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wooden porch warping


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This wooden porch has been expanding and warping since installed two years ago. It was 2 1/4 KD T&G pine, initially not primed, however when swelling was indicated the bottom was liberally coated with latex paint. The top has had good quality porch paint, oil based. The swelling has become so severe that three boards popped up like a tent. Also several boards are warping across the width even though the boards were milled with a groove on the bottom side. The floor is about 2' above grade. The house is 80 years old and has no vapor barrier. Could the lack of the vapor barrier be the cause? Could there be any other causes? I have always thought that KD lumber had a limit to it's expansion.

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Hi,

A picture would help. Is the underside of the porch completely closed in or can air circulate beneath it? If the underside is an enclosed area and there's no way for moisture to escape a vapor barrier will help to cut down on the amount of moisture but probably won't completely eliminate the problem because you've got a self-inflicted wound there.

Painting the underside of the porch with a latex paint after it's already been put down without being primed or even sealed probably didn't do a whole lot of good. The only reason you'd want to seal the underside of a porch is to stabilize the wood against uneven curing - you already realize that - but painting it after the fact only means that now there are narrow bands of unsealed wood where moisture can get into the wood. Instead of the boards swelling and shrinking uniformly along their entire length and on all sides, these are now absorbing ambient moisture through those narrow bands of unpainted wood in the joints and on the face of the joists whenever the humidity changes. Those areas closest to the unsealed wood are expanding and contracting more than the sealed areas and this is causing enough torsional movement to cause the floor to buckle.

Kiln dried lumber is just sitting there whining, "I'm thirsty." If you leave it out in the weather unsealed it will take on moisture and expand. Paint it with a film-forming finish once that happens and now it can't uniformly expand and contract with changes in humidity. It would have made more sense to have sealed the underside with something that can breath like Thompson's Water Seal and then to use a translucent stain on the surface.

Perhaps you should think about screwing it down. It'll still struggle to move but the screws might be able to keep it in place anyway. It's winter, so it will have expanded to the max right now with all of the exterior moisture on it from rain and snow. Drill and countersink, secure it with stainless steel screws, fill the countersink recesses with side-grain wood plugs that are glued in place, sand smooth, paint and hope for the best. Check this and this out.

ONE TEAM - ONE FIGHT!!!

Mike

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Originally posted by Raymond Henrie

This wooden porch has been expanding and warping since installed two years ago. It was 2 1/4 KD T&G pine, initially not primed, however when swelling was indicated the bottom was liberally coated with latex paint. The top has had good quality porch paint, oil based. The swelling has become so severe that three boards popped up like a tent. Also several boards are warping across the width even though the boards were milled with a groove on the bottom side. The floor is about 2' above grade. The house is 80 years old and has no vapor barrier. Could the lack of the vapor barrier be the cause? Could there be any other causes? I have always thought that KD lumber had a limit to it's expansion.

Sounds like flat-sawn, second-growth wood. Kiln dried or not, flat sawn wood expands & contracts significantly more than vertical grain (quarter sawn) wood of any species. Second-growth wood also tends to expand & contract more than old-growth wood and it's more likely to cup & crown as well.

If it's a small porch and you're on a budget, I'd suggest pulling up the floorboards and reinstalling them with a credit-card-sized gap between them.

If you're rich, just replace it with some nice quarter-sawn old growth material from Canada.

- Jim Katen, Oregon

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