Permeable laying course - Flags and slabs

[Chris Fry]

Hi All
I am keen to pick your brains on the permeability of a laying course for 900x900 Indian Sandstone Flags. To gain planning permission I have to preserve permeability for two trees in a front garden. I will be laying a geotextile on the ground surface, then stone aggregate to attain levels, then Cellweb Geogrid - back filled with angular stone. I was intending to use 6 parts sand to 1 cement for the laying course, and grout with a porous resin bound product. My question is how permeable is my laying course, if permeable I am happy, but if not, do you have experience of an improved laying course that may give me additional permeability ? I do not want to fall foul of a very very overly concerned tree officer !

Many Thanks in anticipation

Chris

[Tony McC]

The bald truth is that, while most site-batched bedding mortars using a coarse sand with cement are not completely impermeable, they are barely permeable.

It's much better to buy-in a proprietary pre-batched permeable bedding mortar, and perhaps surprisingly, they work out to cost not much more than the price of mixing your own sand/cement mortar. They also have the added advanatage of being demonstrably permeable, so you can show the packaging to the planners and they can't really argue!

There's a case study on the website here - although it's a commercial project in the case study, the supplier of the mortar is more than willing to supply smaller domestic projects and DIYers.

[Chris Fry]

Thanks Tony - I like the idea of evidence for the planners. The case study again very helpful

Cheers