Joined: July 2004
||Posted: 08 Oct. 2019,11:45
We wouldn't ever rely on waterproof jointing to prevent moisture ingress to sub-surface rooms. Even the very best mortar products/additives cannot guarantee 100% watertightness, and you need only the slightest movement of one flagstone to cause a crack in the jointing and hey ho! You're back to square one.
The correct methodology would be to install a waterproof Damp Proof Membrane beneath the paving, then re-lay said paving on a waterproof bedding material with waterproof jointing, if so desired, but, in all honesty, it's the DPM that provides the security of watertightness, so I'd be inclined to re-lay using a more conventional method.
You can buy waterproofing additives for mortar, but, as I've said, it will be only a matter of time before the jointing (not the waterporoofing) will fail.
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