Sub-base for sandstone slabs - 1200mm x 600mm slabs on clay soil.

[owensr]

We are intending to lay about 120m2 of Sandstone Slabs that are 1200mm x 600mm on a clay soil to form a patio (pedestrian traffic only).

Had various opinions on sub-base. This site seems to indicate we should have 75mm of compacted MOT and then a 20mm sand/cement bedding.
One contractor thought we should have a 100mm mot then 100mm screed (with steel mesh in) with slabs laid directly on top.

My thinking was to do 50mm MOT (to falls) then 100mm reinforced concrete slab then 40mm sand/cement bedding.

Any opinions?
Soil is clay, but quite wet (was not going to do till May), but is well compacted down, but does dry out in summer and crack.

[mike builder/landscaper]

100ml of type 1 is fine for pedestrian traffic, then you can lay your flags on mortar. the mix should be 5 parts sharp sand, 1 part yellow sand and 1 part cement. If the flags are exactly the same thickness then you could screed the area and lay them on a dry mix of sharp sand and cement mixed 6 to 1.

[Mikey_C]

^^as above

but if you really want to over engineer it go for 150mm of mot and put geotextile under the mot, but there is no need to involve concrete slabs. also for the record the minimum depth of mot than can be laid is 75mm (not the 50mm mentioned) which is covered somewhere on the website

[GB_Groundworks]

Read and re read this and get your contractors to read and reread it infact print it out and insist they do it this way. For non collaborated slabs

http://www.pavingexpert.com/stonpv03.htm

You want 75-100mm dtp type 1 laid over geotextile, not weed control fabric. Well consolidated with a roller or whacker. 3 or 4 passes etc not just a once over then bed the slabs on a moist but not wet(as in not like brick laying creamy mortar) 1 cement, 1 soft sand, 5 or 6 sharp sand/ coarse sand/ etc

Laying them on concrete is only really for driveways then imported sandstone not really recommended for vehicular traffic. I'd want to review your contractors previous work not just photos if they are unsure about how to do it.

Now if the ground is really crap like it's been filled or ex dump or lots of ground water etc then that might justify laying them on a concrete raft.



Edited By GB_Groundworks on 1297889455