Can I lay slabs directly onto sand?

[GAZ2785]

Hi there, I don't post here often but the advice dished out is usually excellent.

At the moment I have an old patio that wasn't laid very well - I suspect the ground wasn't compacted properly and there probably wasn't enough sand laid down.

Anyway, I am disposing of the old slabs and will be hiring a wacker plate to compact the ground. My question is, can I then lay the slabs on a bed of new sharp sand?

I read online a lot about people putting down hard-core and using mortar, etc. However, the patio won't have vehicles on and only very light foot traffic. Also, dare I say it's not nesseccarily got to last a lifetime!

Do I *really* need to go to all the extra lengths of using hardcore, membranes and mortar or would just sharp sand be ok? My concern is not only the extra cost but also the huge task of having to dig out additional ground to make way for the hard-core. The slabs in question will be either 450mm x 450mm or 600mm x 600mm (not decided yet).

Thanks for your help.

 

[r896neo]

It is very little extra cost but a lot of extra work. You don't need membranes.

The existings slabs did not move because the soil was not compacted, Compacting soil will have very little effect, a proper compacted sub-base is put in to distribute the load evenly. If it was just about compaction why would you ever need a sub-base, just use a 3 ton vibrating roller on the soil.

You obviously know how it should be done and the risk of doing it without sub-base so what kind of reply do you expect to get?

 

[GAZ2785]

Thanks for your reply. I have decided to enlist the help of a helper and am going to put down hardcore as a base, followed by sand and mortar. Seems like the best way to go.

Can I ask how deep the hardcore should be, and how deep the sand should be? Also with the hardcore, is there anything I should ask for specifically - i.e. millwaste?

Thanks,

 

[r896neo]

MOT type 1- tell them what its for and they should send you the right thing.

check out paving expert . com for the details its a very good site.

 

[foxwoodlad]

Hi there, I don't post here often but the advice dished out is usually excellent.

At the moment I have an old patio that wasn't laid very well - I suspect the ground wasn't compacted properly and there probably wasn't enough sand laid down.

Anyway, I am disposing of the old slabs and will be hiring a wacker plate to compact the ground. My question is, can I then lay the slabs on a bed of new sharp sand?

I read online a lot about people putting down hard-core and using mortar, etc. However, the patio won't have vehicles on and only very light foot traffic. Also, dare I say it's not nesseccarily got to last a lifetime!

Do I *really* need to go to all the extra lengths of using hardcore, membranes and mortar or would just sharp sand be ok? My concern is not only the extra cost but also the huge task of having to dig out additional ground to make way for the hard-core. The slabs in question will be either 450mm x 450mm or 600mm x 600mm (not decided yet).
save old flags ,

excavate patio get rid of topsoil , run wacker over excavated area. lay old flags in hole (no bed material) smash flags up so not rocking. blind old flags with hardcore roughly 4inches. compact with wacker plate . lay new flags on 25-40 mmm bed of semi dry to wet mortar, leave a 10mm joint between flags . when finished point with yellow building sand / cement mortar and busket handle jointer. make sure you allow water to run off paving and into garden etc. allow a fall of about 20 mm per linear metre
Thanks for your help.

 

[GAZ2785]

Thank you for the replies. The old slabs have now been disposed of and the top soil dug out.

I've had 2 tones of MOT type 1 delivered and I must admit it hasn't gone far - think I am going to need another 2 tones, possibly 4 tones.

However, there has been a bit of a disaster. I used a wacker plate on the ground before putting down the MOT, and there was a springy patch in the middle that just wouldn't go away. In the end we dug a hole to find out what was going on, and found a plastic bag buried under the ground.

It turns out it was a MASSIVE plastic bag - and not laid flat either, but all screwed up. We had to dig a hole about 5 feet wide in the end to get it out. My concern now is that we put all of the soil back (much of it was clay) along with some smashed up bricks because the hole was quite deep. We've wackered the area several times but if you jump up and down on it your heels still sink several inches into the ground.

It would seem the 2 options are:

1) Keep pounding it with a sledge hammer, feet and wacker plate all day long until it feels solid

or

2) Re-dig out what we removed and fill it with more MOT. If we remove what we originally dug out, I suspect it's going to swallow at least a ton if not 2 tones of MOT so this would be a more expensive option.

What do you guys think?

Quite why the idiot who lived here before decided to dig such a big hole and bury such a big piece of plastic under the patio is beyond me.

Thanks,

 

[r896neo]

The proper way to deal with it would be to fill it with stone or general rubble, plate it like hell and then top it off with a 4 inch layer of lean concrete.

Unless you have a 1 ton vibrating roller it will not compact a deep fill like that, when compacting you need to do it in layers, for a normal sized vibrating plate 2-3 inches at a time max.

Are you certain it wasn;t some sort of membrane around a soak-away? how far is it from the building

 

[GAZ2785]

Thanks for your reply.

We did fill it gradually and use the wacker plate every few inches. It's already taken 4 wheelbarrows of millwaste which will go to waste if I dig it all out.

I don't think it was a membrane - it was clear rather than opaque and kind of screwed up in the centre of what would have been the old patio. There is a drain pipe that goes into the ground from the guttering - the bag was maybe 3 meters from that, so not right next to the house. It really did seem like it was dumped there in a random fashion rather than laid out properly.

 

[GAZ2785]

Hello, it's me again with another question

Half of the sub-base is down, but not yet compacted. The remainder of the patio is now wet through from heavy rain. It's sunny today, but forecast rain for the rest of the week.

The chap helping me is keen to scoop off a soggy layer and put down a few more tonnes of MOT to help firm the whole thing up and provide drainage. I should add that the half which already has the uncompacted MOT down feels pretty solid, it's the other half that is wet as you can imagine.

On the other hand I don't want to keep buying tonnes of MOT just for it to sink into the ground!

What's your opinion on continuing with this project in such wet conditions? Is it asking for trouble or does it not matter as long as enough MOT is put down and it's compacted enough? I'm not really sure what to do for the best. The temptation is to leave it all to dry out but as I mention it's forecast rain all week and could be all next week too, so it might never dry out!

Thanks,