Concrete Slab Drainage

[supertom44]

I'd like to build a deck outside my back door, however the concrete slab/pad that's there currently has been damaged over the years and the drainage doesn't work.

The main issue is the channel that's there has sunk in the middle so water ends up sitting there.

I don't really wanna remove it all as it's quite thick and that would be one hell of a job.

Wonder what the best way to resolve this issue is, thinking maybe if I dig up just the drain channel and then redo that part? What's the best way to lay concrete so that it slopes towards the drain?



PS: I will make an access hatch in the decking for the drain cover.

 

[ChaiLatte]

Please make a wider shot that actually shows the drain into which the water needs to flow.

The rectangular iron cover is an inspection cover (the clue is in the writing cast into it), and is not supposed to free-flowing like a gully grating. If, in the past, water disappeared into it, that is purely chance, not design.

The first thing to do is a level survey. Water cannot flow uphill, so make sure the drain outlet is actually the low point in the whole area. If not, it might be an option to lower the drain a little to minimise necessary work to the existing hard standing (and if you are to cover it with a deck, no-one will see it or trip over it).

Build your deck on top of impermeable packers that are higher than the water will ever be (so the deck legs do not rot) and let the sunken slab be someone else's problem. There is no water there now, so it does get away somehow eventually.

 

[supertom44]

Doh, including the drain would help haha.

Excuse the mess, the drain is located at the far end, circled. There's a dip in the middle channel which is the main cause of issue that I need to sort out. Not sure best way to do so.

I wasn't expecting water to flow into the inspection grate.

 

[ChaiLatte]

Thanks for the picture. Just make sure everything to the left of the red rectangle is higher than the red oval.

For that, I would clean it well (jetwash), prime with PVA or SBR and use a 4:1 sharp sand/cement/PVA-modified water mix to screed out the badness.

If you work on a minimum thickness of screed of 5mm, you can cut out just that part of the existing concrete so that minimum thickness is achieved (in other words, you do not want the screed to taper out to a feather edge).

You can put some woodscrews into the channel and adjust their height as a guide for your screeding straightedge.