Brick patio over existing foundations

[James Carmichael]

Hi,

We have recently installed a brick and timber greenhouse in our garden. The builder dug trench foundations down to the level of grey clay, a little over a metre.

I'd now like to lay a small (4 ft x 12 ft) brick paver area to the front of the greenhouse. The concrete for the trench foundations of the greenhouse extend out in to this space by about 8 inches, so I'm going to end up with a position where some of the bricks are on the existing trench foundations and some are on top of whatever I put in now.

As we are in a clay area and there are quite a few medium sized trees around, I'm a bit concerned that the two different sets of foundations might lead to the patio cracking - or the two parts sailing away from each other over time!

What's the best approach? Am I worrying over nothing and should just lay to normal depth hardcore and sand/cement? Do I have to dig down a metre again to extend the trench footings? Is there some magic way (rebars?) to stitch the two parts together?

Many thanks

James

 

[r896neo]

You could dowel into the side of the concrete and lay a new concrete sub base but this is really unsuitable for block paving as it will not drain.

How high is the concrete relative to the finished paving height?

If it was me and levels allow i would dig out your paving area to the same depth as the top of the concrete and then lay a geotextile membrane over the lot then apply your hardcore sub base. This will at least mitigate the settlement. Assuming the new area is not made up ground/ fill there is little reason it should settle too much if it has a decent 100mm compacted hardcore sub base.

If you really want it to be certain then dowelling into the concrete and pouring a new pad for the paving would be best. If you do this you will need to lay the paving rigid on a sand and cement screed rather than a straightforward sand bedding.

 

[^woody^]

Am I worrying over nothing and should just lay to normal depth hardcore

Yes and yes. It's flexible paving and will accommodate any movement, even though it will be minor in this case. It certainly won't move sideways!

 

[James Carmichael]

Thanks both. To answer r896neo, the concrete footings for the greenhouse actually come up to about ground level, so I'd be laying the pavers straight on top of it with a bed of sand and cement. (This leaves them, by design, proud of ground level. That's what I want.)

So, interpreting, I think I hear "dig down as normal to firm ground, lay compacted hardcore base, (optionally) pour new pad for paving, lay with sand/cement on top". (I'm not a great fan of just sand bedding, as I've always found - probably my lack of skill - that is a magnet for weeds!)

 

[r896neo]

If the concrete is high like that then a new pad of concrete to match it is a better bet. It can just be semi dry lean mix concrete 100mm thick. something like 5:3:1

To lay block paving rigid on mortar you need to prepare the area and spread your semi dry screed, gently compact it by walking all over it and then screed it flat.

Ideally you should lay the paving without walking on the laid pavers or you could sink one slightly into the bedding.

If the area is too large to allow this then prepare the same way and then lay your paving but walk flat footed and walk on boards/planks.