How to lay patio slabs on top of block and beam?

[mashie]

Hi,
what sort of screed do I need to be safe from bounce wrecking my limestone flags, laid on block and beam 3 feet above garden level

I have a block and beam patio about 2m x 6m with a 4m radius quarter-circle on the end.
Most of the beams are 2.3m or shorter.

I plan to lay limestone flags with a narrow gap (5-10mm). The flags are neatly sawn (3mm tolerance on 30cm).

I've had conflicting advice - do I need a 3cm screed to prevent bounce?
I believe I need some sort of screed to enable the falls
Also, what jointing compound?

I'm changing builders half-way through the project - surveyor doesn't seem to know about block and beam.

thanks! Mashie

 

[lonner]

They need to be on mortar, not a screed.
If you have bounce in the block and beam, I'm not sure a sand cement mix will stop it.

Geofix for joints or maybe fine grit joint

 

[mashie]

thanks -- it doesn't bounce much but the original builder seemed to think we had to put a screed on it. We planned on more a grout than mortar between the slabs. It's Stonemarket pyramis gold limestone slabs - they keep changing their recommendations on how to lay it, which doesn't help!

 

[freddiemercurystwin]

Had you considered laying them on pedestals instead, no worries ever about cracking then. And b/b floors often have a little bounce, it's perfectly normal unless the floor is ooverspecified to start with.

 

[mashie]

I'm looking pedestals up --- v interesting, I like it ... given there is a 3 foot high void / soil beneath the patio, where do we send the water? my brain is going all heath robinson.... (suspended under patio drainage)...I am already worried that I cannot see what happens in the void (half full of soil) under that patio.

 

[Roystone]

I'm looking pedestals up --- v interesting, I like it ... given there is a 3 foot high void / soil beneath the patio, where do we send the water? my brain is going all heath robinson.... (suspended under patio drainage)...I am already worried that I cannot see what happens in the void (half full of soil) under that patio.

@mashie did you sort this. What did you go with, I have the same decision to make.

 

[mashie]

Sorry, I thought I had posted about this. Yes, SORTED! and very happy with the result. Together with my husband laid 90 sq m of large limestone slabs on Wallbarn pedestals ranging in height from 5mm to 44mm. We laid drainage slopes first with concrete, using some fibres for strength in the thin slabs (a lovely chap gave us a sample as you cannot buy the right type of fibre in small quantities). We used a 1 in 100 drop by forming a patchwork of drainage slopes in different directions. Because of the pedestals this didn't matter as we still achieved a flat top surface above the drainage. 1 in 100 is a bit shallow (the longest drainage channel was in fact 1 in 140) but we figured there was no danger of slipping on pooled water on the surface, as it would be below the slabs and could take its time to drain away. Part of the patio had to be laid on 7mm rubber pedestals which are not height adjustable, because of our very small height allowance. So about 3m x 6m is "conformal" ie the slabs in fact slope at 1 in 100. We don't notice this at all. The people at Wallbarn were really helpful and gave a lot of advice - plus their price was better than the resellers. Total cost was not bad as we didn't need any grout or mortar (or professionals, although we hired a labourer for 3 days, to show us how to lay concrete and to help moving the piles of slabs before we started). We used straps to move the actual slabs (14- 28 kg) which worked fine. I don't think we'd have been skilled enough to position slabs on mortar as well as we could on the pedestals. We did a lot of height adjustment using 1mm shims - you can put shims on any corner of the pedestal. A 180cm spirit level was hugely useful in getting the top level. The gap between slabs is very consistent and 2mm which looks very good. To transition to steps (which were laid on mortar) we used a suitable glue for one side of the last patio slab and left one side on the pedestals. This is a bit of a bodge but Wallbarn couldn't think of anything better (if one edge of a slab is fixed and the other can move, obviously you can have problems with the mortar breaking).

I don't think the "professionals" could have done such a good job. We had a 7m curved edge which looks fantastic because it is fully level, and it looks neat that the level along the house walls is consistent. The money saved on "professionals" went on some good tools, really pleased with the lack of dust when cutting the stones using a shroud + festool cutter. Happy to provide more details if you need any info.

 

[Andrew Sharratt]

@mashie Sorry to resurect an old post but we have exactly the same problem. Do you happen to have a pic of the drainage slopes? I cant quite picture how you did those. Thanks

 

[mashie]

It's been a while, but here is the plan we used. The fibres we used were from Adfil

 

[Unclesam12399]

Omg this is an amazing idea, using block and beam and the pedestals!! Thanks for the details… I got a bit lost about the drainage? Can the block and beam itself be laid with a 1/80 fall? Also could a dpm plastic sheet be laid on the b&b so it doesnt get wet or does it not really matter.

 

[Unclesam12399]

It's been a while, but here is the plan we used. The fibres we used were from Adfil

See above, thanks

 

[mashie]

Hi @Unclesam12399 - not sure you could lay direct on block and beam (I am completely untrained btw) -- I think the challenge would be setting up the beams - they are designed to be square to each other I think. If you allow enough space for screeding (which our builder didn't do) then that's pretty easy to do on top of the block and beam. You just make one end of the screed higher than the other, so there is a drainage slope. In terms of the drainage I'm sure angled block and beam would work but you'd have a lot of joins that weren't properly square... I really enjoyed this project. Compared with 'professional' quotes we saved £12,000 and none of the professionals was prepared to use the pedestals. I don't think you need a dpm - it depends a bit what's underneath but essential the block and beam, and the screed, form a "reservoir" that can hold a fair amount of water and will release it over time, if it's breathable. As long as most of the water drains off and is managed properly (drains or proper soakaway) this shouldn't be an issue. I'd be more worried about creating an impermeable membrane which could leave to wet areas where the moisture cannot drain.