How to Fix a Deck that Swells in the Middle

Find out how frost could be pushing up one of the 4x4s under your deck, causing it to rise up in the middle during winter. Get tips on digging a new hole for the post that's three feet below grade…

LESLIE: Don in Michigan is on the line with a decking question. How can we help you?

DON: Hey, guys. I’ve got a 20-foot by 18-foot Trex deck.

TOM: OK.

DON: A first-floor deck, so it’s about 5 feet off the ground.

TOM: OK.

DON: And in the winter, I get a rise in the middle of it.

TOM: When you say a rise, you mean it sort of swells or what?

DON: Yeah. It sort of seems as if it’s pushed up right in the middle, about 4 feet from the house; straight up, right underneath one of the 4x4s that holds it up. There are 4x4s under the middle and then around the periphery. And it – I mean only in the winter and it rises up about 2 inches.

TOM: Right. So, I wonder if the footing that’s right under that isn’t deep enough. Because if it’s not deep enough, when it freezes, it’s going to raise that 4×4 post. And as it raises the post, it’s going to push it up and pick up the deck at the same time. That footing has got to be 3 foot below grade so that it gets down below the frost line.

DON: Interesting, interesting. None of the others do that but this one might.

TOM: Yeah. Maybe it was the one that was the hardest to dig or something and it stopped a big short and – yeah.

LESLIE: So they just gave up. “That’s good enough.”

DON: Yeah, it’s the one closest to our house. Yeah, interesting.

TOM: What you’re describing is – could be the frost cycle.

DON: Oh, OK. Terrific, terrific.

TOM: Alright?

DON: So I guess the idea would be to maybe dig that post up and reset it or would it …?

TOM: Reset it, exactly. Reset it, yep. Mm-hmm. Yeah and the nice thing about the Trex deck is you could probably pull a few of those boards off and work from the top if it’s easier to get in that way or – can you get under this deck, since it’s about 5 feet off the ground? So you’ve got enough room to get under there and work?

DON: Yeah. It’s where I store my lawn tractor and …

TOM: Oh, OK. Alright. So then, yeah, if you can get down there and – you know, what you want to do is dig a hole right next to the one that we think is not deep enough. And then, once it’s plenty deep enough, you could sort of move the post over there, assuming there’s no seam on top of it.

DON: Oh, yeah. No, no seam.

TOM: OK?

DON: Terrific, terrific.

TOM: Alright, Don. Well, good luck with that project. Thanks so much for calling us at 888-MONEY-PIT.