Laying next to an existing patio

Dakkon · Post: # 24575Post **Dakkon** Sat Dec 08, 2007 3:00 pm

Hi

I need to extend a patio, however I'm worried that when I dig out the existing patio will sink! The current patio is layed on sand over a heavy clay subgrade material. Its a light to medium use patio so was intending on laying it on 40mm of 8:1 semi wet mix of cement.

My questions are:

Will the existing patio sink and if so is there anything I can to prevent this?

Is the base thick enough? (note that I'd rather not lay an MOT sub base due to cost)

Slabs to be used are 60mm X 600mm x 20mm

Cheers

dig dug dan · Post: # 24577Post **dig dug dan** Sat Dec 08, 2007 3:10 pm

Is the base thick enough? (note that I'd rather not lay an MOT sub base due to cost)

do this at your peril. your new patio will sink if you don't use a sub base, as will the old one given time. a disaster waiting to happen

You don't need mot type one. use crushed concrete, its recycled and cheaper.

matt h · Post: # 24582Post **matt h** Sat Dec 08, 2007 4:26 pm

AGREE WITH DAN NEED GOOD SUBBASE. IT isnt expensive if you only have to do it once !:p

Post: # 24586Post **Tony McC** Sat Dec 08, 2007 4:42 pm

If the existing patio has not sunk to date, what makes you think it will do so when you excavate for the extension? Are you concerned that some of the bedding will wash out or trickle away? If so, then accept that this nearly always happens and the usual "fix" is to let it, and then when you come to lay the new paving, you lift the sunken/dropped paving at the edge of the excavation and re-lay that while you're at it.

Dakkon · Post: # 24590Post **Dakkon** Sat Dec 08, 2007 5:00 pm

Hi Tony thanks for the info that answers my question. Yes I was worried that in the process of digging out some of the sand would fall away.

With reference to the base, is there any way of getting round laying a sub base i.e. by increasing the thickness of the concrete being laid, if so what sort of depth would be suitable? I don't own a wacker plate but do own a cement mixer so it would be a lot cheaper to order an extra ton of sand than it would be to hire a wacker and a ton of mot/hardcore.

matt h · Post: # 24591Post **matt h** Sat Dec 08, 2007 5:04 pm

the idea is that you are laying a permable base to facilitate drainage a concrete base wont allow for that, or have i got the wrong end of the stick?

Stuarty · Post: # 24603Post **Stuarty** Sat Dec 08, 2007 8:43 pm

A days hire of a wacker and a ton of crushed concrete isnt going to break the bank to be fair. It will be more expensive to rectify when it all sinks and you get trips all over the place.

It doesnt matter how much use a patio gets really, if the subgrade is going to settle, it will do it in its own time, extra weight only accelerates the settlement.

J.D · Post: # 24621Post **J.D** Sun Dec 09, 2007 1:25 pm

I agree . Even if you use MOT you should be able to get it less than Â£30 a ton . I pay Â£27 per ton delivered (incl VAT). Most hire companies will hire you a wacker for a few hours and wont cost that much. It pays to do it right first time.

Post: # 24627Post **Tony McC** Sun Dec 09, 2007 4:12 pm

My own opinion is that a sub-base is NOT always necessary with a patio. I accept that it's often a better construction when a sub-base is used, but some patios/pathways just don't warrant all that additional excavation and importing of materials, especially when the sub-grade is firm and stable.

However, if levels need building up; that is, if the flags would need to be laid on more than 50mm of laying course material, then a sub-base is cheaper and easier than building up with additional sand/cement.