Flags Direct On Clay

[JonD]

This post is a bit of a continuation of one I dropped on the 'Drainage' section.
I'm about to lay new 3x2 (by 2" thick) flags to make a bigger patio at the rear of the house. The intention is to lay directly onto the clay soil as was the case with the previous (same size) flags which have all been taken up pending intallation of the new batch.
Question: because it's clay soil underneath, should I just use grit sand to get a level rather than a mix of sand and cement?

[84-1093879891]

The choice is yours. Personally, I'd use clean grit sand, but for most DIYers a weak sand/cement mix (10:1) is much more forgiving of partial compaction and uneven bedding.

This is nowt to do with clay soil or any other sub-grade - it's just easier for DIYers!

[JonD]

Thanks again for the comments, Tony.
You might recall my submission in the drainage section re 'Low Level Patio'? It occurs to me that if the flags were laid on a dry sand/cement base, there might be less chance of natural water dispersion after rain. On that basis, I don't intend sweeping a dry sand/cement mix over the slabs once they're in position. Does any of that make sense, or will there be negligable drainage effect below the flags? In other words, would it be best to seal the flags and let any rain water run over the top and on down to a point dictated by the patio incline? The previous patio flags were laid direct on the clay and there weren't any flooding problems, although the new patio will sit about six inches lower when completed.

[84-1093879891]

Regardless of how you joint the flags, the majority of surface water will run across the surface and so you need to allow for some drainage collection point.

If you're not going to joint them using a sand-cement mix of some description, what do you plan to use? Dry sand?

[JonD]

Interesting question.

I had in mind to dry joint the flags with the idea that rain water would seep down between the cracks and disperse below.

However, the 'below' is compacted clay so it may be that any water seeping down will tend to sit there?

If I thought the clay had any drainage properties at all, I'd lay the flags on a grit sand base (no cement). But if we're saying the rain will run down the flag surface anyway, it might be best to lay them on a 10:1 dry sand/cement mix, then sweep in 5:1 between the cracks whilst hoping the suface water will drain through the slits in the retaining wall.

[84-1093879891]

Using sand joints will allow water to penetrate the pavement and, rather than just sit there, this water will disturb and transport the unbound bedding material, leading to settlement and movement of the flags.

An unbound bed is ok as long as the joints are sealed (this normally means mortar-pointed) or if you are certain that any surface water finding its way into the construction has a rapid escape route via the sub-base or a drainage composite. Allowing surface water to find its way to an unbound bed is one of the quickest ways to ruin a pavement.

[JonD]

I guess that settles it then.
I'll go for a 10:1 sand cement mix for the base layer and 5:1 for the joints.
Is it better to lay the flags when the clay soil is wet or when it's dry....or somewhere in between? Or doesn't it matter?

[84-1093879891]

Whether the clay is damp or dry doesn't make much difference, but if the surface is wet and claggy, what we call 'slutchy' in Lancashire, then the slutch should be scraped off to expose firm, non-slutchy clay before placing the bedding material.

[JonD]

Thanks Tony lad......the retaining wall's now up and I take delivery of 42 flags tomorrow morning, but guess what? It's started raining again :biggrin:

[84-1089053830]

It's been glorious here today -September is already a better month than August was!