Worrying wet patch
March 16, 2022 9:05 AM   Subscribe

I just moved in to a new flat and I am planning to put in a new laminate floor. When I lifted up the carpet, I found what looks like a wet patch in the corner.

It's next to an outside wall and it's on the first floor of an old brick building. I've uploaded a couple of pictures here. Does anyone have any idea what could have caused it? Is it likely to cause a problem if I put in a new floor?
posted by Chenko to Home & Garden (18 answers total)
Quick clarification question: Is it currently wet to the touch?
posted by mekily at 9:17 AM on March 16, 2022

Is it actually wet? Does it still feel damp at all? Is there evidence of damp in that bit of the wall? It COULD be an old stain from a fish tank/animal pee/ roof leak / anything really.

You'd have to just keep an eye on it to see if it's a current wet problem or an old mishap.
posted by stillnocturnal at 9:18 AM on March 16, 2022

Response by poster: It's not currently wet to the touch. I think it might be very slightly damp, but I might be imagining things.
posted by Chenko at 9:23 AM on March 16, 2022

Best answer: A $15 wood moisture meter (the pin type) might help shed some light on this. Test that wet-looking spot and some other dry-looking areas elsewhere on the subfloor for a comparison.
posted by MonsieurBon at 9:23 AM on March 16, 2022 [5 favorites]

Just to clarify for American responders to this question, this is the UK, so first floor means second floor. :-)

If it's the window at the top of the picture, I can see there's an air brick for the sub-floor, which is a good thing, as this creates air circulation and helps prevent rising damp. Check local damp-proofing companies online and see if there's a local one that will give a free inspection/quote.

Also, did you get a copy of the mortgage valuation report? Or did you have a more detailed Home Buyer's report? In both cases, the surveyor should have checked for visible damp, and might possibly have done readings with a damp meter. It might be worth checking with whoever inspected the property.
posted by essexjan at 9:55 AM on March 16, 2022 [1 favorite]

If you press a bit of toilet roll or kitchen roll against it, it may help clarify whether or not it's damp right now. (Another good way to tell if something is damp is to touch it with your lips instead of your fingertips, but that's a more palatable option for drying laundry than for a mystery stain on the floor!)
posted by ManyLeggedCreature at 10:14 AM on March 16, 2022 [2 favorites]

I once rented a place (ground floor apartment in two story brick apartment block) that developed a soaking wet patch in the carpet & the concrete slab underneath that wouldn't go away. Property manager brought in a plumber to diagnose it -- if i recall correctly, one of the tricks the plumber used was pulling away the carpet then using a small blowtorch to dry out the concrete slab and see if new moisture was coming from anywhere -- it was -- before starting to hammer-drill holes into internal cavities to zero in on the cause. Turns out you can make a glorious amount of mess in a couple of hours with a hammer drill.

From the picture, it looks like you don't have a concrete floor, so maybe blowtorching it would _not_ be a good idea.
posted by are-coral-made at 11:12 AM on March 16, 2022

Well, don't just install a laminate floor without finding the cause of the damp. There are several tide marks on the floor, which suggests it's happened more than once. Is that mould in the corner? If it is, it suggests that there was moisture around there after the area was last decorated. Looking at the outside, the inverted 'V' mark and the different style of windows and other stuff suggests there was once another part of the building joined on there. I have no idea if that's relevant.

Maybe it's already mended, maybe not. Is there a neighbour you could ask?
posted by StephenB at 2:01 PM on March 16, 2022 [1 favorite]

Moisture meters are pretty cheap, and probably the only way to tell if this is just an old stain or something to worry about. It looks like you have some hardboard or something laid over the existing floorboards. That stuff does soak up water really well, and will stain. I'd expect to see staining on the ceiling underneath (I don't know if you can get access and take a look).

It looks like the windows have been replaced fairly recently, so it's possible there was just a leak around the old window that caused the stain.
posted by pipeski at 3:29 PM on March 16, 2022

Response by poster: Thanks everyone! I got hold of a wood moisture meter. With the caveat that I don't know what I'm doing, I got a reading of 33% on the wet patch and 10% on the dry areas
posted by Chenko at 3:50 PM on March 16, 2022 [1 favorite]

Can you mark the photo of the exterior where you think this bit of floor is?

Definitely don't put down the new laminate til you figure out and solve the problem.

You can tape white paper, tissue paper or facial tissue to the wall & ceiling around the area to see if maybe new moisture is dripping or something (the paper will get ripply if it gets wet). Make sure to use white, as coloured paper can stain when it gets wet.

The previous tenants may have spilled something on the day they moved out (plant? drink? fish? etc) so you could also try drying the area with a hair dryer til the wood reads 10% moisture, and then waiting til a day or two after the next big rain to see if the wet area re-emerges.
posted by nouvelle-personne at 4:37 PM on March 16, 2022 [1 favorite]

33% is higher than the range at which electronic moisture meters are accurate. These meters are actually glorified ohmmeters; they measure electrical resistance, and give readings based on the typical resistance at various moisture percentages. Anywhere above about 28% there is liquid water present, and the resistance is so low that the meter can’t measure it accurately. In other words, a reading of 33% is a wild approximation that just means “wet.”

I think you have an ongoing source of new moisture there. Modest spills dry out pretty quickly at normal indoor humidity and temperature levels.
posted by jon1270 at 4:43 PM on March 16, 2022 [2 favorites]

Best answer: It definitely looks like an on-going leak, based on the tide marks. It's hard to tell where the wet spot is in relation to the outside of the building, but there is a brick coping that runs along just under the windows with a long section that appears to be missing or has been broken off. This seems a likely culprit. Yes, it's going to be a problem if you lay a new floor over it. Maybe not immediately, but at some point.

Really, the only way to find the source is going to be opening up the wall and tracking back. if the interior walls are plasterboard, it's relatively simple to remove the skirting board and cut out a section of the plasterboard so you can see inside the wall. Start by cutting a section out about 300 mm high and see what you can see. You may have to wait until it rains (or use a hose, but this often doesn't properly simulate rain) and observe water actually coming in through or down the wall. It may be obvious because you can see water tracks and see daylight through a crack in the external wall, for example.

If you could mark on the exterior photo where the floor level is and roughly where the wet spot is, that would help in advising you where to start, but my best guess is that removed brick coping between and below the windows. As StephenB says, there are signs that there was a section of building that has been removed. That aligns with the removed coping and the area may not have needed to be waterproof originally but was not properly waterproofed as part of the building changes.
posted by dg at 5:25 PM on March 16, 2022

Response by poster: If you could mark on the exterior photo where the floor level is and roughly where the wet spot is, that would help in advising you where to start

It's to the left of the top window, close to the removed bricks
posted by Chenko at 5:43 PM on March 16, 2022

Then that seems like a good place to start - if you don't want to open up the inside wall (eg if it's brick), you could do some waterproofing on the outside in that area, sealing off any gaps, then wait for it to rain and monitor closely. It might help to dry the floor thoroughly (eg with a hairdryer) so you can more easily see if any water is still coming in.
posted by dg at 6:02 PM on March 16, 2022

This will definitely cause a problem, especially with laminate. Do you own the place, and if so do you have homeowner's insurance?

If I had to guess, I would say you have a leak from up top running down the inside of the brick. This used to happen at the place I worked when the roof drains would get clogged. If you are on the top floor (it looks like maybe you are in the photo?) this would be a good place to start.
posted by oneirodynia at 6:54 PM on March 16, 2022

Best answer: > It's to the left of the top window, close to the removed bricks

I hate to point this out, but this black-ish area sure looks a lot like a hole in the wall.

It's impossible to tell for sure based on just a photo, but I surely would be inspecting that area and anything else for a few meters in every direction on that wall for anything that looks like a hole or crack. The "hole" I circled is along a line where the bricks have clearly cracked and separated before, and then been repaired, so it wouldn't be surprising at all for a new hole to open up along that line again.

If it is as simple as a hole in the wall allowing water seepage it might be as simple as some caulking to at least stop the water intrusion in the short term. Long term handling of the cracking issue might be a hole nother kettle of fish, though.
posted by flug at 10:17 PM on March 16, 2022

Yes, after looking at the enhanced photo flug posted above, my guess is that it's penetrating damp from rainwater ingress. Relatively easy to fix, with proper pointing/filling of the hole, so it looks like a job for a builder.
posted by essexjan at 4:57 AM on March 17, 2022

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