Underfill

[JohnBeeBee]

Hi,

I had about 8 indian sandstone flags taken up by a contractor to check the footings on my property. He's put the flags back down but there appears to be a lot of space underneath the flags and very deep wide joints as one expensive tub of jointing mortar (VDW 800) didn't go very far. Is there anything I can pour in to fill some of the gaps and joint depths before I point the rest with the expensive stuff.

It only took 2 tubs to originally point the whole patio and so I'm reluctant to use more than 2 tubs just to redo a small area.

Thanks

[Tony McC]

It would be better to lift the offending flags and re-bed them completely, rather than rely on trickling-in some filler.

However, if you wanted to avoid lifting them, then the best method would be to make a slurry of grit sand with cement at around 6:1 and pour that into the open joints. Then, use a hosepipe on low pressure to "wash" in the slurry, with the aim of sluicing it into as much of the void space as possible, adding more of the slurry as required until you sense that no more can possibly be swallowed by the paving. Use a thin stick or cane, if it helps, to push the slurry into the voids.

Make sure you wash off any spills or splashes before it has a chance to set.

[Bob_A]

Hi Tony I've not heard you mention that before. I've got Indian Sandstone that's pointed with Rompox Eassy and over the years a great deal of it has been power washed away leaving one or two flags not rocking or moving but I can 'feel' not hear a small click when I walk over them, maybe due to freeze/thaw. If I remove the remainder of pointing around those flags would this method work before I repoint with a 2 part GFTK?

[Tony McC]

It's not likely to fix a rocking issue - it's more a technique to fill medium-to-large sub-surface voids without lifting the flagstones.

There has been a host of other threads about fixing rocking flags with finer-grained slurries, SBR-enhanced grouts, even adhesives, and they would be better options for what you describe. But still, I'd rather fix the rocking properly before re-jointing, than rely on a bodge that may well fail after you've spent a deal of time and money on jointing compounds.

https://www.pavingexpert.com/faq_fix_loose_flags

[Bob_A]

Ok I see my mistake. I was aware of the methods you mention but mistakenly thought this was an additional method.
Sorry to the OP for gatecrashing

[JohnBeeBee]

Thanks Tony I'll give that a go. I'm not up for lifting the flags and removing the existing bed and I don't think it will take much to fill.