new mysterious hump (bump?) - concrete basement floor

May 4, 2020 6:16 AM

In the past 6 months or so, a sizeable hard hump has developed (?) underneath the laminate floor in our basement. What the heck could this be? Bonus question - who should I call to help investigate and fix this?

Details on the hump: the hump is about an 1 1/2" taller than the rest of the floor, sort of cantaloupe size in circumference. It can be felt underneath the laminate and causing the laminate to rise and bounce nearby. The location of the hump is sort of in the middle of the basement, 6 feet from the exterior wall.

Details on the house: 100 year old brick house, finished basement with exterior water proofing and interior french drain done in the last 10 year. No water issues in the basement. No trees on the property - but the next door neighbors have a huge, old deciduous tree that is 60 feet away (no idea what it is, maybe 80 ft tall).

Current theory - some kind of foundation shift (though why so localized?) or a wayward tree root.

Current plan - looking to pull out replace the laminate floor this summer, (won't cover up the bump, want to remove it and fix the underlying issue) but want some theories now as to what this could be, what to look for when we can see the concrete, who to call etc.

posted by walkinginsunshine to Home & Garden (15 answers total)

Does anything come out when the bubble is pricked with a pin? Air vs water might give some idea of urgency and next steps. We had water bubbles under laminate without having water issues in general. And I defer to the experts for if this is a good idea to do now if other steps can't happen for several months.
posted by beaning at 6:33 AM on May 4, 2020

Response by poster: Beaning - no water comes out. It's not a bubble per say, it presents as a hard unmovable hump under the laminate floor.
posted by walkinginsunshine at 6:39 AM on May 4, 2020

There are fungi with fruiting bodies hard enough to do this.
posted by flabdablet at 6:43 AM on May 4, 2020 [1 favorite]

We used to have a 100 year-old house with an unfinished basement which showed signs of hydrostatic pressure. It showed up as wet spots on the floor, but not so much that it would have come up through a pin hole.
posted by yerfatma at 6:45 AM on May 4, 2020

I'm not sure speculation will get you far. I'd cut the center of that piece of laminate out right away as it is ruined in any case. Then you can see what's there. The likely options are something growing, or expansion of in place materials due to moisture.
posted by meinvt at 7:29 AM on May 4, 2020 [8 favorites]

I'd first call someone like me. I was an all-around carpenter and sometime-mason for 30 years. First thing I'd probably do after looking the whole basement over is cut out the bubble, as suggested. Then, if it was something beyond my capabilities or expertise, I would suggest the next expert. I wouldn't have charged much just for that.
posted by Hobgoblin at 7:58 AM on May 4, 2020

I would use a small nail to pierce it. The floor is concrete, but there could be a small crack allowing air to get in and create a bubble. This is low-effort, low-risk, so that would be my 1st step.
posted by theora55 at 8:07 AM on May 4, 2020

Not enough expansion space left around the perimeter probably.
posted by humboldt32 at 8:18 AM on May 4, 2020

Response by poster: Some points of clarity: There is no bubble. There is a new-ish, hard mass under the laminate. It's not air.
posted by walkinginsunshine at 8:22 AM on May 4, 2020 [1 favorite]

If you thump it and it sounds and feels solid, versus the bouncy area you mentioned is nearby, then it's something pushing up the concrete from below, and the strategy of cutting out some laminate and dealing with it is the answer.

If it does not sound and feel solid, then it could very well be the lack of expansion space at the perimeter mentioned by humboldt32.
posted by beagle at 9:03 AM on May 4, 2020 [1 favorite]

2nding flabdablet. I've seen large toadstools pushing up through thick asphalt, which is a different material but in a 100 year old basement, I wouldn't expect the floor to be more than an inch or two skim coat of concrete. You could start by drilling an exploratory hole but that sounds too much like the 11 minute mark of a horror movie. I'd pull back the laminate before calling anyone.
posted by bonobothegreat at 9:29 AM on May 4, 2020 [2 favorites]

If there's a bit of damp under the concrete it could also be efflorescence or calthemite or perhaps concrete cancer.
posted by flabdablet at 10:32 AM on May 4, 2020

Please let us know what it turns out to be after you find out!
posted by Larry David Syndrome at 11:27 AM on May 4, 2020 [7 favorites]

Response by poster: Ok, I cut open the floor to discover an old floor drain, broken tile and chunks of thinset. The tile has a bit of a bump to it, but I think the broken pieces added to it ? Not sure. This will be painful to fix. Guessing it will have to be broken up further, and filled with concrete.... open to suggestions. Thanks everyone!
posted by walkinginsunshine at 4:01 PM on May 4, 2020

If this hump has grown noticeably over the past six months, then the most likely scenario is that the basement floor is seeing pressure from below by water-induced ground swelling, and that the weak spot caused by the old floor drain is where that pressure is causing the most movement.

How sure are you that the french drain is still actually draining? And what's the ground like outside the house, where the external waterproofing treatment will certainly be interrupting the natural flow of ground water?
posted by flabdablet at 10:33 PM on May 4, 2020