The Brew Cabin

Posted: **Thu Sep 15, 2005 3:21 pm**

Having laid 3/4 of a large patio I am now feeling a little worried as slabs are beginning to lift (after two weeks), particularly on edges.

The stone slabs have been laid on type1 sub base with a bed of mortar of around 40mm. Ratio of mortar mix has been 3 - 1 (builders sand to cement), but noticed something strange when mixing - it was like the water was separating from the mix leaving the mix looking quite sandy.

Anyway, kept remixing to get the right consitency, thought everything was alright. But now edge stones have become loose and this is without any traffic across them. When you lift one of the slabs it looks like the slab has been floating on the mortar rather than bonding. This is despite every slab having a good beating down.

Now I'm worried that the whole patio could work loose.

Do you think this is the mix problem I mentioned? ...and is it likely the whole thing could start lifting?

It's like the building sand is more coarse than it should be.

I've also realised (having read your site) that I should have been pointing straight after - At the moment none of it is pointed, do you think this will help fix the problem?

Thanks in advance for your help!

Posted: **Thu Sep 15, 2005 4:08 pm**

Building sand is the wrong material - you should have used a grit or coarse sand.

Anyhow, it's hard to say what might be the cause of the perceived problem without actually seeing it. Flags don't have to be stuck down, though. As long as they can't move and are fully supported, there's no advantage in them being 'glued' to the laying course material, so, if they are loose when prised up, but not moving when walked upon, I wouldn't worry.

Posted: **Thu Sep 15, 2005 4:55 pm**

Thanks for the comments....

What about the pointing, will this also help with the rigidity of the patio?

Thanks.

Posted: **Sat Sep 17, 2005 3:34 pm**

Slightly, but flags aren't (and shouldn't be) dependent on the jointing for their integrity. They are bed dependent, and so the bedding alone should be sufficient to support them and prevent any rocking. The pointing should be little more than a matter of filling the joints.

Posted: **Sat Sep 24, 2005 6:20 pm**

We find 10mm ballast good.

Posted: **Sun Sep 25, 2005 3:50 pm**

10mm ballast isn't available throughout the country, though. It's reasonably popular in the aggregate-poor SE of Eng-er-land where they have buggerall else, but most places north of Watford and west of Southampton don't use it.

Posted: **Sun Sep 25, 2005 8:59 pm**

How about 20mm ballast or mixing a similar sized stone into sharp sand?

The Brew Cabin

Loose stone slabs - Stone slabs are lifting from mortar base