I would like some advice regarding the pros and cons of laying a sandstone patio over concrete foundation.
I have designed a terraced garden for a client in London. There are two large terraces; upper terrace approx 30m sq and lower terrace approx 14m sq. The terrace walls will be rendered block work.
As this is part of a large refurbishment with extension of the existing house the builder is going to build the block walls and create the terrace foundations for the patio from breaking up the builders waste ie old bricks, plaster etc and then laying a concrete skim over the top. I do not know the depth of the concrete skim at present.
I specified in the drawings that whatever sub foundation was used then l would put a 100mm MOT Type 1 on top then build from there. It seems that this is may be ignored and it seems that the builder is suggesting the concrete is being used as the patio foundation. There will be approximately a minimum of 4 months between the laying of concrete and the laying of the patio.
What are the implications of laying over concrete? Would there be any long term stability issues with this method? Would it be beneficial to add MOT on top of the concrete maybe to a lesser thickness than 100mm to aid stability?
Any thoughts please welcome.
Craig
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A good concrete base is the best base for sandstone however his idea is probably to help get rid of any rubble he has left over and i think it is a dodgy idea from your point of view.
You need at least 100mm of concrete or it might break up and probably an expansion joint in the 30sqm one.
Whilst a thoroughly compacted sub base of hardcore brick or concrete rubble only (no plaster!!) under concrete is ok as long as it is broken up into small pieces it is only being done probably to suit him and the real question is can you trust him to do it right? I have worked on a lot of sites and the stuff that gets buried is criminal, rotting timber lucozade bottles old radiators etc.
I'm sure you and plenty of people on here have worked on new builds where you have to relay sunken 'patios' laid on this type of rubbish!!
You have lessened some fears regarding laying directly on concrete and reinforced many more... the ultimate issue it is outside my control which can lead to issues. I will give the client some additions to the specification provide and a list of concerns based on your input which reinforces concerns l have already mentioned to the client regarding the importance of a proper base and shortcuts by builders.
The client does seem to have good quality builder with reasonable landscaping experience; l could be being over paranoid but ultimately the division of responsibilities could lead to issues that l don't want my client to suffer from.
Thanks for the advice and your time...
Have a word with the builder, A good builder will be able to put this base in to a top specification & level to suit without having to bat an eyelid
Thanks Lawn Lover,
I have discussed further with the client; supplied some recommendations, construction diagrams and specifications. She will be discussing further with the builder on Monday... l do agree a good builder will be able to produce a top specification level foundation... unfortunately the trade does have a poor reputation rightly or wrongly! The builder does seem to construct high quality houses and extensions but better for us to reach agreement now than have issues later.
Thanks again
I would recommend a meeting with all 3 parties present to ascertain what needs to be done by whom & emphasise the importance of the base for a patio. Back this up with a written specification, take a photo of the condition of the base before doing any work, & most importantly, if the base looks inadequate do not start work. Once you adopt the work, your professional reputation is on the line. That said, most builders are good if they know what is exected of them, you (meaning us) have just to be strong enough not to be pushed into a situation you know may cause problems.
My main concern would be:- Given that the builder has probably finished most of his work, why is he going to the trouble of concreting the area? Maybe hardcore and wacker but cant see many builders would go to the trouble of concreting if the intention is to lay a patio?? Think I would prefer it to be left this way then an investigation of sub base is far easier.
I agree with Steve, above. Certainly wouldn't agree to any guarantees if not laying the base yourself. If there is movement the builder is unlikely to accept responsibility, regardless of how nice a chap he is.