25mm crusher run - Sub base 25mm crusher run

[alexrobinson]

Thank you for the fantastic website.

I have laid 100mm sub base for my patio (riven indian sandstone paving slabs). I laid it last weekend, good dry weather on top of almost waterproof clay. Hired a wacker plate and wacked it all down and it was super tough, like a concrete base! so I was really happy, It is now Thursday and we have just had a day of heavy rain and my nicely compacted crusher run has gone all mushy, I sink into it, it's like it has the consistency of heavy slump concrete. Will I have to pull all this 25mm crusher run up and put in 50mm crusher run to make it more stable? Or do you think it has gone mushy just because it is the first rain on it and when it dries out it will set harder?? Worried if I do have to pull it all up because I have 5 tonnes of it in and thought I was about ready to start laying the flags??

Hope you can offer some of your expert advice. Thanks very much, Alex Robinson

[Tony McC]

Why has it gone mushy? Has it been laid on a clay low spot with little or no chance of drainage? A sub-base needs to be able to drain for it to perform properly.

And why have you used a sub-base for a flagstone patio? There's no need to do so, unless build-up levels require it.

[alexrobinson]

Thanks for the reply Tony, I must be confused, I thought I read on this website that I needed 100mm of sub-base for a patio. No I didn't need to build it up, I dug out the 100mm of soil to allow for the sub-base. Yes underneath the Sub-base is almost waterproof clay, but I have sloped the land to drain off into a bottle gully linked to the main rainwater drain, so I was really surprised when it became so waterlogged and sloppy. Turns out the crusher run came from a local quarry and it is renowned for having loads of "dust" and not many stones, I asked a local builder who has had the same trouble with Crusher Run from this quarry. He said it does bind together when it dries out and each time it dries get harder. He was right it did harden, but to be sure and for peace of mind I ordered some 50mm crusher run and some good quality 25mm crusher run. This weekend I dug out the old crusher run and put in the new 50mm wackered it down and topped it off with good quality 25mm crusher run and wackered it down. Nothing like re-doing a job this weekend that I thought was adequate last weekend.

Anyway the problems seems to be from the poor quality crusher run from our local quarry. I hadn't realised that I didn't need a sub-base, but surely it must be better now with a good quality 100mm sub-base.

I will re-read your website because i don't know where I got the idea that I need 100mm sub-base?? I will give it a go laying the Indian Fossil Mint flags, but it I find it is out of my range of my abilities will have to call in an expert, but at least the groundwork and preparation is taken care of.

Cheers for the excellent website, Alex Robinson.

[Tony McC]

There's an FAQ explaining the reasons why a sub-base isn't always necessary for patios, depending on type of paving, and why it is that they only need be included with flagged patios when levels need to be built-up.