Loose flagstone flooring

[Tom-H]

Hi, From the advice here I’ve bought an old 1820’s weavers cottage which has been sympatheticly restored.

It ticks all the checkboxes with all signs of cement removed, so lime plaster and pointing throughout! I have to say there’s not a single sign of damp anywhere, the house is bone dry.

Downstairs the flooring is the original flagstone flooring. These have been lifted and electric underfloor heating installed. I presume these have been laid on a lime floor as the pointing is in lime.

However I have about 3 flags that move around when you walk on them. I didnt notice this when moving in a few months ago but have since had the underfloor heating on which the previous owners didn’t use.

Quite a few other flags sound hollow but are solid.

So I’m going to have to lift these flags, so what is the recommended lime mortar mix for rebedding these? Or is something else recommended that will hold up better to being heated?

Thanks

 

[Socksitis]

I do not have an answer, but will share the questions I had for having a limecrete floor with UFH and then slate slabs on top. The slate slab supplier was hesitant when I suggested the slabs would just be bedded in lime mortar. He said they would 'ping off' and the slabs would need to be stuck down, so I called his glue man who told me they would need to be glued with his product and if used at a diluted rate, would be breathable.

I then look at the Ty Mawr site and they do a product that can be used to glue floor slabs - Adhere Cal I think it was. I spoke to them and they said this was the product to use.

I then spoke to Mike Wye who I use and they said they did not see a problem with the slabs moving, if bedded in correctly.

I have asked on here and the responses were similar in that there is no issue with not using a glue.

I know it does not really answer your problem, but it may help?
A

 

[Flyfisher]

How are the flags 'moving' when you walk on them? I presume some sort of rocking rather than lateral movement?

I don't really understand why flags would need to be stuck down. If properly bedded on a solid floor then they really shouldn't be able to move in any direction (except if they are pulled upwards!). I suppose it's possible that the UFH has caused the bedding mix to expand/lift/crack, which might create a small 'ridge' on which a flag could wobble slightly - but I'm really just guessing.

Here's a recent thread about tiled floors with UFH that might be helpful: http://www.periodproperty.co.uk/forum/viewtopic.php?f=1&t=16175

 

[88v8]

Our sitiing room floor is limecrete, new in 2011 just before we moved in.
No UFH.
We laid ersatz Painswick slabs, reconstituted stone, each one moulded from an original slab. They were a pig to lay as the sizes are random. Laid direct on the slab, no screed.

Being reconstituted and made in a mould, the bottoms were flat and afforded no grip for the lime mortar. A few weeks later they began to come loose. The whole lot had to come up. After some conferring with the manufacturer, we relaid then with cement mortar, keeping the cement back from the edges of each slab. In that way, the floor could still breathe through the lime pointing. It is after all the pointing that breathes rather than the stone.
Six years later, OK and breathing well.

It takes time for lime to gain its full strength. Weeks. Months. I can imagine it debonding if the ufh is switched on soon after laying. Worse with large slabs due to the dimensional change. Walking on them too soon is a big aggravating factor.
I think if one doesn't put on a continuous layer of glue/cement, but leave it back from the edges of each slab, the breathability will not be undermined.

[the manufacturer of the slabs subsequently changed the moulds so that the bottom was ribbed to give the mortar something to grip.]

Another cause of lime coming adrift could be failure to clean the slabs before laying, perhaps aggravated by laying them dry; I always think it best that anything being laid in lime should be slightly damped in the same way that the background should be damped before lime plastering, but I'm not sure this was done with flooring in ye olde days.

Ivor

 

[plasticpigeon]

I tried bedding some cement stone window sills in lime mortar and they didn't staick at all. They stuck fine with a 1:1:6 mortar mix. I wonder if reconstituted synthetic cement stone just doersn't stick with lime. Bricks certainly do and so does stone. I'm not sure of the mechanism that woudl cause a mortar to work or not work, but I would wager that stone slabs would be happy bedded in lime screed, and as people have said it doesn't really need to be stuck down there just needs to be no gaps under the stone and they needs
pointing up well so that they cannot move.

 

[Tom-H]

Thanks for the replies!

The flags rock slightly depending on which side you stand on and the mortar is cracked around the edges. One of the flags that moves is cracked from when it had been lifted, so this may also be an issue.

They are the original sandstone flags so it sounds like in theory there should be no problem with the lime and UFH

I will contact Ty-Mawr but will probably stick to a NHL lime mix and use the tricks suggested to make sure they are fitted correctly this time

 

[MatthewC]

... I have about 3 flags that move around when you walk on them. I didnt notice this when moving in a few months ago but have since had the underfloor heating on which the previous owners didn’t use.

Quite a few other flags sound hollow but are solid.

From what you say it sounds as though they weren't well re-laid after being lifted - if some sound hollow then there is no join between the stone and the slab. A chap who laid some smaller tiles in the church here told me to mix up some neat NHL (I think he was using 3.5) into a paste and smear it thinly onto the stone, w; this then guarantees a good join between the stone and the slab. Sadly his mate didn't do that with the larger ones and some of them sound hollow...!

As has been mentioned above, it does depend on how long was left between laying the slab then the stone and walking on the floor, and between laying the stone and switching on the UFH. If in doubt allow an extra fortnight or more!