have issues, there's no way they'll stand behind the product. Could be a very expensive mistake.