Grain Direction When Laying Deck Board
I recently built a small ‘floating’ porch for an out-building. The porch is just resting on some concrete pavers laid on compacted crushed stone. The wood is all pressure-treated southern yellow pine. It was straight, but also pretty wet. I debated whether to let it dry before starting, but was concerned it could warp, twist, etc., so I went ahead and started. I treated all the cut ends with ACQ preservative, and fastened everything with HeadLOK screws and Simpson hot-dip galvanized nails and hardware. I put joist flashing tape on top of all the framing. The deck boards are 2×6. I butted them together tight, thinking they would shrink, which they have.
My question is about which is the right way to lay the deck boards. I set them all so the end grain arched ‘upward’, thinking that if the boards cupped, they would follow the grain. Some boards have cupped slightly, but in the opposite direction from what I hoped (see picture), with the edges up. Did I lay these boards upside down? At least the cupping is slight, and does not pose a trip hazard. Thanks, Don Christensen
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don_christenson,
The rule of thumb I've always heard is Bark Side Up. I would have laid them the way you have, and don't know why they have defied our expectations. I'll be interested to hear what others say.
I'm thinking it's drying differentially, the top side is drying faster than the bottom which would cause it to curl like that. The top is facing the sun, the bottom is facing the ground.
Depending on how much ventilation is under the deck, it might even out in time as the bottom dries out, or it might always be that way.
Thanks Malcolm. I forgot to mention before that, in the interest of economy, I used DeckMate screws to fasten the deck boards. They felt pretty cheesy, with minimal thread, compared to the HeadLOK screws. If I were doing it again I think I would just spend the money on the HeadLOK's. I suspect even the 16d hot-dip galvanized nails may hold tighter than the DeckMate screws. I don't think they caused the issue I raised previously, although I did have to go back later and sink a few of them in deeper.
I've heard many people claim to install decking 'bark side up' or alternatively 'heart side up' but I've never heard a satisfactory explanation for either method. P treatment soaks better into sapwood so i suspect that could be one reason behind bark side up. I'm not convinced wood movement dictates one way or the other. I generally just put 'good side up.'
I agree with DC that you may have differential drying (the top, when baked by the sun, dries much faster). That said, the direction your boards have cupped is actually in line with how wood moves. If drying, the end grain lines tend to straighten. The reason is that the tangential shrinkage is greater than the radial shrinkage. Tangential is along the growth rings, so basically picture all those rainbows as being pulled tighter (if drying) and how would the wood move.
The differential drying will cause cupping in shorter time frames than the movement from overall board drying (it takes some time for that thick a boars to come into equilibrium).
This goes with "best side up": https://www.jlconline.com/how-to/framing/q-a-bark-side-up-or-down-with-decking_o
Yes, that is an excellent summary.
I don't think there's a guaranteed correct way. I tried following the conventional wisdom, and found some boards cupped up and some down. I flipped the up cupping boards the other way, and a few months later they were cupping up again. The problem is wood will expand and contract differently depending on the conditions. So no matter what some of the boards will be cupping up at least some of the time. I think the best strategy is to try to get the boards to cupping down when the get wet. That way water will tend to roll off. When it's dry out, it matters less.
If money was no object, you could commission some quarter sawn decking. That should keep cupping to a minimum.
When wood dries, looking at the end grain, the growth rings attempt to flatten out. When wood gets saturated, the rings tend to curl up. Deck boards usually arrive wetter than their average in-service condition so the rings will tend to want to flatten out. I prefer to put the heart side up--frowny-face, not smiley-face--so they are less prone to cupping when exposed to drying sun and air.
However, in most cases, the bottom of the deck boards will be more damp than the top side, so some cupping is nearly inevitable. For that reason, I first look at which side of the board is more attractive, and importantly, does not have pith, which will eventually pop out or split the board. If both sides are good, I put the heart side up. If only one side is good, I put that side up.
Thanks everybody for your comments. I learned a lot. There's more to this than I initially thought.
I agree that the top is drying faster than the bottom; the gap between boards is wider at the top. There's not a lot of ventilation underneath. I wanted to discourage wildlife and aggressive country weeds from taking up residence, so the side boards come pretty close to the ground. Also didn't want to provide a path for wind to get underneath. I don't know if or when drying will equalize, I guess time will tell. Right after I finished construction (late May), it got real hot and dry here. I'm in Colorado, so it's dry generally. UV is strong here too.
PS: I tried to attach a couple photos from construction, but don't seem to be able to today.
Don - I'm also in Colorado (currently ~ Ft Collins). When we moved into our house there was very little ventilation under our deck (N side of house) and the joists were rotten. That deck was about 10 yrs old, and the joists were redwood. We replaced the joists with pressure treated lumber about 25 years ago, and at the same time replaced about half the boards that had closed off the underside of the deck (from deck to ground) with lattice on all 3 sides to improve ventilation. There's likely code for required sub-deck ventilation but the bottom line is that even here in Colorado, you'll likely want openings that provide for under-deck ventilation, with some sort of fencing to keep out critters (our small-opening lattice has kept out the local cats, rabbits, skunks, and raccoons).
Thank you John for your comments; sorry, I didn't see your post earlier. In my case the side boards are structural; there are no posts (see picture of under-deck framing), so I don't think I can replace them without starting over. Fortunately, my porch is on the west side of a brick-clad building, and it gets baked pretty regularly. I may be able to cut out some slots and cover them with hardware cloth. Have to figure how to make it look OK. For what it's worth, there was a somewhat similarly framed porch there before that was just resting on bare dirt. I have 4-5 inches of crushed rock. The old porch was definitely at 'end of life', but even after40+ years (that I know about), the framing under the deck was still solid except where it was in direct contact with the dirt (mostly clay). Thanks, Don Christensen
If you look at this photo what is happening is the wood shrinks along the yellow line making the yellow line about 15% shorter as a result the board distorts in the direction of the blue arrows.
I think at this point flipping the boards seems like to much work for the amount of benefit.
Walta