Screeding for block paving

[23vc]

Yet another block paving question I’m afraid.
A few years ago I put type 1 down on my drive and compacted it, but left it as-is. I’m now looking at block paving it and getting my edgings and levels in place, but the natural contour of the type1 is a bit “dished” ie the sides have a slight bank upwards. This hasn’t really struck me as being a problem, as it’s the natural lie of the ground, but I’m now wondering if this is a bad idea and I should try and build things up to make a completely flat plain? I guess screeding is easier if I did it that way, but presumably still possible with a bit of a contour? Either way, I think the footpath at the boundary will still follow a bit of a dipped line.

 

[noseall]

Yet another block paving question I’m afraid.
A few years ago I put type 1 down on my drive and compacted it, but left it as-is. I’m now looking at block paving it and getting my edgings and levels in place, but the natural contour of the type1 is a bit “dished” ie the sides have a slight bank upwards. This hasn’t really struck me as being a problem, as it’s the natural lie of the ground, but I’m now wondering if this is a bad idea and I should try and build things up to make a completely flat plain? I guess screeding is easier if I did it that way, but presumably still possible with a bit of a contour? Either way, I think the footpath at the boundary will still follow a bit of a dipped line.

Our screed rails are lengthy and straight. They naturally iron out any humps or hollows, when screeding off the sand. It makes sense to fill out any humps or hollows with hardcore, simply to cut down on pockets of deep sand.

A lot of the guys are using finely crushed limestone now, instead of screed. I'm guessing it will crust a lot harder than screed sand.

 

[catlad]

As per nose. A lot of people have moved over to grano round here which looks like crushed slate.

 

[23vc]

Thanks. I guess what I’m trying to ask is, currently the compacted MOT follows the natural camber of the previous drive, but am I better off filling the middle out with more MOT to make it flat as per the blue line? I’m kinda thinking either would work, but if I fill it out it’ll make for much easier screeding?

I’ll look into the grano/limestone dust thing too

 

[cdbe]

It's been a while since I did mine (at the time I studied www.pavingexpert.com and so became an "expert").

You haven't said what the deviation is, but if I recall the standard laying bed is 50mm +- only a small deviation because it sinks when you whack the blocks down whereas the MOT doesn't and you need it to sink evenly.

If I'm not being clear - assuming it compresses by 15%, 50mm will sink 7.5mm and 100mm will sink 15mm.

I aimed for +- 10mm on my MOT - this made the screeding and laying pretty easy despite making do with lengths of timber decking instead the professional kit of rails etc.

To make matters worse, with a dip in the middle, unless your subbase is top notch the weight of your vehicles may make the middle sink even more.

 

[23vc]

Yeah, all good points, and I think I’ve still worded my original question badly! Basically, if you assume the bedding layer is a consistent 40mm or whatever, am I better with my finished job following the black line (cambered) or the blue line (totally flat)? I’m pretty sure the blue line is the way to go. It’ll involve filling up to 100mm around the middle point with MOT, but that’s fine.

 

[Charlie George]

Set your levels all out with string lines, you may find you need to reduce the MOT in some areas, so do that first before ordering more MOT,
add your block height and bedding layer ie 100mm to top of line.

 

[^woody^]

Flat, with a suitable fall for drainage. Don't worry about any minor variation in the bedding sand, particularly at the un-trafficked edges.

Never have dips. Chambers are OK but not the opposite.

 

[23vc]

Flat, with a suitable fall for drainage. Don't worry about any minor variation in the bedding sand, particularly at the un-trafficked edges.

Never have dips. Chambers are OK but not the opposite.

Thanks. Do you mean never have it shaped like my black line, but you could have a ramp/incline if needed?

 

[23vc]

Thanks all. I’ve got the string line out and it’s only really significant (100mm dip) in one place - the rest is within 20-30mm of the line. So I’ll just adjust the MOT as needed when I get to that point. But I’ll set all my kerbs to a line and not attempt to have them follow any dip/contour.

 

[^woody^]

Yes, not like your black line

 

[Cheslea2010]

You need 50mm of bedding layer which historically was Sharp Sand but now people are using 2-6mm Clean Stone for SUDS compliance

 

[robinbanks]

Yea just bear in mind if using 6mm bedding aggregate that when you brush your kiln dried sand between the blocks to lock them in place, the first rain will likely see the kiln sand wash between the bedding and disappear from the gaps. I used a bit of lime mixed in as a binder for mine in the end

 

[Cheslea2010]

We don't use KD sand anymore for brushing in. It's 1-4mm Joint Filling Aggregate which is a crushed granite and is SUDS compliant.

 

[23vc]

Thanks - mines draining to a soakaway I’ve dug, does that mean I’m ok with sharp sand? Any pros and cons of each? Was planning on doing sharp sand but always open to options