Dealing with standing water on oversized slab

[oliwoodings]

With the majority of our garden building finished, I'm now turning my attention to landscaping around to tidy everything up. However, first I need to deal with the fallout from a mistake I made very early on in the build: an oversized slab.

 

At the time of having groundworkers in to lay the slab, I was planning a wider, shallower building with decking at the front, and I'd asked for the slab to be sized to go under this decking (in hindsight, this was silly and was always going to cause me headaches with water runoff). Then, after the slab was done, I decided I didn't like the layout so completely redesigned it 🫠 I ended up building a longer, narrower building, with doors down the side. I'm very glad I redesigned it, but it's left me with a long wide strip of slab sticking out on the entrance side, and much narrower strips all the way around the other 3 sides.

 

The main problem with this is that water ends up pooling on the exposed slab, meaning the slab is constantly wet (it didn't help that the groundworkers did a shite job of levelling the slab, and then trimmed the DPM right down even though I asked them not to 🙄). This used to actually wick back under the building, but I've mitigated this for now by running stixall round the perimeter between the dpm (under the sole plates) and the slab. Here are some photos (including the standing water):

 

 

 

 

 

This is the current build up of the walls, floor and slab (where the slab sticks out the most, at the side of the building):

 

For the wider strip running down the side of the building: I'm currently thinking the easiest way to both mitigate the pooling and make the side look nicer is to lay 20mm porcelain tiles, bedded at an angle so water runs off the edge of the slab onto the plumb slate, and sealed against the building. I'll also bond the edge of the DPM to the side of the slab with stixall so that water can't track under the slab. For the tiles, I'm planning on using 600x900 ones and cutting them down length-wise to fit, with a 20mm overhang. Here's a drawing:

 

 

 

For the narrow strips on the other 3 sides: I'm planning to bond the side of the DPM to the slab with stixall and then apply 200mm flashband tape, tucked under the housewrap and lapped over the dpm (might not be the prettiest thing, but it will be hidden by planters eventually and I think will be an effective way to mitigate the problem). Here's a drawing:

 

 

I've got some questions on all the above:

1. Does this seem like a sensible approach in general?
2. Would you prime the slab before tiling, if so what primer?
3. Any recommendations on exterior tile adhesive + grout, and tips for laying on an angle (it will be my first time tiling!)?
4. I'm thinking of having the tiles overhang the edge by 20mm, is this sensible?
5. What would you use to bond the dpm to the side of the slab? Stixall?
6. Would you bother tiling the side of the slab as well, or just top up the plumb slate to hide it?
7. Should I clean and prime the slab with anything special before applying the flashband?

 

Thanks in advance

[Russell griffiths]

Bite the bullet

disc cutter, cut the concrete through as close to the building, break it up and chuck it in a skip, dig the ground out and lay a row of slabs slightly lower than the rest of the concrete. 

[Canski]

As Russell said but incorporate a gravel strip around the edge of the cut off slab. This non separation is going to cause you more and more problems. 

Edited March 15 by Canski

[oliwoodings]

43 minutes ago, Russell griffiths said:

Bite the bullet

disc cutter, cut the concrete through as close to the building, break it up and chuck it in a skip, dig the ground out and lay a row of slabs slightly lower than the rest of the concrete. 

 

30 minutes ago, Canski said:

As Russell said but incorporate a gravel strip around the edge of the cut off slab. This non separation is going to cause you more and more problems. 

 

Thanks for the suggestion. It's something I've been considering since I know it would be the best option long term, but I'm really worried about doing it for a few reasons:

- The swa runs right along where I'd need to cut

- The chances of accidentally catching the cladding seem pretty high (I'm not well-practised with a disc cutter)

- The slab has got rebar in it, which seems like a daunting (and messy) task to break up enough to shift it to the front of the house

- My wife actively uses the building for work 5 days a week (massage therapist), my chances of completing this all in a weekend seems slim

 

Any thoughts based on the above?

[Alan Ambrose]

That looks quite a tricky-ish problem and I think you’ll need a big-arse petrol cutter to get through a 100mm slab, which means you won’t be cutting v close to the building. Maybe with an operator if you have not done it before - I haven’t either, but I’ve helped keep the disk wet with a hose while someone else did the skilled work  . You could trim off most of the biggest bit though within say 150mm of the wall. Should cut through the rebar also. Is that the swa lying loose by the edge? Can you pin it up or remove it temporarily? The cutting will be noisy and a bit dusty and take maybe 30mins, but everything else’ll be quiet so shouldn’t disturb anyone. 

 

Personally, I don’t hate the flashband solution.

[oliwoodings]

5 minutes ago, Alan Ambrose said:

That looks quite a tricky-ish problem and I think you’ll need a big-arse petrol cutter to get through a 100mm slab, which means you won’t be cutting v close to the building. Maybe with an operator if you have not done it before - I haven’t either, but I’ve helped keep the disk wet with a hose while someone else did the skilled work  . You could trim off most of the biggest bit though within say 150mm of the wall. Should cut through the rebar also. Is that the swa lying loose by the edge? Can you pin it up or remove it temporarily? The cutting will be noisy and a bit dusty and take maybe 30mins, but everything else’ll be quiet so shouldn’t disturb anyone. 

 

Personally, I don’t hate the flashband solution.

 

Yeah that's the SWA, i intentionally didn't clip it yet in case I need to tuck anything behind the housewrap as part of these mitigations.

 

Glad to hear you don't think the flashband option is completely horrific 😅

 

It sounds like there are issues with the tile approach I'm suggesting (hence multiple people saying to chop it all out) but they aren't obvious to me. Would anyone mind highlighting what's wrong with it specifically? Could water get down through the grout/adhesive or something?

[Temp]

Perhaps you could cut a few grooves in it to act as drainage channels. Bit like you have on a kitchen draining board.  

[Alan Ambrose]

I think most tile adhesive won’t last through a frost cycle or two and the tiles are likely to be slippy too? I would expect frost to be problem also for a cement fillet. Maybe a bunch of waterproofing with some gravel grid on top? Paint on Bitumen maybe?

 

Maybe there’s some ideas or some components you can borrow from flat roof design?

Edited March 15 by Alan Ambrose

[saveasteading]

Crazy idea?

Extend the building to the slab size in timber. The tiniest extension ever can have doors all the way and can be used as storage for garden stuff.

Or the same but just 600 high, with hinged sloping cover .

[oliwoodings]

1 hour ago, Temp said:

Perhaps you could cut a few grooves in it to act as drainage channels. Bit like you have on a kitchen draining board.  

Considered this as well but I reckon it would be pretty hard to do this well, especially to totally eliminate the problem.

 

1 hour ago, Alan Ambrose said:

I think most tile adhesive won’t last through a frost cycle or two and the tiles are likely to be slippy too? I would expect frost to be problem also for a cement fillet. Maybe a bunch of waterproofing with some gravel grid on top? Paint on Bitumen maybe?

 

Maybe there’s some ideas or some components you can borrow from flat roof design?

I was under the impression that exterior rated porcelain tiles are actually more grippy than a lot of other outdoor hard surfaces. I've been looking at stuff like this: https://www.wickes.co.uk/Winterburn-Slate-Black-Matt-Outdoor-Porcelain-Paving-Tile---600-x-900-x-20mm/p/288674 stuck down with something frost-resistent and flexible like https://www.screwfix.com/p/mapei-keraquick-wall-floor-rapid-set-flexible-tile-adhesive-grey-20kg/561fu

 

1 hour ago, saveasteading said:

Crazy idea?

Extend the building to the slab size in timber. The tiniest extension ever can have doors all the way and can be used as storage for garden stuff.

Or the same but just 600 high, with hinged sloping cover .

 

I love the lateral thinking! But I don't think it will work design or cost wise - the exterior doors from the two rooms in the building go straight out on this side.

[Andehh]

We have a 20mm tiled outside patio, and laid a lot for the concrete slab to make it work!!! Tiles don't get slippy, though never had snow on them so can't comment for that situation.

 

I'd just tile over the top of it.

Edited March 16 by Andehh

[Dave Jones]

hire a push along cutter, less mess and no chance of debris flying up and dinging the cladding. Then a cheap titan breaker from screwfix.

[oliwoodings]

2 hours ago, Andehh said:

We have a 20mm tiled outside patio, and laid a lot for the concrete slab to make it work!!! Tiles don't get slippy, though never had snow on them so can't comment for that situation.

 

I'd just tile over the top of it.

Looks awesome! Did you lay the whole thing to fall? If so, how did you do the fall?

[oliwoodings]

I've just stumbled across Marmox sloping backer board! Looks to be exterior rated - can be used for patios, balconies etc. After inspecting the installation instructions, it looks like it is designed to create a proper frost-resistant waterproof layer under the tiles, especially when used in combination with their tape and sealant. And it can be bonded directly to concrete with standard exterior tile adhesive.

 

 

If I used the thinner end of the boards across my ~700mm strip, then at it's thickest point I'd still be left with a 10-15mm airflow gap under the cladding (3mm adhesive + 19mm backer + 3mm adhesive + 20mm tile = 45mm, the gap is 55mm at its smallest).

 

For the overhang, I've also found these exterior exterior edging strips which should neaten it all up.

[Andehh]

6 hours ago, oliwoodings said:

Looks awesome! Did you lay the whole thing to fall? If so, how did you do the fall?

Yes, whole thing to a fall via the slab, though for your area.

 

I still think tile it with a small fall, and make a feature out of it... Especially if you have two doors there as well!.. Its not like you'll use the space for anything else or be able to grow anything there.

[oliwoodings]

11 minutes ago, Andehh said:

Yes, whole thing to a fall via the slab, though for your area.

 

I still think tile it with a small fall, and make a feature out of it... Especially if you have two doors there as well!.. Its not like you'll use the space for anything else or be able to grow anything there.

Thanks! I've found a low-slump thick bed tile adhesive I think might do the trick without faffing with sloping backer board: https://www.uk.weber/weberset-thick-bed . Given I've only got to get a slope over one tile width, I reckon it could work, especially with a jig to help lay a bed to fall.

[Andehh]

Yep, as long as you have a slight fall and good water management/DPC you'll be OK. Can seal the grout for extra measure but we havnt bothered to do so yet.