Advice needed on levelling before limestone flags are laid

[TinyFlat]

Hi
I have a patio with a concrete base which has depressions in some areas. The highest level of the concrete seems to be around the edges where the concrete meets the walls (patio is enclosed by 4 walls) and it seems to have sunk in the middle over the years.

How would you even this up before laying limestone flags? I also have two doors (one from a bedroom and the other from a living room) that open out onto the patio so i am conscious I need to stay below the threshold to avoid any damp problems and both doors open outwards (it's a basement flat too).

I've been told a levelling compound needs to go on top of the concrete. Someone else (limetsone supplier) said use the depth of the mortar to level . I am really confused and would value some professional opinions from here as to how you would do. The slabs i want to use come in 22mm and 17 mm thickness.

Many thanks

 

[Dumbo]

If the patio has 4 walls how does rainwater escape . Is it in a drain in the middle which suggest it hasn't sunk but that is how it was designed . Also there are very few levelling compounds which can be used externally . I only know of one made by ardex bur there may be others

 

[TinyFlat]

Hi and thanks for reply. Yes, there are 2 drains, and yes the concrete is sloped around these two drains (as it should be to allow or drainage) but that's not what I was referring to when I said it's uneven. The concrete has depressions in it as you move away from the walls. The patio has not been touched since the 1980s as the person I bought the flat from lived there since then.

 

[J Sid]

if you say top of flags are 15mm down from the damp course what is the distance left to you concrete base?

 

[Old Mod]

An image or two would help a lot.

 

[TinyFlat]

if you say top of flags are 15mm down from the damp course what is the distance left to you concrete base?

Hi
Sorry I don't understand this question. The mm I referred to in my post are 22m or 17mm and these are the thickness of the slabs available to buy in the limestone I've chosen.

What i want to know is what do you guys use ,or put on top of concrete, (that is uneven) when you lay limestone flags? Would you have to dig up the existing concrete? Or would you put a leveling compound on top of the concrete or as the tile supplier suggested would you lay mortar on the uneven concrete and make sure that is level?

I don't have pictures at the moment as not home. The patio is 3 meters at each side and 7 meters length. So 21 square meters in all and the depressions in the concrete I'm referring to are maybe 10mm- 20mm depth which means I get some puddles when it rains. There's about 3 puddles.

 

[WetSaw]

Use the thicker slabs where the depressions are!

 

[Time's Ran Out]

Fix the limestone slabs with a dry sand/cement mix, slurry the backs and concrete and take out the undulations in the screed - assuming you have the depth to the damp course!

 

[TinyFlat]

Fix the limestone slabs with a dry sand/cement mix, slurry the backs and concrete and take out the undulations in the screed - assuming you have the depth to the damp course!

That makes sense to me. I think I will have enough depth if i use 17mm thick slabs. Thanks, John.

 

[Time's Ran Out]

Minimum depths for a screed so may have to go the thick bed adhesive route for outside work.

 

[J Sid]

simply, how many mm from concrete slap to damp course?

 

[TinyFlat]

simply, how many mm from concrete slap to damp course?

Ok, i understand. I can't measure at the mo, because I'm not there but i will. From memory I had 50-60 mm where the concrete meets the wall with the doors in but when i looked at the opposite wall the concrete there looked higher. So I think I need to take highest point of concrete to damp course. I'll measure tomorrow when back and update the thread. Thanks.

 

[J Sid]

ok, if that the case you can lay as @timeless john has instructed.

 

[Dumbo]

A damp proof specialist will tell you should have your slabs 150 mm below your damp course or you will need a pea shingle gully around your patio .

 

[TinyFlat]

A damp proof specialist will tell you should have your slabs 150 mm below your damp course or you will need a pea shingle gully around your patio .

If I don't have enough depth I'll have to dig the concrete up or gully. I hope isn't going to be the case. I've got someone over tomorrow who has a laser level so I will be able to check again by taking a measure from the highest point of the concrete across to the level of the dpc and after allowing for 15mm seeing how depth much I have to play with.

Thanks to everyone on here for responses.