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I just spent a boat load of time and money building a deck. I laid down 5/4 x6 red merante mahogany for the decking. I fastened it tight with 2.5 inch stainless trim head screws. The deck is on a Northwest exposure up here in Rhode Island so it does get some sun. The problem is that the boards are now cupping. When I put a straight edge down across a board there is about a 1/4″ gap in the middle of the board. The water just sits in the middle of it when it rains. I fished the deck with jabots Australian Timber Oil right after I finished the decking. I would love to hear people thoughts on what the problems may be or any solutions there could be. Most people don’t really notice it, but it bugs the hell out of me
Than
John
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Could be a few different things, kind of hard to say without looking.
First off though, if you sealed the top of the boards after you installed them, then the bottom of the boards will absorb more airborne moisture than the top, which will make that unprotected face swell more than the sealed face, which can make boards cup.
Another thing that might be happening is the sun is cooking the top of the boards and drying them out, so the bottom contains more moisture, with the same result.
Are the screw heads pulling through the boards? You might have to pull all the boards up, seal all six edges, and refasten them. But out here in the Pacific Northwest, I can't imagine using trim head screws on exposed deck boards. I have always used galvanized box nails or bugle head screws.
*John - I've had the same problem here (with a client's decking) but the cupping was restricted to random boards (replaced). This is part of the deal with meranti, I think.Jeff
*Another factor may be the moisture content at the time of installation. It may have been well below the ambient moisture content of exterior wood in your locale. You sealed out the moisture on top and the bottom was free to absorb.Hardwoods actually take considerably longer to equalize moisture levels than most people think, even outside. Your lumber may have been dried for interior application. It probably won't correct that much over time.It's a drag, but you could unscrew them and turn them over.jim l
*Not sure of the design...if the deck is raised high off the ground, or down low, just off the ground.Before you start unscrewing, consider trying to minimize the moisture hitting the wood from below. If the boards are still cupped after they've had time to acclimate, if it'll work out aesthetically, consider putting a sheet of plastic under the deck, covering the ground below. You can cover the plastic with a few inches of stone.Remember, when you coat wood, try to treat all sides equally.Good luck!
*I have a Mahogany deck that is 4 years old what kind of finish have you guys used? How well have they worked? I have to reapply every summer if not the decking turns gray by mid May (I have used only penefin oil) . I have the same cupping problem also, but I'm sure that is do to having only treated the top and sides the past 3 years, getting underneath is near impossible. I did put finsh on all 4 sides at the time of installation.Thanks Larry
*John / Larry,We do a ton of installations with Pau Lope (harder and more stable than mahog.)and, without sufficient air movement below the deck surface, we will see this in extreme cases (mainly on 1x4 & 1x6 NOT 5/4 installations). Double check to make sure that gutters or sump pumps do not drain into the area as this was a problem in 2 cases we know of. Penofin now has an "Exotic Hardwood Blend" that is brewed especially for mahog., teak, Pau Lope, etc. It blocks 99%of the UV and is a ONE COAT system!!! Experience to date is FANTASTIC.With the orig. blue label I could still, very easily, sand through the finish...NOT the case with the new hardwood blend. It soaks in far better than the orig. formula. Seems to bead water better too!
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I just spent a boat load of time and money building a deck. I laid down 5/4 x6 red merante mahogany for the decking. I fastened it tight with 2.5 inch stainless trim head screws. The deck is on a Northwest exposure up here in Rhode Island so it does get some sun. The problem is that the boards are now cupping. When I put a straight edge down across a board there is about a 1/4" gap in the middle of the board. The water just sits in the middle of it when it rains. I fished the deck with jabots Australian Timber Oil right after I finished the decking. I would love to hear people thoughts on what the problems may be or any solutions there could be. Most people don't really notice it, but it bugs the hell out of me
Than
John