Subfloor hump over joist tied into cantilever

Question

Just bought a house and I’m swapping out floors. There had previously been carpet in this area, but I’m laying down LVP so I need things reasonably flat.

There is one part of this room that has a very pronounced hump, to where I could feel it through the carpet before I tore it up. I have access to a crawl space underneath it, and it looks like the joist that’s crowning up excessively is tied into a portion of the house that’s cantilevered out to a short deck.

In a simpler situation, I would just pull the subfloor and plane the joist flush with the others. But now I’m not sure if it’s humped because it’s just got a pronounced crown, or if the cantilever is cranking it up. If it’s the latter, I’m worried that planing it down would weaken it.

What’s the move here? Sister and then plane? Just plane? Put carpet back down?

Joists are 2x8 (7-1/4" actual), 16” OC, with 3/4” subfloor.

Note the deck itself is not cantilevered, just a bump out on the exterior.

Answer 1

It's the cantilever. When I was first in construction the rule for cantilevers was 3:1, or three times inside what's outside, but I've seen many that weren't even close to that. Now they don't even allow that, for obvious reasons. Too small a ratio and you have huge torque loads on the girder joist.

Sistering should help prevent further movement in your case. I'd try to introduce some pre-tension downward if possible. Do your planing first rather than putting anything in the way.

1. Open the subfloor over the area to be repaired. Cut down the center of joists, or plan on installing blocking to support any edges. Give yourself enough room to work comfortably.

2. Lay a straightedge across the floor at various points to determine how much you'll plane off, and mark that on the joist. To avoid extrapolation error, lift the straightedge until the gap is equal on both ends, then measure the gap.

3. Do your planing. Keep people and heavy loads off the deck while you do to minimize movement. Obviously you'll have to plane the cantilever joists as well, tapering out to zero at some point.

4. Sister the new joist. To introduce some pre-stress downward, rip the joist to ~½" shorter than the original, then fasten it at the center, flush on top. Fasten one end, also flush on top. Now wedge or lever down on the other end to create a gap to the subfloor. Fasten in place. You can use wire or steel strapping and lever off the bottom of the original joist. Use plenty of structural screws or bolts to be sure they don't shear or bend.

5. Replace the subfloor. Use construction adhesive and 2½" screws.

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What an amazing coincidence. I did exactly this repair for a friend over this past weekend. The entire end of the house was cantilevered. In this case the ratio was more like 2:1, and the doubled girder joist was humped about 3/8". This created a pronounced ridge across the living room.

I used a powered hand planer to remove the top of the joists*, then we installed engineered hardwood. It came out beautifully.

_{* The material removed was negligible--joists vary by nearly that much anyway. Also, I got the impression that all movement had ceased a decade or two ago, as there was no evidence of recent movement in the adjacent walls.}