WTF is living on our house?

April 12, 2011 7:45 AM

Is this mould? We're stripping down our bedroom to redecorate. We've found these yellow blobby stains on bare unpainted plasterboard (drywall for Americans) behind the old fitted wardrobe.

It's mainly contained to one entire piece of plasterboard (upstairs, inside of an outside wall, away from window, no leaks from above), though there are small blotches where overhead cupboards were. There's no spots or marks on painted areas, even the small amount on the vertical join.

We've tried bleach and rubbing it down, and neither have shifted the marks which makes me think it might not be mould. We don't want to cut out the whole piece of plasterboard and replace if we don't have to, as that means hiring a professional - and not having a bedroom for a few more weeks than we planned!

Even if it's not mould, we'd like to try and figure out what it is. Any ideas?

posted by tweemy to home & garden (11 answers total)

Glue stains?
posted by stormpooper at 7:53 AM on April 12, 2011

If we're talking normal plasterboard (as used in the UK), then the surface is usually grey paper. You shouldn't be using bleach on that, or be rubbing it down.

Are you sure there isn't a skim coat of plaster on top?
posted by le morte de bea arthur at 7:55 AM on April 12, 2011

That looks an awful lot like condensation marks. Is it damp in that room at all?
posted by LN at 7:59 AM on April 12, 2011

Is the blobby-stained part contained to roughly the same perimeter as the wardrobe? What's the wardrobe made out of? If it (the wardrobe) has been treated with a lot of stain, it's possible the marks are discolored areas where oil has leached out of the wood.

In my experience, (light, surface) mold (like in crappy apartment bathrooms) has always come up with some bleach and scrubbing, so if you tried that already I really don't think it's mold.
posted by phunniemee at 7:59 AM on April 12, 2011

Those look like water stains (which can cause mold, obviously, but it appears this hasn't gotten that far).

This is what mold looks like.

This is what mould looks like.

We had a decent amount of mold in our bathroom. We did a Potemkin village kind of rehab (fix up cheaply until we can attack it later) on the bathroom. Figuring we'd replace the wallboard later if we had to, we cleaned with bleach and primed with Kilz. That was a few years ago and the mold hasn't returned. Whatever you choose to do, ventilate and wear a mask.
posted by Terminal Verbosity at 8:10 AM on April 12, 2011 [1 favorite]

then the surface is usually grey paper. You shouldn't be using bleach on that, or be rubbing it down.

Yes, it's grey paper on top of the underlying gypsum. We only did a small test area near the floor, behind where the new wardrobes will go. We don't want to bleach the whole lot, as you say it will damage it. It's not been skim plastered, alas. Basically, they mounted the wardrobe and wall-mounted cupboards directly to the plasterboard (and screwed into the breeze block behind). Then painted around them in white undercoat, then a horrible pink. Someone later wallpapered over the exposed areas.

It's not particularly damp in the bedroom. We do have a little classic black mould that's in the bathroom, on the tile grout. That comes off with some elbow grease easily enough.

The old wardrobe and cupboards were laminated chipboard, and the stain is constrained to where the side of the wardrobe was - the back area, on the 90deg adjacent wall is clear, though also bare plasterboard. There is also small handsized areas behind each of where the cupboards were, on the same wall as the main stain. Both wardrobe and cupboards were stand-off from the wall slightly - they weren't directly against the plaster, apart from the thin side bits.

We're happy to just prime and basecoat, then paint like the rest of the room, but don't want to just paint over mould!
posted by tweemy at 8:12 AM on April 12, 2011

For the cost of a piece of drywall, I'd be inclined to cut a section out to see what is going on inside the wall. If it is clean, you were going to paint anyways, now you just have to patch a bit too. If it isn't... yikes... but at least you know...
posted by csmason at 8:16 AM on April 12, 2011

I've added a couple more pictures to the album;

this one shows where the wall-mounted cupboards were;
and this shows the wardrobe area (side is the stained part, back wall was the rear of the wardrobe)

Thanks for the ideas so far. If it's water stains, can it have come from inside, i.e. condensation, or does it mean there's water leaking through from the other side?

I'm not sure our DIY skills are up to hacking out exploratory sections - or rather repairing them afterwards, we've certainly damaged the plasterboard a bit getting the three inch rusty iron nails out that were holding on the skirting!
posted by tweemy at 8:34 AM on April 12, 2011

Fixing drywall (plasterboard) is dead easy. Don't be afraid to cut out a small section so you can get a looksee back there.
posted by Aquaman at 8:43 AM on April 12, 2011

If they are water stains, they formed by condensation between the old cupboards and the wall surface. If the water source was inside the wall, like a leaky roof or burst pipe, you'd see a vertical stain like someone had just peed on the wall.

Patching drywall is really easy, by the way. You just need some wood, screws, tape and mud. Actually, I've seen ready-to-go screens for quick patches, but haven't tried them myself.
posted by Terminal Verbosity at 8:45 AM on April 12, 2011

Here's a good drywall how-to (with pictures!) if helps.
posted by phunniemee at 8:51 AM on April 12, 2011