Thin sub-base or thick bed? - Is a thin sub-base better than none?

[IanGlyn]

Dear Tony

First let me say that no amount of praise and thanks for your web site would be sufficient - what an amazing undertaking and so incredibly useful. Thank you very, very much for your endeavours.

So that's the buttering up done, now for the very humble request for advice from you or the community of paving professionals and enthusiasts you've established!

I'm paving a 30m2 back yard (as a Midlander I can't bring myself to use the 'patio' word), about a third of which will be within a newly built (but not yet complete) outhouse. Sadly for me I didn't pick up on your pages until well after I'd done the groundwork (foundations, drainage, that sort of thing), which was completed about 9 months ago. The top soil in the area was dug out and the ground beneath well compacted. The sub-soil is a sandy-loamy-clayey structure and it is now compacted so well that whilst it does still drain it does so very slowly. I allowed 75mm depth for the paving (100mm in the outhouse area, the floor of which will be slightly higher than the yard). Now I've started researching the paving and found and studied your web pages, including the section on sub-bases. Rats!

I don't think my budget, bank balance or hernia will run to bringing in yet another skip and digging out a further 75mm for an MoT-1 sub-base. However, the 'traffic' on the area will be, and can only be, very light, so the glimmer of hope is that the faq page at http://www.pavingexpert.com/faq_subbase.htm#reqd suggests that I might just get away without it given the light traffic and well compacted, top-soil-less sub-grade. But given the slow drainage of the sub-grade I can sort of see the sense of having a sub-base. I'll be using 25mm calibrated flagstones (Indian sandstone) so have 50mm for any sub-base and bedding (75mm in the outhouse area).

The question (finally) is therefore: would anything be gained by putting in a 20-25mm sub-base (constructed of 10mm gravel to keep to the 'half depth' rule with some sharp sand filler) and then a 25-30mm sharp sand/cement bed or would I be wasting my time and be better off just putting down a 50mm bed? Or, horror of horrors, should I be digging out another 75mm of subsoil?

I have a couple of other questions but I'll post those separately another time to keep the subject matter distinct in the discussion group.

Many thanks
Ian

[seanandruby]

read this
Always best practice to use a sub base but as shown "the rules can be bent."

[Tony McC]

10mm gravel with a sand filler really isn't a sub-base. Once you go into the fizzix of how sub-bases work, you'll understand why - all to do with interlock - and realise that, while it's an OK-ish way to build up levels, it's not actually a load-bearing layer in its own right.

If you've only got 75mm to play with, there's a good argument to be made that 75mm or concrete as a bed is a better choice than 50mm of gravel and 25mm of mortar. However, if you prefer to work on a 25mm mortar bed, then I'd just use 50mm of 10mm gravel and not worry about the sand filler - it does nothing other than fill the voids and, with a mortar topping, there's nowt to trickle down into the voids anyway!

Some contractors would put down the 50mm of gravel then use a sand blinding - just scatter grit/sharp/coarse sand over the surface of the gravel and then use a plate compactor to 'settle' it in and fill the topmost voids. That's fine, but, for a midlands "back yard" it might be considered over-engineering.

If the sub-grade (the ground) is firm, then I'd go with the 75mm concrete bed. I can't see any significant benefit in creating a flaky 50mm layer of gravel: concrete is so much more reliable.

[IanGlyn]

Thank you very much for your sound advice, Tony, it does make complete sense. Thanks also to Sean for his reply, the page it directed me to wasn't entirely relevant but of interest nonetheless (I had already looked at that page but a closer look answered another question for me!).

Given the advice and having looked again at the page on sub-bases, I've decided to approach it at follows...
In the yard area, where I only have 50mm or so beneath the flags, I'll follow Tony's advice fully and use a thick bed with no sub-base.
In the outhouse, where I have 75mm or so beneath the flags, I'm less sure about being able to handle a bed of that thickness. I'll therefore build up the level a little (30mm or so) with a bonded 'sub-base' using a lean concrete mix and then use a 50mm bed on that after giving it 2 or 3 days to cure.
So I'm a happy boy (or will be when the job is done) :;):

Many thanks again
Ian

[Tony McC]

Let us know how you get on.....I don't envy you starting a project in this weather!

[IanGlyn]

Ah, but you don't know how my projects go - it's at the planning and deliberation stage now and that can take weeks! Weather should have warmed up by then. My approach may yet change, at least with regard to the yard, so there may yet be another post or two.
Thanks, Tony, I will let folk know how it goes. Always assuming the post is still on the list when I'm done!

[Tony McC]

It might have moved down the listing a bit, but I never delete posts to The Brew Cabin - a permanent archive of man's unrealistic expectations and delusions of grandeur!