Best way to waterproof a rain gulley

30 Jun 2014
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United Kingdom
I have a sunken patio which runs the length of the back of my house (Edwardian townhouse), and a raised terraced garden above it accessed by steps up and off the patio. Rain runs off the garden and onto the patio, which for reasons I shall not go into, has been paved with concrete slabs up to the house wall, and laid above the surface of the ground floor. Some years ago I paid a builder (lunatic) to cut in plastic drainage channels along the side of the house to channel the rain and run-off into the drains, one each at either end of the patio. Unfortunately the mortar mix he bedded the channels into failed and reverted to sodden sand, wicking moisture into the house wall. Not only that but the joints between the separate pieces opened up, so any water coursing down the channels toward the drain dropped through and further saturated the wet mortar/sand beneath it, forcing even more moisture into the sand trench touching with the wall. The ground floor is obviously suffering as a consequence. I have since removed the lot and am now left with a lovely trench along the house wall about 6 to 8 inches wide, 8 inches deep, and well below the interior floor surface which I plan to treat more like a roof valley or gully. My question is, can I just line it with a screed of normal mortar, angled of course to the drains? Or can I/should I add a further coat of rubber roof type paint lapped up the sides to make it completely impervious? Appearances are not important, absolute water tightness is. The gulleys will remain open.

First post by the way! Treat me nicely, and nice to met you all.
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Can you leave this new channel open, Alex, or will it fill up with water?
I had a similar issue, and my trench is 9" deep and 6" wide.
I've covered the top with wire mesh to keep leaves and animals out, and it has dried my below dpc area a treat.......whereas before there was a tendency for things to be damp down there, now there is none.
John :)
No, its going to be angled down so any rain and run off will sluice straight into its own drain at the end of each run. So once any rainy weather has ceased the gully will only stay wet for as long it takes to evaporate off, it won't contain any standing water at all. Am I worrying unduly? Having suffered damp problems for so long, I'm anxious to sort it out in one go and get it right first time. Perhaps the coat of rubber paint is going too far?!
I think your cement screed should work just's worth keeping an eye on things during the winter though, just in case the cement cracks with the frost, and likewise any exposed bricks.
You could of course set some channel (guttering) within the trench which could help to cast the water away quicker.
John :)
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Ok, one last query. Is it viable to use loose sand to pack out some small depressions in the base of the gulley before doing the skim (basically to use less mortar to complete the job). Not to line the whole thing, just the odd hole here and there.

Then I think I'm ready to make a start........ :confused:
For what you'll save, I'd go for full mortar......any loose sand is likely to be picked up by the mix anyway. If you are concerned about adhesion, just paint a layer of PVA mix on, and it will stick well.
John :)
With rain forecast for the weekend and the bottom of much of the excavated trench bare soil and rubble (aka Edwardian foundations) I was anxious to start before any rain fell. So, I got half a dozen sacks of ready mix mortar and slung a load down just to cap it. Trowelled it into shape a bit then rubbed it down with a bit of hessian backed carpet to give it a bit of a groove as it started to cure.

So at present its all undulated and I've not even considered sloping it, so to give it a bit of fall to encourage faster coursing, I'm going to start with a high point in the centre of the trench and let it fall away to either side to a drain at either end, to share the load when it does rain. Is it best to build the mortar up in layers to the final height or can I just whack it all down in one go and just give it more time to dry out. Also, in case it should rain before the current layer is totally dry, will it cause any/too much/irrevocable damage?

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