Screeding Rails - Block Paving

1 Jun 2005
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United Kingdom
Hi, I'm planning on block paving my drive and front lawn. I'm going to get lads in with a digger to do all the scraping off and then I will put the crush down myself and level it etc.

When it comes to screeding the sand, I have read the excellent site but one bit confuses me.

I am going to use 2m long pipe conduit as the screeding rails. How do you set them in the sand to the correct level and wack the sand without disturbing the rails ? You can't knock them in when the sand has been compacted so I assume you put them in on semi compacted sand and top it off with loose sand to level the rails to the screeding height. Then presumable only wack the sand in between the rails. But wouldn't this leave the sand under the rails, once removed, a weak point causing the bricks there to sag ?

I know you can cut a notch in a timber screeding board to support one end on a paver but the drive is wide so the other end of the board will have to run on screeding rails.

Any advice is welcome on how best to setup the rails so they are at the right height and don't move when screeding.

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You can use a rail that is the desired thickness of sand for example i have some 32mm steel pipe for this and you can lay them direct on the sub base.

The other more common method is to spread a thin layer of sand roughly in a line where you want to place your rails, say 15mm of sand and compact it. Then lay your 20mm conduit down on top and settle it into the sand with a wiggle here and a tap there. This will give you a depth of sand of approx 30mm once you wiggle it into the sand by a few mm. Check it is laying flat with a straight edge and check there are not floating areas where it is not fully supported by the sand and then carry on. If you leave something weak like 20mm electrical conduit unsupported the vibrating plate may flick out or at least move the rail which will cause all hell with your levels.

If your conduit is not very straight then you can use a length of straight timber to create a nice level spot and then place your conduit in this. Make sure if you have a slight bend in your conduit that you compensate and put the bow up in the middle. The weight of you leaning on it while screeding will take the bounce out. If you put the bow down then you risk creating a dish
You don't compact the screeded sand rather you throw an amount of sand down and compact it into the sub base. You then set the rails down and add more sand to do the finer screed work. Ideally the rails should not be in contact with the stone as this will limit your height flexibility.
Once your rails are set, you add more sand between them and drag it off. Slide the rails down, fill the gap that has been left by the rails and carry on.
P.S. We use iron gas pipe as rails. You need something strong but not too thick.
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We used the same site as you for info but we just used some old plastic conduit we had lying around, packed a bit of sand maybe 40mm round it on top of the existing whacked 100mm+ hard core and leveled it off with an old floor board. Then we pulled out the bars carefully, dropped the blocks on top, then whackered it afterwards.
Seemed to work as so far it hasn't dropped.
I was thinking of using these metal conduits from Screwfix "DETA CLASS 4 CONDUIT TUBE GALVANISED 20MM X 3M" hopefully they will be strong & straight enough. But then this week I saw a paving firm using thin flat plates as rails. He carefully placed a block under a string which was at finished level, scraping away any of the compacted sand that was proud and moved along under the string brick by brick. The sand was now flat under it then he placed the flat bars under the string and used them as his screed rails, lifted them off when screeded and no holes to fill in and tamp down. Pretty sure he wacked all the grit sand down before it too as he was knelt on it.
To put it another way there's not much point whacking the sand as it's such a thin layer and you'll need a bit of give in it when you whack the blocks to the same level. Since it's going to cause such a pain for the rails, it seems better not to. As far as i know, the sand layer is only to give it a bit of flexibility not to provide any strength.
I have only block paved once - for a rear patio and not a drive. I whackered down MOT and then a thin 5-10mm sand blinding to roughly level out. I then placed my screed rails (22mm copper pipe) onto a narrow line of sand and set to my correct levels making sure they were straight and true. Filled in the middle with sand and walked on it to compact somewhat. Checked my screed rails again and screeded off excess sand. Layed the blocks so they were about 5mm high then whackered them down after. Final couple of passes with kiln dry sand and Bobs your father's brother...
Just looking at Wacker / Compactor plates, wondering what size to get as they come in different sizes, a small wacker will take forever up & down the crush, a heavy one is perfect for the crush but will it be too much for running over the finished blocks ?
I just hired a small one and did 2 layers of 50-60mm each. It takes longer as the plate is smaller and you have to do everything twice, but it was my own time and hiring was cheap.
The one I used has a raised area on the base to increase the compaction on the final run over. This lets it confirm to the standards for council work or something. No idea the exact inns and outs of the standards but it did an excellent job on both the hard core and blocks. The blocks only needed the briefest of runs over though.
John - thanks for that I'm tempted to buy a used wacker then I can take my time as I have 66 m2 to do on my own otherwise the rental could add up to a bit.

As an aside, does anyone know if these are Red pavers or Brindle, I would have said Brindle but they look too red to me
Does anyone use the rubber pad on the wacker when doing blocks?
I didn't, but they recommend it. The problem is the scrapes don't look good on day one, but they soon wear off, so for diy it's fine
You do small areas at a time and leave a generous layer of kiln dried sand on the blocks as you compact. This way the plate doesn't leave scratches really. Then brush it over to your next areas and continue.
My block paved patio had those fine scratches after wacking with no pad. They soon wore off but if I do a drive or patio in my new house I'd use a pad.

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