Waterproof jointing for paving slabs (flagstones)

[motco]

I have a paved balcony over part of a garage. It was relaid and a new balustrade fitted but the builder (now defunct) persuaded me that a coat or two of Cromapol would work as well as a membrane at keeping water from penetrating to the garage below. Needless to say it did not. Despite re-grouting water still penetrates through the joints. I have tried a siloxane coating (Enviroseal) with limited effect. I think what I need is a flexible waterproof jointing compound such as expansion joint mastic, but I am reluctant to choose this route without some expert guidance. Relaying the flags over a membrane would, of course, be the best answer but this is impractical unfortunately. Thanks in anticipation.

 

[mikeey84]

Unless there is a fall I can't see, I reckon you will also be getting water going under the flashing, as, as far as I know, it's not really meant to be used like that.

I'm not sure this is an option out there that would last longer than a couple of seasons.

Maybe mortar with a waterproofer in?

I take it the garage was strengthened before the slabs were added? That's a fair amount of weight...

 

[motco]

Thanks Mikeey84, you're right about the flashing, there is no fall and it was pouring in through there after the flags were re-laid. I have pumped several tubes of lead sealant under the edge and that seems to have fixed that leak. It seems that mortar is too brittle and it separates from the flags' edges and where ponding occurs, seepage also occurs. The garage roof is a concrete beam structure similar to Bison beams so there's no problem there. I was drifting towards a mastic such as expansion joint material and possibly a clear two-pot epoxy lacquer over the whole surface but I'd like an expert view before committing to that - hence this post. Again, thanks for your reply.