Bedding sandstone - Advice on laying slabs

[SJB]

I am laying some 600x600 sandstone paving,honed, sawn and calibrated to 20mm on a damp screed of sharp sand and cement strength 1:8. I have taken a lot of care ensuring the slabs are evenly supported, firm and not rocking when layed but have noticed that subsequently some of them have a hollow sound when walked on as though they are floating rather than solidly fixed although they are not moving.The slabs are relatively light compared to concrete paving. What am I doing wrong and how can I fix it? Any advice gratefully received. Steve

[lutonlagerlout]

I always lay sandstone on a wetter mix with SBR added to the mix to aid adhesion
also I do not believe 1:8 to be strong enough

1:6 is our usual gauge cement : screeding sand

just have a bucket of clean water and a clean brush nearby for any splashes

cheers LLL

[neatcon]

What is SBR??

[mickg]

styrene-butadiene rubber

SBR

[johnty]

cant say i have used it myself either

[lutonlagerlout]

I didnt use it till around 5 years ago John

but it is great gear for steps and coping stones

it really does stick them like $%&* to a blanket

LLL

[mikeindorset]

Sandstone is also porous, and lighter colour stone is susceptible to staining, from the iron content of sharp sand used in the underlying slurry or screed bed. It is best to follow advice on pavingexpert.com, and as LLL suggests, seal the back of the slabs with a paste made from SBR and white cement before laying.