The Brew Cabin

Tony, I have read that it is not always necessary to have a subbase layer if the soil it quite established. Would it be possible for you to define how well compacted and established it should be to not need a sub layer.

I can give you a brief description of the soil now if it helps. When dry, the soil seems quite solid under the feet, even if "jumped" on. However, when the great rains came down the other week it became quite soft (which I suppose is common sense!!!). I had to build the small walls through out the garden and with walking on it quite a bit it was noticed a bit of sinking, but not by loads mind.

Would this not matter once flagged, cause I cant imagine much rain water getting to the soil once down.

If the soil if sufficent for loading, then would it be a good idea to put down some strong membrane and then the sharp sand layer on top to strengthen it all up, or would this cause problems

The flags size by the way is 450 x 450 x 32mm and to be layed with gaps between and filled with a filler.

Russ

any chance of a hand???

A hand with what? Laying the flags? Not with my knackered spine!

Defining the competence of a sub-grade is fraught with difficulties, as the ground varies so much from place to place and any description-based definition is almost certainly doomed to fail. That's why a standard test, the CBR, is used to determine the suitability or otherwise of a sub-grade.

However, for a patio, we needn't get too concerned, as we're not anticipating massive loads. If the ground is firm and stable when dry, then it's probably ok - if, when you jump on it, it wobbles around a metre or so away from where you are, then that's more of a problem.

If you have any concerns, then using a geo-membrane would help, but, for 95% of all patios, laying the flags on a cement-bound bed, such as the 10:1 mix of grit sand and cement that I advocate, is more than adequate.

Tony, Sorry to here youve been ill again, No worries about the delay one little bit mate.

The ground is pretty much stable when dry, but I will give it a further good compacting with a whacker before I put the 10:1 mix on top. I was thinking of using a Geo-Membraine as well to ensure there is no mixing of the 10:1 with the sub layer. Ive looked on the geo-fabrics page but a bit confused on what to actally get. I was thinking of just using the woven fabric you can get in most stores now-a-days, but would this be stable enough. I looked at the thicker more durable woven stuff you can get (think is a polyester type material, seems pretty stong) but I was wondering if the bed might slip about on the smooth surface.

Any recommendation would help alot. Also, what thickness sharp sand would you recommend as a bed with out a sub-base.


one more thing!!!! Reading you jointing page, if there is the slightest of movement in the flags, does the joint not crack???? Is there not a flexiable jointing material

You can get a roll or two of Patio Partner from the Terram mini-website which is ideal for your sort of project, or you can get mini-packs (50m²) of Terram 1000 (same product, but different colour and different name) from better BMs for around 75 quid.

Lay your paving on a bed not exceeding 50mm. I like to add a sprinkling of cement to sand bedding beneath patio flags, as it helps stabilise everything.

And you're right, if there is any movement in the flags once they're laid, then the jointing will crack. This shouldn't be too much of a problem as flags are supposed to be firm and stable on the bed and not rely on the jointing for their strength, unlike, say, blocks or setts.

However, in practice, especially with smaller patio flags, the jointing can contribute hell of a lot to the stability of the flags, and so, if there is any movement, a rigid jointing medium, such as mortar, will crack. But if you use a flexible medium, you either pay a small fortune for a sealant such as Nitoflor, or you use an unbound medium, such as sand or splitt, which allow vegetation to establish itself on the pavement.

You have to decide which is best for you. With stone or riven-effect flags, I prefer mortar jointing, but with some of the polished or shot-blasted flags, a 3mm flexible sand-filled joint is fine.

Just wondering tony, does the woven fabric decrease the structural integraty of the base???
I only ask cause they currently have a hugh roll (god knows where they got it from cause the roll width is about 6meter!!!!) of a woven (I think) grey fabrics, it is stronger than the cheap thin stuff you get from wilkinsons.
Im pretty sure its woven (never come across non-woven!!!!), it kinda looks like the fibres are just mixed randomly (rubbish explanation!!!!).

And beens as they have loads, was wondering if I could make use of it save them spending £50 - £75.

Cheers for help Tony
Russ

If it's a permeable geo-membrane, then it should be ok. Even if it's a weed barrier, it won't do any harm, but it probably won't last more than a few years as the sub-base will pierce it.

If it's an impermeable membrabne, such as a dpm, then it could cause problems. :(

A non-woven permeable membrane looks like the stuff on the Geo-sheets page, a random orientation of fibres that are thermally bonded (techno-speak for hot glued).

And who's "they" and "Hugh"??? ;)

If its the top picture on the geo-fabrics pages then I think thats the one, Its pretty thick (could kind of define the thickness as you can crease (if thats the right spelling) it). So that should be ok to use???

You say no more than 50mm of sharp sand as the bed, with some cerment....10:1. Im I right in saying the thiner the bed the more solid (as long as its not to thin) or dont it really matter.


"Hugh" is that local chap known as "spelling error!!!!! ;)
and
"They" are the womans parents....so I suppose I best to at least a half decent job!!!!

Russ

(Edited by Rusty at 10:43 pm on May 14, 2003)


(Edited by Rusty at 8:44 am on May 15, 2003)

The membrane you have sounds fine, Russ. :)

50mm is the maximum thickness for a bedding layer, and 30mm is the minimum, although you can go down to 12mm if it's a mortar. Like a lot of other things in life, you don't want too much or too little. 40mm is ideal. :)

Well, I'm glad "Hugh" know who they are! ;)

Tony,

Grand, I will initally set it out a 50mm then, then whacker it down and finally screed off to 40mm..........sound a plan???

Thanks for your help, Wish me luck this weekend, saying that if the rain keeps on it looks like I wont be laying the flags as I presume its best not to do it in the rain!!!! :(

Russ

To create a 40mm screed, you need around 70mm of sand, that's compacted with the plate to around 45-50mm, and then you have an excess that can be screeded to final level.

Working in the rain is not a problem, unless there's mortar involved, but if it's chucking it down, then it's best left. Fine drizzle is not too bad, and showers are tolerable, but steady, non-stop, soak-you-to-the-bones rain, such as we have today, is awful and you're better off seeking shelter in the alocal lehouse while revising your strategy! :)

Well, what a weekend, chucked it down every day and with the ground as water logged as it is I didnt even think about attempting to prepare the ground for flagging. So carried on building the next level walls.

You say use 75mm of sand and then plate down. I have 3 tonne of sharp sand, do you think I should order some gravel as well or will the sharp sand alone (with a 10:1 cerment mix) be ok??? I have loads of 20mm gravel (a wee bigger than the recommended 6mm!!!!) ;)left over but I can imagine that is a bit to big.


Russ

The sand alone will be fine. Adding gravel is fine if you're using individual bedding, but when you're planning to create a screed, the inclusion of gravel can 'drag' on the surface of the screed, creating 'furrows', which aren't the end of the world, but can sometimes be a problem when laying smaller elements, such as 300x300 'flags' or block pavers.

Obviously, a 6mm gravel wouldn't be too bad, as it will mix in with the coarser particles of the sand, but a 20mm gravel would cause much more of a problem and is best left as a coarse aggregate for knocking-up concrete.

If you're using the 10:1 bedding mix, don't make up more than you will cover with the paving in around 3 hours, as the cement starts to go off as soon as it comes into contact with any moisture.

It's not much of a week for flagging, really! :(

Your right there Tony, :( sick of the weather now

I didnt realise you only have rougly 3 hours working time with a 10:1 mix. It it possible then to prepare one aera first, lay the flags then prepare another aera.
Will the vibrations through the ground from the plate whacker not knock the flags loose or anything

Russ