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.

 

[TinyFlat]

Hooray measured yesterday with laser, I have 70mm up to where DPC starts.

I'm going to use a mid-grey limestone (not as dark as kotah - tumbled ash grey) on 21 square meters patio. I am thinking it's easier (?) to use the mortar to joint and I'd like some kind of contrasting colour (light grey or beige?) between the slabs. How do you get the light colour, is it the sand or the cement or both? I'm using 600 x 400 size. If anyone has any suggestions for patios they've created please offer up and tell me what you've used.

 

[Blunt Tool]

Use soft sand, and if you want a sandy cream colour use snowcrete ( white cement) and you can get yellow colouring to get the colour you want. Remember the ratio you use of quantities for a consistent colour or mix dry and more than enough for area. If you want grey it is Portland cement (grey) and soft sand and you can use some snowcrete to lighten. Again remember ratios or dry mix more than enough for area. For pointing a ratio around 2 to 1 or 3 to 1 for soft sand and cement.

 

[Blunt Tool]

Once you mix it dry take a small amount mix with water and let it dry to see finished colour to make sure you are happy with it

 

[TinyFlat]

Once you mix it dry take a small amount mix with water and let it dry to see finished colour to make sure you are happy with it

That's great - thank-you. Think I will go for light grey.

 

[One Day]

Once you mix it dry take a small amount mix with water and let it dry to see finished colour to make sure you are happy with it

Would you add a touch of sbr to the mix?

 

[Blunt Tool]

Would you add a touch of sbr to the mix?

Personally no, messes about with drying. But then if I was pointing limestone slabs I would be leaving mix dry then on a dry day with little chance of rain I'd take a small watering can and wetting the joints with it then using a soft brush sweeping dry mix into joints and compacting to make sure joint is full then sweep away all excess then wait and the water will rise into the dry mix then finish joint with a finishing tool. That way no mess and no stains on limestone slabs! Thats how I would do it

 

[Dumbo]

Personally dry pointing I think I rubbish . How many exterior patios do you see with grout falling out the parts of my patio that I did wet are solid as a rock whereas other parts that were done by my brother in law are falling out . I told him to do it mixed with water but he did it his way because that's how all the builders do it

 

[Blunt Tool]

Personally dry pointing I think I rubbish . How many exterior patios do you see with grout falling out the parts of my patio that I did wet are solid as a rock whereas other parts that were done by my brother in law are falling out . I told him to do it mixed with water but he did it his way because that's how all the builders do it

Never had a problem so can't comment or get into it

 

[TinyFlat]

I'm getting a bit confused about something. Reading on paving expert website and they seem to recommend, sharp sand grit sand or coarse sand to mix with cement. Is soft sand not recommended?

 

[Blunt Tool]

I'm getting a bit confused about something. Reading on paving expert website and they seem to recommend, sharp sand grit sand or coarse sand to mix with cement. Is soft sand not recommended?

Depends what size joint you are going to give them, anything under 8mm soft sand, anything above sharp sand as bulks out joint better. But remember soft sand will give a smooth finish joint and sharp sand a coarser finish joint.

 

[TinyFlat]

Depends what size joint you are going to give them, anything under 8mm soft sand, anything above sharp sand as bulks out joint better. But remember soft sand will give a smooth finish joint and sharp sand a coarser finish joint.

Thank-you. It needs to be soft then.

 

[SJPurdy]

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 .

Hooray measured yesterday with laser, I have 70mm up to where DPC starts.
QUOTE]

So the existing concrete is already 80mm to high and you want to raise it further!

 

[TinyFlat]

The existing concrete is not 80 mm "high". I am talking about the distance between the concrete and where the damp proof course starts. The level of the concrete is 70mm below where the damp proof course starts measured from the highest point of the concrete on the patio and using a laser level.

 

[Dumbo]

The existing concrete is not 80 mm "high". I am talking about the distance between the concrete and where the damp proof course starts. The level of the concrete is 70mm below where the damp proof course starts measured from the highest point of the concrete on the patio and using a laser level.

I think he gets that