The Brew Cabin

I have an area prepared for block paving & now i have put down the edging course, i will have some areas that will require 3" or 4" of bedding sand rather than the usual 2". Is it ok to lay blocks on this thickness of sand?? Is there any limit to the thickness of bedding sand.

Turbonutter

You must not lay blocks on sand that's more than 50mm deep. It will shift and settle and the blocks will go with it.

You should regulate your sub-base layer by adding additional granular material to it and then compacting to achieve a surface that will accommodate a bedding layer of uniform thickness, and not more than 50mm.

if u had the choice/preference of laying blocks onto a sub base of 150mm of mot and 50 mm sand or 175mm of mot and 25mm of sand which would u prefer to go with???

175/25, no doubt.

After almost 25 years of block laying, I've seen what happens when too generous a bed is used. Back in the late 70s/early 80s, it wasn't uncommon for blocks to be laid on sand that could be up to 100mm thick, over sub-bases that undulated like the Peak District, mainly because gangs being assigned to block laying duties by Local Authorities had no real experience and were guessing at a best approach. These pavements tended to develop problems pretty quickly, and gradually, as the laying standards were developed, gangs realised that the key to succes was to have a well prepared sub-base, that was nearly pefect in profile and levels, and a laying course of uniform thickness, somewhere around 30-50mm, but generally 30-40mm.

We knew that we could achieve a high degree of compaction with the sub-base, and so it was the sand that was the 'weak point' in the construction. By keeping the sand to a minimum - just enough to provide a cushioned bed for the blocks, but not so much that it could suffer from differential settlement, we now construct pavements that exhibit little, if any, settlement over their design life.

Actually, given the choice, I'd use a slightly more generous laying course, as I reckon 25mm is probably just too little to cope with variation in the sub-base and provide cushioning for the blocks, so I'd opt for 165/35, if I could! :)