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Posted: Mon Apr 14, 2008 8:43 pm
by newbuyer17
Apologies in advance but I have quite a few questions on a patio project I am doing and was hoping I could get some advice from people in the know.
I have an existing patio which I want to extend to approx double its size. This is a raised patio, which is what is causing most of my concerns.
What I have currently is a wall of rocks which do not appear to be mortared together. Inside these is a bed of soil (which is very clay-like) and sand and slabs laid on top. It actually had a small flower bed at the front which I have already dug out. It appears to have withstood the test of time fairly well and hasn't sunk too much.
http://kjpublic.dynalias.com/patio3/IMG_4674.JPG
http://kjpublic.dynalias.com/patio3/IMG_4667.JPG
http://kjpublic.dynalias.com/patio3/IMG_4673.JPG
http://kjpublic.dynalias.com/patio3/IMG_4677.JPG
http://kjpublic.dynalias.com/patio3/IMG_4666.JPG
and my hardcore:
http://kjpublic.dynalias.com/patio3/IMG_4676.JPG
I intend to move the wall forward a couple of metres and fill in the extra with hardcore I have. Note that the wall doesn't support any weight vertically. I will then fill in the top 150mm or so with sand and a little cement before laying the new slabs on. I will remove the old slabs and replace the whole thing, but not sure whther to keep the existing base over half of it or start from scratch across the whole area.
Not for my many questions:
1. Will the wall not be pushed forwards by the hardcore forcing it forwards. This hasn't happened with the soil and sand so far, but I dont quite understand how.
2. Should I mortar this wall instead of loose building it?
3. I wasn't going to dig foundations for the wall - It currently just seems to sit in the soil.
4. Should I try to keep as much of what I have or start again over the whole area? i.e. is it easier to mix and match or will it never sit level?
5. I dont quite understand how the sand wont just drip through the gaps in the hardcore over time, leading to sinking? How do I avoid this, or will it really not happen?
6. I will put a gradual slope on it to get water to flow away from the house, but wont this just lead to the lawn getting waterlogged. I think at the moment the water drops through the cracks in the paving and is absorbed into the bed of soil/sand underneath - probably 2 ft deep.
I've probably overlooked lots so any other advice is greatly appreciated.
Thanks in advance
Posted: Tue Apr 15, 2008 12:38 am
by matt h
qu 1. this existing wall has been built well in that the stones have been well meshed togrther. Your soil and sand back fill have merely been retained by it, and as there has only been light traffic over the slabs there has evidently been litle movement. Over a larger area the sideways push will be greater, but if the wall is constructed as now, and you crush your hard core fine enough you should have no problems. Its the interlocking of the wall stone which gives the wall its strength. And the gaps provide natural water drainage. If you've ever been to Minorca there are many of these walls built which have stood for centuries.
Qu2 Mortar would help strengthen the wall but would retain water and then you'ld need to insert some sort of drainage behind the wall
Qu 3 depends how far you are extending patio really.Normally I would say lay a base then build up , but as this has no real height i would say compact area and lay straight off ground as before. As long as the base is firm you should be ok.imho
Qu 4. It really depends on what you want as finished effect. I f you want it to look rustic then duplicate what you already have, but remember the rest has settled over the years and your extension may need a few extra passes with the compactor to compensate for this. The soil looks quite loamy to me and the sand has been added as a binder and to assist in drainage
QU 5 IT will sink over time, but if well compacted should take long time, sharp sand compacts well and allows good drainage. If your hardcore is crushed enough and compacted the gaps are much smaller
Question 6 If you have sand between your slabs you should have natural drainage. If your lawn doesnt get waterlogged at the moment , I dont see how this extension will affect it any more than existing.
If you are intending heavy traffic over the patio, then none of the above apply... take it all out and start again, and go to main site re bedding etc. Dont forget to ensure your finished level is min 150mm below dpc
Posted: Wed Apr 16, 2008 8:12 am
by newbuyer17
Thanks very much for a quick reply. In response some more questions:
2. I was going to just use a dot/dab of mortar, so I'd still get the drainage - is that OK?
4. The dreaded compactor word. Do I really need one of these? I was hoping to do without hiring one and just use a sledgehammer or whatever to compact stuff down a bit. I also figured using one of these was even more likely to push the wall forwards. As you assume, it will only be light traffic i.e. table and chairs out there.
6. In terms of drainage I believe the current patio allows water through the joints, and into the soil below. I will probably grout these so as to not allow water through, but to have it run off the front. Therefore I'll have 4M worth of rainwater rolling off the end in one place. Or is that not the way to do it? Would this cause flooding on the lawn?
Thanks again for your advice on this.
Posted: Thu Apr 17, 2008 1:52 am
by matt h
see main site reference landscaping and walls, which explains how it all works. If you dont compact you will get greter settlement later. Stop trying to save pennies, it'll cost you pounds. Better off using a semi dry mix screed and bedding on that imho. Your current patio drains ok so why alter the spec?The other alternative is to incorporate drains into your extension, again all explained at length thanks to Tony's patient and well laid out main site.
Posted: Thu Apr 17, 2008 5:26 am
by seanandruby
You have quite a depth there. you will need to wacker in your crush, sledge wont do at all. Also dotting is forbidden. I think you need to give it a lot more thought as to what you want, ie steps, falls drainage. It would be a nice little job to do.
Posted: Sun Apr 20, 2008 5:03 pm
by newbuyer17
Thanks All,
Thats clarified a few things for me, and made me more confused about a few other bits : )
Firstly "Also dotting is forbidden - Sean and Ruby" - I think you misunderstood what I meant I wasn't going to use dot-dab on the flags, just on the wall to give it a little extra strength. Is this OK or not worth doing? I dont want to use full mortaring on it as it will look rubbish.
"Your current patio drains ok so why alter the spec? - Matt". Unfortunately by extending I am altering the spec. The current one seems to have a sub base of just soil and sand. My half will be done (better) using crushed hardcore. My dilemma is whether to leave the existing half alone or try to rebuild that too. I definitely would prefer not to excavate the whole thing, but instead of lifting old flags and dropping down new ones I could just lift the last 150mm or so and use the same screed bed? Or would I be better to leave it be and simply replace the existing flags on the same base?
Someone suggests using Semi Dry mix screed. However,
"The flagstones can be bedded directly onto a coarse, grit sand bed, approx 40mm thick." - from the bedding layer page.
Do I need a full on screed or can I just lay on sand and cement mix?
I'm also a bit confused about the cement bit? Is it laid wet so it dries quickly and holds the flags in place, or laid dry so the slabs sit in place and it hardens when it gets wet with rainwater?
I'll be using old slabs, bricks and other concrete smashed up as fine as possible for the sub base. Can the screed go straight onto this or will it just drop through the gaps? Should I mix soil.sand in with the hardcore? Or just put down some sort of aggregate later between the hardcore and thr screed?
As for the compactor I will bow to your greater knowledge and hire one, but it will be just one weekend only. Can I hire it for a day and compact from the top, or do I need to do it a layer at a time? The former would be better, as I'm trying to do this a bit at a time as a DIY project.
Once last thing - weed membrane - where does this go? Just under the sand/cement layer or on top of it? I dont want weeds finding their way up through the grout lines as they do now. What about my existing patio and question 1 above - if I kept that could I lay slabs onto a weed membrane?
Thanks again all for your advice and continued patience.
Posted: Sun Apr 20, 2008 7:18 pm
by seanandruby
Get someone in to do it if i were you. you sound clueless. 
Posted: Sun Apr 20, 2008 8:36 pm
by newbuyer17
Inexperienced, Yes.
But isn't that the whole point of a forum? I've stumbled across and heard lots of contradictory advice, and I'm keen to do this as well as possible. I could go ahead and do what I think is best, but thought I'd consult people who have more experience.
Is anyone else willing to give me any more constructive advice on this one?
Posted: Mon Apr 21, 2008 11:20 am
by Rich H
The single most important thing is the sub-base. Yes, you can hire a Wacker vibrating plate compactor (or equivalent make) for a day or for a weekend. You need to lay no more than 100mm of your well broken up hardcore at a time. Don't leave significant voids. Run the wacker over it at least eight times and then repeat the process until you've reached the laying height. Obviously the wall needs to be built and sound before any of this. If the wall is built well enough it will stand the above process just fine.
If you do this then there will be no voids for the screed to fall through. Make the mix as Tony describes - the bedding course doesn't need to be wackered if you're using a mix, just well tamped before the final screed and the slabs consolidated with a mallet.
Don't make the mistake of thinking a load of bricks and old slabs piled on top of one another and walloped a few times with a lump hammer constitutes a sub-base. It doesn't.
Posted: Wed Apr 23, 2008 8:29 am
by newbuyer17
OK, thanks. I'll do the hiring.
My main concern now is what to do with the existing half. I'm fine with the new half, but this old half has stoof the test of time and is fairly level.
Do I leave it as is, lift the slabs and put new ones down. Or am I digging the whole lot out and doing it 'properly'? As I said it has been like that for years so is there any need to mess with it?
Or is there a halfway house that would be better - lifting the top 6 inches off only?
Thanks for everyone's help.
Posted: Wed Apr 23, 2008 8:45 am
by Rich H
If it's sound then you don't need to. However, from your photographs the slabs look ancient and the joints have a lot of weeds. I'd take out the slabs and the bedding course and lay the new, enlarged area as one.
Posted: Wed Apr 23, 2008 9:30 am
by matt h
agree with rich..the end result will look much better and there is no point in half doin a job. Stop trying to cut corners.. you will regret it in the long term otherwise.You need to compact in layers. There is no need to cement between the stones on the wall. READ MAIN SITE. It explains layering etc.
Posted: Wed Apr 23, 2008 12:39 pm
by newbuyer17
Thanks,
But again I'm a bit confused??
According to the main pages the bedding layer/course is just the final layer of sand/screed right? So Rich suggests just lifting this thin layer and compacting a new one on top of the existing subbase, alongside the new half?
Is that correct, or are you suggesting lifting the whole existing one up all the way to the ground i.e. about 2ft and treating the whole area as one? - which seems to be what Matt is saying??
I understand what you are saying about cutting corners, but equally I dont want to do work that isn't necessery. Especially as it wont have vehicles on it.
I know it is mainly down to my not understanding the termanology, but I'm getting confused by contradictiory statements.
Apologies again for my continued confusion - just trying to do a good job without going over the top, and never done any of this before.
