With the landscaping industry improving its standards, best practices constantly changing and new products continuously coming on to the market, it is important as landscapers that you keep up to date with these changes.
I was laying Indian sandstone from the mid-nineties until I finished landscaping last year. Although Indian sandstone was considered a budget paving slab, that is no longer the case with prices almost doubling in the last 18 months from £18 per square metre to £30 per square metre. This is almost touching the price of the cheaper end of the porcelain range. Also, sandstone prices are set to rise further in 2022.
Back in the 90s most Indian sandstone was 30mm to 50mm thick and by the 50mm thickness they were quite a lump to lay on your own. These days most sandstones are 18-21mm thick and calibrated. Around 12 years ago Indian stone suppliers realised that at 30-50mm they could get around 13-15 square metres in a crate, but if they made the slabs thinner, they could get 20 square metres in a crate. This was a great idea for them as businesses as this means more profit for supplier, but it brought issues to the contractor’s table especially when sawn and honed sandstone came on the market.
With the actual weight of riven sandstone now reduced by a minimum of a third, this meant the paving slabs no longer had the weight to hold them to the mortar bed and de-bonding became a problem. It immediately affected cracking, which allows the ingress of water and brings further issues with freeze-thaw de-bonding surrounding pavers. Industry Best Practise now is to slurry prime the backs of all paving to prevent de-bonding of the slab from the mortar bed.
Several key points when laying Indian stone:
- Lay on a 30-50mm mortar bed.
- 5-1 strength mortar bed, 4 parts sharp sand, 1 part builders’ sand, 1 part cement. This is my preference, and at a later date, we will discuss hybrid mortar beds and their benefits.
- Lay riven sandstone to 1-70 fall rate
- Lay sawn and honed sandstone to 1-80
- Always mix your crates of sandstone don’t just empty one crate at a time or you may get a patchwork quilt affect due to colour variations. Always wash the dust off the back of the slabs from the calibration process. Dust left on the back of slab may prevent the slurry primer properly adhering to the back of the slab
- Slurry prime the whole of the back of slabs with premixed British Standard primer. If you leave a ring around the edge you may get what is called the picture framing affect (see figure 1)
- A full contact mortar bed is essential, which means the whole of the back of the slab is in contact with the mortar bed. There should be no trowel marks in the mortar bed, as in the picture below (see figure 2), as these hollows can hold moisture which can cause freeze-thaw delamination issues, collect salts which may cause an efflorescence bloom or reflective staining. This can be a big issue, especially in sawn and honed sandstone (see figure 3).
- Try using a float to prepare your mortar beds as you can achieve a much flatter mortar bed which helps greatly with full contact.
- At the end of each day of laying slabs insert brick ties into the mortar bed to tie the next days paving to the first days paving and continue this procedure until at the end of each day until the patio is fully laid. This will help prevent day joint separation on clay sub strate.
- Use a Grabo when lifting the slab onto the mortar bed as this will prevent the slurry primer getting all over the face of the slab.
- I would always recommend sealing the whole of sawn and honed paving before laying with a breathable sealant. Take great care to follow the instructions on the product, especially reference temperature and ambient moisture.
- Never clean mortar stains with brick acid, especially with Kandla Grey, as this was will bring iron deposits or rust staining to the surface of the slab (see figure 4).
- Always use a product specifically designed to clean sandstone.
- If laying light coloured sandstone especially sawn sandstones I would use a white cement which has reduced iron and heavy metal content. White cement is also a lot more expensive, so I would factor this in at the quoting stage.
- Grout using a slurry grout such as Flowpoint.
Fig. 1 
Fig. 2 Fig. 3 
Fig. 4
