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Do i need to apply sbr to underside of indian sandstone patio slabs?

I plan to put down 100mm type 1 MOT and whacker plate it with a 1:80 fall sloping away from the house, followed by 50mm mortar which will be 4:1 sharp sand to cement, as long as this is all correct, please can someone advice if I actually need to apply SBR to the underside of the INDIAN SANDSTONE slabs prior to laying them on the mortar bed? I've tried looking online but get various answers which also depend of the type of stone used, and some saying it'll be ok if you just wet the underside of the slab with water first which will stop the slab absorbing all the moister out of the mortar . A definitive answer would be much appreciated.

6 Answers from MyBuilder Stonemasons

Best Answer

I think it's just to stop the initial suction so it wont dry to quickly, if you apply sbr you could do all the stones then lay them. If you use water then you will have to damp every stone down as your going to make sure they all stay damp as there being layed.
If you use a feb in your sand and cement mix also it will keep it more plyable for longer


Answered 5th May 2020

In my opinion the answer is yes !it only takes seconds to do each slab give's better adhesion /blood bridge from slab to mortar ,can also stop any mortar staining seeping through to top face of slab .


Answered 3rd May 2020

In my personal opinion as long as you have a good solid level bed of mortar and is of a damp consistency but not to wet i brush the back with water on all the Indian sandstone being bedded, you can lay straight down, never had a problem..


Answered 21st Dec 2020

With Indian sand stone and any natural
Portland, forest of dean etc, I always brush a cement slurry to the bed of the slab either white or grey cement depending the stone used. Make sure the bed is full not a dot and dap.


Answered 5th May 2020

Firstly sandstone, whether Indian sandstone or not sandstone will vary in its porosity, and ideally a sandstone with low porosity, is preferred for external paving, I would personally apply a bonding coat of PVA and cement to guarantee a bond between bedding material and stone especially on thinner materials, as they are prone to popping free, and of course a full bed, no mortar dabs.


Answered 6th May 2020

Depends on the current climate. As it's currently hot we normally give thinner slabs, like indian sandstone, a quick brush with a pva and water mix. Helps with suction as previously mentioned by another comment. On thicker slabs we have scored the underneath as well as pva'd them before for extra grip. Works, no complaints.


Answered 5th May 2020

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