Waterproofing an outbuilding

[Mark A]

Hi chaps,

There is rendered block outbuilding in our garden which was built so that the rear and one side is part of a retaining wall holding back about 5ft of ground.
It's generally damp in there anyway, but after heavy rain water puddles on the floor and it will remain wet for weeks, especially this time of year.

I would prefer to knock it down and build a nice insulated timber shed the far side of the garden, but I don't have the time right now.

Is there anything which can be painted over the internal walls and floor to hold back the water? It must be quite hard-wearing (shed is used to store building materials, mowers and garden tools) and *not silly expensive* as it only has to see me through to next year.

Cheers,
Mark

 

[Mark A]

Some pictures

Apart from the fresh paint and two louvre vents in the back wall it's all original 1960s bodgemanship.

 

[MikeG.]

Not successfully, no. The best solution is on the outside, not the inside. If there is any prospect of digging a channel out alongside the walls down to the level of the current floor level, then let me know and I'll describe a strategy for you.

 

[Mark A]

It might be hard work to dig out but I would be happy to give it a go.

What do you suggest, Mike?

 

[MikeG.]

For an ad-hoc short term solution rather than a full tanking, I would suggest digging out as narrow a channel as you can manage down to below floor level, then propping some corrugated plastic (there are cheap polycarbonate roofing sheets available at most "sheds" and builder's merchants) against the wall, corrugations running up & down the wall (and the top of the sheet above ground level). Lay a perforated 4" pipe (the cheapest is to buy ordinary 110 dia. plastic drain and slit it with lots of short cuts from a mini angle grinder, top half of the circumference only) at the bottom of the trench surrounded by pea shingle. Back fill the rest of the trench with rubble/ hardcore, with lots of voids. This arrangement should keep most of the water off the walls, but it will only work properly if you can find a way out for the water you'll collect in the drain. If there is a slope away from the building you could just let the pipe discharge onto the ground, or you could connect to an existing surface water drain or soakaway if the levels and locations work. Or you could lead the pipe to a sump and drop a pump controlled by a float switch into the sump to pump the water away when levels rise.

It isn't perfect, but it's a pragmatic medium term solution whilst you focus on designing a proper outbuilding elsewhere on your site. The corrugations work well in directing water down to the drain, and allowing some air in to help dry the wall. Eventually, all French drains will clog up, and the rubble voids will fill with "fines" and lose their effectiveness, but it should make a massive difference for a few years.

 

[Mark A]

Thanks for the advice Mike, I appreciate it. If the weather is dry tomorrow I'll dig an exploratory hole to see what's behind the retaining wall.

In other areas of the garden the ground consists of 6-12 inches of loamy soil before hitting solid clay. Would that make any difference to how you would approach this?

Cheers,
Mark

 

[MikeG.]

Not really.

 

[Trevanion]

I did the exact same thing that MikeG described for my shed earlier this year as the water kept coming through the concrete block wall off the lawn which was on a higher level than the floor level of the shed. There was already a pipe with holes in underneath the ground with stone on top but it wasn't working as it should. I dug about a 1 foot wide by 2 foot deep trench along the wall clearing out all the old dirty drainage stone which wasn't letting the water through, tore out the rotten blue DPM and took the pipe out, this is when I discovered the bottom layer of blocks had turned into black sludge from the sitting water (Acid rain perhaps?) and it was a miracle the whole wall hadn't cracked and moved along with it. I quickly dug out all the rotten blocks and poured concrete down into the length of the trench and sat the pipe on top of that joining it back into the existing pipe running underneath the shed, I then poured more concrete between the wall and the pipe to create a little ramp off the wall down into the pipe and let it all set up. I tanked the wall with white tanking and rendered over that and the concrete to allow the water to run down the wall into the pipe and then painted the wall which I'm not sure would help or not but I thought it wouldn't hurt to try. Filled the trench back up with new clean stone and turfed the top again. So far after the latest heavy storms, it's been very good, A little water has come in but considering the level of the storms was pretty intense I would call it a success. Whether it will last is another thing but I won't find that out for another 20 years.

What really surprised me though was that the DPM had rotted away and could be torn easier than paper, which more than likely is what caused all the water to go through the blocks as it was rendered into the wall above the lawn and went underneath the pipe. I thought the plastic would still be going strong. I also thought that of the blocks but they'd turned into mush so I have not an idea, to be honest.