Sandstone what am I doing wrong?

17 Apr 2015
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United Kingdom
laid two patios with riven Indian sandstone flags. Laid on 5to1 sharp sand and cement with plasticiser.
wettish mix slightly stiffer than a wet bricklaying mix. Continuous Solid bed and pointed next day with 3to1 mortar mix.
Looks great initially but it's not bonded to the back of the Slabs in that you get a hollow sound when you knock them and getting hairline cracks on the pointing.
After the first time I thought my issue was too dry mix but second time was wet and still got the problem but not as bad.
Any idea what I'm missing as otherwise the job looks good. Should i Sbr back of each slab as I go?
Cheers for any tips
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A hollow sound is not a bonding issue, a hollow sound means there are unsupported areas, i.e. its not quite a full bed or a long shot but the stone itself is delaminating.

Have you lifted some of them to check the quality of the bed?

I think you also may have suffered from the very hot weather. Flags should not be bone dry when laying and should be misted to keep them damp in hot weather once laid. Otherwise the slab sucks all the water out of the mix too fast leaving it not bonded. It can also cause picture framing as it sucks up cementy water into the flag.

Lastly a bond bridge is a sure fire way to prevent picture framing and not bonding or as a halfway house you can simply add perhaps 100ml of sbr into every gauge of mortar.

As for the hairline cracks around pointing that is either because the flags are rocking slightly or the pointing mix dried too quickly and shrunk a little. This can again be due to very dry flags sucking water away quickly.

Also adding lots of plasticiser to a pointing mix can cause shrinkage and sometimes people fall foul of this when using a pointing gun.

What mix did you point with and how?
Thanks for the reply
That's the thing when I lifted some up the bed was like a screeded floor ie pretty good, which was why I'm sure it's a bonding issue
maybe hollow isn't the word but you can tell it's not right when you knock it
pointed it with 3to1 building sand. Did use some plasticiser as I gunned it in
Yeah it was hot and the Slabs were dry. Sounds like misting them with the sprayer might have saved it!
For the pointing try plastering sand especially if using a gun. You dont need as much plasticiser and it gives a much harder mortar too for long term strength.

Are you sure you didn't cause slight rockers by walking on it too soon? A 5:1 mix should be fine to walk on the next day but a slightly weaker mix and laying late in the day might not have been ready to walk on the next morning?? Just a thought?

I always keep sandstone misted with a hose even if its not hot. Just a belt and braces thing. Using sbr in your bedding mix should be fine really rather than priming every flag. Granite slate etc need a bond bridge but not normal sandstone.
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I walked on them same day after a few hours which I thought was ok to do, guess I may be wrong!?
The Sbr I will do in future.
Next day is an absolute minimum in my book. you could risk walking in the centre of large flags after 4-5 hours if you absolutely had to but why take the risk. 6:1 or stronger is good for the next day.

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