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Posted: Thu Jun 01, 2017 9:54 pm
by bodgeitandscarper
Its been a couple of years since Ive done paving (got a bad back)and im trying to lay my patio! Ive laid 8m2 so far and ive checked it today and it seems a few of the 600x900 sound hollow in one corner, the rest of slab sounds solid. Should i lift or could the pointing sort it out? I am struggling to lay the big ones due to my back and this maybe the cause, due to not dropping it equally onto the bed. Ive used a moist mix and sbr.
Posted: Fri Jun 02, 2017 10:32 am
by Tony McC
Hollow sounds usually indocate a void beneath the flagstone, bit because a flag acts like a drumskin amplifying the sound, the void can actually be relatively small. So, hollow sounds don't always indicate a problem of any real consequence.
I'm working on a report right now where 70% of the flags sound hollow when tapped with a stick, but when a ranndom sample were lifted to see what lies beneath, the voids were found to be relatively small, smaller than, say, a golf ball, but because of the drumskin effect, they sound as though there is a mineshaft beneath.
The only reason to lift and re-lay a flagstone is if it is moving. If it rocks when straddled or moves when walked upon, it needs to be lifted and re-laid onto a fresh bed.
Relying on the pointing to resolve rocking or movement is not always successful. It *can* work in some instances, but each case has to be judged individually. If there is significant movement/rocking (and how you define "significant" is tricky) then lift & re-lay is the best option, but if it's only a couple of millimetres of rocking, then flooding the joint with a SBR-cement slurry, and rocking the flag to draw in the slurry to fill any voids or gaps beneath, can often work a treat.