Laying purstone greyfell - Questions about full mortar bed and sand

[andyken]

I hope someone can assist me with a couple of questions about the laying of a patio using Stoneflair Greyfell natural stone.

The work is being done by a largely competent general builder as part of a much larger project. I have two specific questions. It is supposed to be laid on a full mortar bed, but the builder only uses mortar around the edges. He does use it very generously, so for a large slab it is five or six inches wide, and for a small slab there is just a small area in the middle that dos not have any mortar beneath it, but this is presumably not a full mortar bed. What are the possible consequences of this approach? The base below the mortar is a few inches of well compacted MOT hardcore.

Secondly, I notice that he has used sharp sand for the mortar in one part of the patio. The rest is made with soft sand. Essenitally he was using up sharp sand that was ordered before I requested the patio be laid on a full mortar bed. Again, I wonder what the conseqence is of using this mix.

Thank you for any help you can give.
Andy

[Tony McC]

I know the method you describe and it's popular with some patio layers. It's less prone to problems than the notorious spot bedding, but, as a void remains beneath the flag, it does not meet the requirement of BS 7533:4 which states that flags should be laid on a bed providing "uniform support".

It's difficult to condemn it for smaller flags, where it might leave a void of only 50-75mm diameter, but for the larger flags, it is, in my opinion, unacceptable and I would not 'pass' such workmanship on any job I supervised.

As the paving is being laid on a wet mix mortar, whether that mortar is based on a building sand or a sharp sand is relatively immaterial; it's all cement-bound, so drainage of the bedding layer is not affected. I'd be far more concerned about voids beneath the larger flags.

[andyken]

Thank you very much for your reply and (as has been said by so many others) for putting together such an excellent web site.

Your comments confirmed my fears and I have stopped the contractor working on this job. Several of the slabs rock and they are not laid evenly, even after taking account of he riven surface. I asked for a full mortar bed BEFORE he started and asked again twice when I saw what was happening but he has been deaf to my requests.

So here are a few more questions: I now have a partly completed patio, no slabs have been cut and no pointing has been done.

Should I get the existing laid slabs taken up immediately or is it ok to leave them until I can get a professional paving contractor, to asses the job and take them up?

Is it likely that the laid slabs can be taken up and relaid? I realise that there may be some wastage/damaged slabs but I am hoping that they are not all written off.

Will the hardcore sub base need to be relaid?

Thanks again.

[Tony McC]

You could take up the already-laid flags, as it will be easiern if they've only just ben laid than it would be giving them a couple of weeks to really bond, but then, it's unlikely to save you a great deal of money. If you do take them up, clean off as much of the mortar as poss, and then clear away the bedding that's left behind, so that the new contractor has a 'clean canvas' on which to work.

You should be able to salvage 90% or more of the flags laid. It's worth holding on to any cut pieces as they might be re-usable.

The sub-base should be OK - let the new contractor decided whether it's fit for purpose or not.