damp-proofing a garage

[Fred]

Hi,

I have a single-skin brick garage. It appears as though the DPM is at ground level! I'm sure the garden has not been built-up so I think this was a mistake by the builders. When it rains heavily and puddles form on the ground, sometimes these seep into the garage.

Inside the garage one wall has been painted, the other is bare brick. I am finally about to line the walls with insulation and plywood but am wondering what to do about the brickwork first. On the painted wall, a couple of the bricks at ground level have peeling paint. That could be due to age (I've never painted them since moving here) but I think the bricks may be a little damp but it is only one or two bricks on the bottom course.

Without digging up the floor and laying a new membrane, what are my opinions? Would painting the walls with a waterproof paint help or would that make things worse? I was thinking either bitumen paint or a special waterproofer. I found this:

don't know anything about it.

Would you paint the bottom few courses only or the whole wall? I was thinking of painting inside rather than outside but would it be worth painting the bottom courses outside too? I think in the past they had black masonry paint on them.

TIA

[js.b1]

If you wait long enough, Tommy Walsh on Quest (38) does a single skin garage.

If the outside ground is higher. Dig a ditch, line with membrane, set in a perforated land drain in gravel. You can get 80mm in 25m lengths quite cheaply on Ebay.

Bitumen paint for walls. Everbuild do a D.P.M. which is bitumen with latex as I recall, costs a bit more tho. Wet the brush with water, then dip in the tin and onto the walls. Walls from ground to ceiling and floor if necessary although I suspect a liquid DPM you can pour is probably better (someone will be along shortly).

Treated battens on wall. Screw treated battens to wall, then infill with insulation, insulation over the top ideally re cold bridging, them board out. The insulation & PB does not touch the floor so any leaks can not wick up the wall as it were.

[john]

I really enjoyed watching that. A good one. Had a wry smile though with the shot (morning) of big Al painting a first spot of DPM on the wall. Come going-to-the-pub time he then proudly shows what seemed an acre of walls and floor covered in the black stuff. It didn't quite ring 100% true :)

[Vernon]

Celotex website advises to place insulation against wall, then batten on top to form a cavity, and cables for sockets etc can run in the cavity. Then put plasterboard or in OP case ply on top.

[NT]

but he makes a boob re the dampproofing. If you put the VB on the cold side of insulation as he did, condensation is likely, and it cant get out. Fit any insulaiton, then add a VB on top.

Dig and backfill with gravel should work if you can provide drainage.

NT

[js.b1]

Walsh did... Bitumen on wall, then p/t battens, rockwool between battens, celotex over battens, foil tape over joints, PB over celotex, taped joints.

If you do not bitumen a single leaf wall you will get chronic water ingress. If you do not tape the joints of the celotex or tape PB, you do not have a vapour barrier.

So basically he was relying on 2 vapour barriers. Fine as long as that in the middles does not fail - and there comes an issue re "drylined socket accessories" in something like a kitchen. The moisture can get in, can't get out of the PB, can't get out of the wall. One reason why I always insulated behind backboxes to remove the cold bridge which worked superbly on a test kitchen (just 6mm Marmox is enough, but I used 12.5mm since I had bits kicking about).

In the middle go p/t battens - and hopefully stainless screws where it is single leaf brickwork.

The only foolproof way would be to turn it into a "pool shower room". Seal with wall with Primer-G / Primer -S / SBR, then Mapei Keraflex, then Marmox in 6-50mm thickness, then polyurethane sealant & tape the joints, then plaster skim with tiles or whatever on top. The problem is that gives too low a U value for most applications, it is however very waterproof. Stick a piece of Marmox behind any backboxes, since it is extruded polystyrene it is tough enough to work ok.

There is no perfect solution, I too do not like even p/t battens in any wall which gets damp and much prefer bonding the insulation to the wall. In that respect I am surprised there is not a "Marmox-PIR" (ok, there is, but rarely found) in terms of a cement & glass-fibre backed PIR which can be bonded directly to the wall with Keraflex. That ensures nothing is getting through your single brick skin (or double solid brick which can be just as bad in certain circumstances).

[NT]

Ah, that makes more sense :) I only saw as far as the bitumen coating and cutting a doorway out.

NT

[Fred]

Does anyone else have an opinion on bitumen vs. "dpm paint"?

I haven't found anyone selling the Everbuild one but Wickes do a rubber/bitumen paint specifically marketed as a dpm and the reviews on its web site are good.

I read here that two coats of bitumen paint would work. Would I need two coats of the dpm paint or can I save myself a coat if I buy the special dpm paint?

TIA

[js.b1]

for Everbuild d.p.m. - there are other suppliers.

Probably the same stuff. It is nothing magical.

See the instructions.

For floor use I recall Tommy Walsh used 3 coats on the "stick-built- glass-box" on Challenge Tommy Walsh, the one with the "French idea" outside CU which was moved inside. For walls I suspect a single coat will do - not read the tin. He wet the brush in a bucket of water before dipping into the D.P.M. otherwise it gets rather tiring to apply... probably a hoard of minions working off camera :-)

[NT]

I dont expect dpm paint would be anything but bitumen

NT

[Fred]

It claims to be a mixture of rubber and bitumen but strangely it is cheaper than bitumen paint, so I wonder whether it contains less bitumen or whether the price is lower because of economies of scale: perhaps they sell so much dpm paint they can offer it for less?

[Fred]

Sorry to reply to myself but I forgot to say:

I already have some bitumen paint, so if it is just as good, I will use that up and save buying a new tin.

One wall is bare brick, so that's straightforward enough but the other wall already has paint on. Can I paint bitumen/dpm paint over the existing paint or will I have to strip the existing paint first?

TIA

[zaax]

Have you working out why the walls are getting damp?

[Fred]

The walls are not damp at the moment. I felt them the other day to check. When it rains heavily some puddles form inside but I wonder whether the water passes through the first line of mortar rather than the first line of bricks? Anyhow, I've never known the walls get damp, only the floor and only then when we have had very, very heavy rain.