Which paint to seal wall against rainwater

[Pamela]

A short wall dividing two flat roofs absorbs rainwater, which leaks from the brickwork into the roofspace below. The top and sides of the wall were treated with bitumen paint which worked for a few years before water got in again. This is the wall:

My handyman will paint it again if I buy some sealant. The original paint was "IKO-pro Flat Roof Renovator" which the tin says is for the flat roof itself rather than a wall:

There is also this IKO-pro Bitumen Paint (now discontinued). Is this sort of thing better for that wall?

I also saw this Thompsons High Performance Roof Seal:

Igot spoilt for choice when I saw an acrylic sealant Polar Leakseal:

I'm confused. Which one is best to use?

[Dave Liquorice]

Remove railings, cap stones and first course of bricks. Cover exposed top of wall with sheet lead that comes down the wall and over the top the roof covering fillet, both sides. Relay new clean bricks and cap stones, replace railings.

As a bit of lead to cover that width of wall and down the sides won't be cheap you could probably get away with just lead flashing along the edges and DMP immediatly above it. Ensure the lead and DPM have a slight fall to the outside. The wall top covering needs to be contiguos to stop water penetrating the wall. If there has to be a joint the edges along the joint need to have a vertial bit of about an inch and an inverted U cap over the top.

"Sealants" will just fail, again. Properly done lead/DPM will cure the problem for good.

[alan_m]

Could it be that the water penetration is where the cap stones have been drilled through to fit the railings.? The bitumen may have sealed the base of the railing posts but possibly these are the most likely to have moved cracking any seal.

[Dave Liquorice]

exposed

Possibly but there are still the joints between the cap stones to fail and let water in.

[Chris Hogg]

Is the damp permeating equally along the whole of the wall, just one end, in the middle? Can you inspect the top/sides of the wall to see if there is an obvious crack in the existing sealant? A place to look carefully would be that fillet of cement flashing that runs along the bottom of the wall and on the ?EPDM rubber roofing itself. It doesn't look as though it was ever bitumened. If you can see a possible crack, brush it out any loose stuff and smother it with a rubberised roofing sealant from a gun. I have used Geocel Roofers Seal successfully for that sort of problem. Despite what they say, apply it in the dry.

[Pamela]

All very true and something to do in spring. Meanwhile the wall needs sealing as rain water is entering and I'm not sure if I should get plain bitumen paint, roofing bitumen paint or acrylic sealant paint.

This was my shortlist:

[David Paste]

FWIW I had a similar problem with a flat garage roof and just used the cheapest bitumen goo I could find and put a reasonably thick layer on. It definitely worked on cracks and small holes but AIUI cracks and small holes indicate that a "proper job" needs doing. It did, however, give me an extra 2 years of leak-free garage action. But I'm glad that it was re-covered properly so I don't have to play the rainstorm lottery anymore.

[Dave Liquorice]

exposed

As a "keep the wet out 'till the spring(*1)" solution I'd just get some reasonable weight poly sheeting and a few bricks(*2) and simply cover the cap stones, bricks along the edges every 2 to 3 feet. With the sheeting an inch or two wider each side so water drips off onto the flat roofs. The delux version would have a batten along the line of the railings to enforce a fall on the sheeting and allow a bit of ventilation underneath. Super delux tape any joins between sheets rather than just having an overlap.

Trouble is we don't really know where the water is getting into the wall. It could be through the joints in the cap stones or beteen the wall and roofing fillet or both... try just the capstones if that doesn't have much effect it's easy to put a bit sheeting under it and down to the roof (weighted down of course) to fully protect the wall/ fillet as well.

The caps stones do have a drip groove underneath the overhang don't they?

(*1) I'm assuming spring 2021, not '22 or '23... B-)

(*2) Such as scrounged empty feed bags from local farmers opened out and spare rocks that are lying around or even a logs from the wood pile.

[Andrew]

Also, there may not be a drip channel on the underside of the overhanging lip of those slabs. All the rainwater pouring off the edge will wick under and down the wall without a drip channel.

Window and Door external cills have a drip channel for a reason !.

Complete removal and the addition of a lead or copper flashing right across the wall below would be the 'proper' fix.

[Andrew]

+1

[Pamela]

A drip channel was put in with an angle grinder. I think the concrete(?) slabs weren't correct for the job and may be porous. Hence the sealant paint.

The amount of water this arrangement let in to the room below during a rain shower was amazing. Until the true cause of the water was discovered, the roof was tanked with fibre glass like a boat (on the near side in the picture) but it didn't help.

[Andy Burns]

wickes liquid DPM

[alan_m]

Would a quick run over with a blow touch re-melt the bitumen and reseal any opened cracks?

[tabbypurr]

just paint more on, filling any sizeable cracks first. For something that may last, paint, add cloth, paint, sprinkle with sand. The latter has at least some movement tolerance.

NT