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Posted: Tue Oct 09, 2007 1:14 pm
by flimflam_machine
Ok, I'm preparing myself for a bit of a kicking on this one because many similar questions have been asked and answered with some exasperation by the regulars. However, brilliant as the site is, it's not completely clear on some specific points, and for a newbie it is confusing on some things that are just obvious to pros. I have read all the main pages and I'm still having some trouble.
The plan is to lay an 4m x 5m riven flag patio on the site of an existing lawn, the patio will be edged with a (loose) gravel path. We'll use individual bedding (40-50mm) since we can't guarantee the consistency of the flags. We've done the calculations for the quantities of sharp sand and cement (10:1) required. We've done some basic building (rough garden walls) so mixing mortar and pointing is pretty familiar.
The question is: do we need a sub base? Obviously we'll take off the grass and the root layer, but I suspect that there will just be more soil under that. At the very least I guess we'll need to wacker it down to level it and provide a stable layer for the bedding material (and weed-kill it just to be sure). Is this good enough or is compacted (top) soil horribly unstable (especially in the wet)? Is there any point in mixing sub-base material into the soil before compaction to make it more stable or is it only worth doing if all the soil is taken out and pure DTp Type 1 is used?
A secondary question: we're planning to relay the gravel paths round the side with a membrane underneath. Is there any problem with laying a couple of inches of the membrane into the middle of the bedding layer to avoid any gaps that weeds can come through.
Again, I've read the main pages and these sentences may mean lots to a pro, but relatively little to a relative newbie:
"This [a sub-base for a patio] will not normally be required for a path or patio unless there is concern regarding the suitability of the sub-grade."
(I don't know what constitutes a suitable sub-grade!)
"Do I really need a Sub-base? The short answer is: if the pavement will be trafficked by vehicles, then yes, but if it will be foot-traffic only, then not necessarily."
Posted: Tue Oct 09, 2007 4:24 pm
by lutonlagerlout
when you say membrane i take it you mean a geo-fabric like terram or multitrack,polythene membrane is no good for this application
as for subgrade,if you have chalk ,sand or very very stony ground thats good,no need for a subbase
however if you have pure clay or topsoil you really need a subbase
we rebuilt a porch last week,the original one had leaned out from the house 65mm at the top,so i assumed it was built on a poor foundation
WRONG!
the foundation was 500mm wide and 500 mm deep of concrete,what the original builder had not done was remove all of the top soil which was 700 deep here for some insane reason
so we kept digging till we were in virgin ground then repoured the foundation and rebuilt it
i know your patio is only foot traffic but if the ground is soft its no good to go on top with just the bedding layer
hope this helps
LLL 
Posted: Tue Oct 09, 2007 4:54 pm
by flimflam_machine
Cheers LLL,
I do mean a proper geo-fabric and not just polythene sheeting, which would, I assume turn your path into a swamp when it rained.
That does help regarding the need for a sub-base. The answer seems to be that unless, when digging out, we quickly hit something that's hard and free draining we really should put a sub-base in. I assume if we do hit decent stony ground nice and quickly we should check it's well compacted down.
Putting bedding material straight on top of normal soil won't work no matter how compact it is due to possible further compaction and subsidence (due to ground water?). Correct?
The laying-patio-where-lawn-used-to-be does seem to be a fairly common project. It might be helpful to have some description on the site (plus pictures) of what constitutes sufficiently good sub-grade that a bedding layer can be put straight onto it without a sub-base. Getting it wrong would be annoying.
Thanks again, FFM
Posted: Tue Oct 09, 2007 5:57 pm
by lutonlagerlout
well FFM you have only given your location as uk in your profile,so i cannot comment on your ground
generally speaking any soil that is black i.e. topsoil or vegative matter in it will give to some extent
as a pro i always use a subbase except in exceptional circumstances,belt and braces IYKWIM
i had to dig a gully into a patio the other week and the patio was laid on grit sand on top of topsoil
it looked like one of those skateboarders parks,slabs all over the shop,i expect it looked nice for 6 months at the beginning
if you get my drift
LLL
Posted: Tue Oct 09, 2007 11:06 pm
by Tony McC
It seems to me that, with regard to the use of a sub-base, you've answered your own question. As noted in the FAQ that you quote, a sub-base for a patio may not be necessary. Occasions when it would be necessary include...
1 - when building-up levels
2 - when the existing sub-grade is not suitable
..try as we might, we can't see your sub-grade. we can't walk on it or poke it with a stick, so we can't really comment on its suitability. That has to come from you.
If you've stripped off the site and are left with a firm, stable sub-grade, then a sub-base may not be necssary. If, however, what you're left with is soft, sticky, moves when walked upon or in any other way unsuitable, then yes: a sub-base may be necessary. If in doubt, use a sub-base.
Unlike block paving, there is no definitive answer regarding the need for a sub-base with patio flags. It's a case-by-case decision, and only someone familiar with the site can make that decision.
Posted: Wed Oct 10, 2007 10:48 am
by flimflam_machine
Thanks again LLL and Tony.
The location is Oxfordshire, in the garden of a modern (70s, 80s ?) house. The garden is pretty overrun at the moment with very tussocky grass and brambles so it may well turn out that by the time we've removed the root layer we'll need a sub-base just to build up the level.
No doubt there will be a bit of poking-with-a-stick, but it sounds like a sub-base is the way forward and since it should come in at less than 100 quid for a 25 square m project it seems like a false economy to leave it out. A case of PPPPPP, IYKWIM! It doesn't help that we're trying to do this job over a weekend so we need to order in all the materials without being able to see what the sub-grade is like.
It might be useful to put some pictures of different sub-grade on the site and describe how each one will stand up to foot traffic if something is laid without the sub-base. A picture of that skate-park patio perhaps LLL? I know it's the kind of judgement that can only really be made with experience, but the feel from the main site is that it's a judgement call, whereas from here I get the impression that the real message is "better safe than sorry".
Posted: Wed Oct 10, 2007 12:02 pm
by Tony McC
Pictures of different sub-grades? While that might seem like a simple idea, there's three key problems:
1 - the sub-grade beneath my driveway is not the same as the sub--grade beneath my neighbour's driveway. The ground changes enormously from place to place.
2 - Extending item 1, there must be millions of different sub-grades in Britain alone
3 - Photos can't tell you how firm or stable is a sub-grade
Use of a sub-base beneath a patio often is, as you say, a judgement call, and choosing to be safe rather than sorry is a valid option, but it would be wrong for me to state categorically that a sub-base may as well be installed in all cases as it can't do any harm: there are more patios that don't need a sub-base than those that do.
Posted: Wed Oct 10, 2007 1:18 pm
by Rich H
FFM, I work primarily in Oxfordshire and Berkshire, where there are two predominant subgrades depending on whether you are upland or lowland. The upland subgrade is generally very thick chalk. No need for a sub-base unless the topsoil covering it is thick.
The lowland subgrade is clay, and will need a sub-base for certain. Even in the clay seems firm, it can easily become fluid.
I make most of my living removing and replacing paving which has been laid onto clay subgrade without a subbase.
The downside of putting a sub grade is that you will need to buy a couple of bags of MOT at around £80 total delivered, and you will need to dig out a little more than you thought.
The upside is that, having done this, you can have complete peace of mind that you've done the job properly and it isn't going to move.
BTW, just like LLL I always put in a sub base regardless of the subgrade, mainly because I price it that way and you can't normally go digging someone's garden up just to do a quote
Posted: Wed Oct 10, 2007 4:06 pm
by flimflam_machine
This site just gets better and better!
Thanks again LLL & Rich H. Sorry to say this Rich, but I think we'll try to avoid becoming one of your future patio-replacement customers. The site is lowland (not up in the Chilterns) and fairly close to a river so it probably is clay. In light of which we will put in a sub-base.
LLL I know a complete sub-grade list would be impossible. But I can't find any guidance on how to make a judgement on it. Even a couple of lines or pictures saying "If your sub-grade looks like this (clay) or this (topsoil) then definitely use a sub-base." might be useful to help newbies avoid laying a future skate-park patio.
Just trying to make a last minute decision on what flags to get. We're looking at simplypaving.com as a supplier, has anyone got any experience, good or bad, with them?
For stone I reckon either Bradstone Milldale Golden Sand, Bradstone Weatherdale Mellow/Weathered Cotswold, Stoneflair Regal Mellow York or Stoneflair Yorkvale Antique Gold. Any opinions/experience on how these look?
Also are there any particular traps or difficulties to avoid if we choose to lay a circle?
Posted: Wed Oct 10, 2007 8:36 pm
by lutonlagerlout
like the boss-man says ,how many photos of mud can the public handle?
chalk is the best in the south followed by sand and hoggin
all the stuff you mentioned is good its your choice on the slabs
and as for circles and problems?
well if you have never done one before i can almost guarantee you will struggle
read the main site for circles
cheers
LLL
Posted: Thu Oct 11, 2007 10:12 am
by Rich H
If you're in the south of the county, AWBS in Henley, Chiltern in Wycombe and Woods Yard in Twyford all stock a wide range of paving and also have displays.