I had an email from a friend of mine who is in a quandary and has asked for some help.
Paul has laid a patio in his back garden outside a shed/chalet where he has a little workshop. Half of the building forms the shed and the chalet part is a sort of outside kitchen come family room for the summer where they eat regularly when the weather is nice.
Until now the lawn used to go right up to a wooden deck which is integral to the chalet.
He dug out an area that he wanted to pave to a depth of 75mm and in-filled the base area with course sharp sand. The sand was consolidated and graded off level.
The slabs were then laid directly onto the sand and pointed in with a 4:1 sand cement semi-dry-mix mortar but just three weeks later he noticed movement in the slabs with considerable movement when he walked on them.
Paul tells me that he was given advice on what to do at the local builders merchants. Advice I consider to be incorrect.
The slabs Paul used were 37mm thick and 450mm x 450mm square. It is my view that these are far too thin for bedding down directly onto sand, especially on a bed of 38mm. I have always been an advocate of using 5:1 sharp sand and OPC (Ordinary Portland Cement) mixed to a fairly wet consistency so it runs quite freely from the mixer. Do not worry that it is wet because once the mortar comes into contact with the paving it will very quickly firm up as the water is 'pulled' from the mixture onto the stone.
It is quite acceptable for pedestrian use only to lay the slabs directly onto the mortar which in turn is directly on the soil providing the soil is firm, but make sure you have a depth of mortar of at least 50mm. However, it is always advisable to provide a consolidated sub-base of type 2 limestone scalpings so that the chances of movement are diminished.
I like to level out the mortar with an ordinary trowel and then fluff it up using the point. When you present the slab, the air is pushed out and the water is absorbed into the slab. You should not have to tap hard, just enough for it to locate your pre-set levels.
If the traffic is to be heavier with say a garden tractor or perhaps a car then it is essential to use 75-100mm of limestone scalpings compacted using a vibrating plate or roller and then at least 50mm of mortar to bed the slabs onto - assess your base depending on the type and regularity of any traffic.
Use a rubber mallet to tap down the paving; it is easy to damage a club hammer handle if you use it constantly. After 24 hours the mortar mix should be as hard as nails and the joints can be pointed in. Be careful as the cement is still green. If you are able to leave for a week then even better.
As far as just sand bases are concerned, it is best to do this only if you have a heavy flag stone like York paving. Heavy stones and interlocking paviors will hold in place quiet easily on a consolidated as sand base.There are also a number of ways of using a sub base support system that acts as a profile which is set in place beneath your paving stones. It is very interesting way to fix your slabs but you must ensure the metal bars are well fixed and cannot move otherwise the problems described above will occur again.
You must also make sure that the thickness of the paving is consistent. Some manufacturers, especially the cheap 99p type of slab will often vary so be careful.
Edited 9th May 2008