The Brew Cabin

Before I nag everyone with questions, I'd just like to say what a great site this is. The layout/design is excellent, the information useful & so far I haven't found anything else like it on Google.

Now onto my queries...

I plan on laying a patio in my back garden. My wife & I both like the idea of slate as the surface & we have a friend who works in the stone industry. However, I'm concerned that the slate won't be thick enough - it's only 10-15mm thick. Is that too thin for a patio?

I'm also confused over whether or not I need a sub-base. Some web sites say it's necessary, others don't. It's a new house, so the back garden is currently covered in a layer of sand. We get lot's of rain here, so I'm currently thinking that digging up the sand & then laying & compacting a sub-base would be a good idea (as a Brit living in Texas, when I say "lot's of rain" I mean massive downpours - we had 8in one evening last Nov!). So, is a sub-base a good idea?

Thx

That slate is a bit too thin for paving work, where we don't like to use owt less than 30mm in thickness, but, if you really want to use it, then it will have to be laid on a full bed of mortar. Even then, I'd be worried about it!

If you use a mortar bed, then you need a base of some form - a sub-base is ok for a patio, but it needs to be properly prepared and thoroughly compacted. A base would be better, given the problem with the thickness of the slate, and for that, I suppose the best material to use would be a concrete, something around 20 Newtons strength (whatever that is in the strange, nay incomprehensible, 'bags' that seem to be used to define concrete strength in the US).

If the gound is reasonably stable, then you could lay the slate directly onto a concrete bed-cum-base and miss out the mortar bedding, but the skill level required for this is quite high and you need to work at a good pace to cover the concrete before it starts to set.

Given the rain you have to contend with, I'd suggest a minimal sub-base, just enough to prepare the ground, then a base of 100-150mm of concrete, and finally, the slate laid on a 20-35mm thick layer of Class II mortar.

How does that sound?

Looking at the slate, it looks more like tile: it's only 12"x12" and the bottom has been machined into ridges. So your answer is what I was expecting, but not what I was hoping for. :-(

I was hoping to lay the slate on a bed of sand so I could easily raise it an inch or so at a later date (there is a concrete patio right next to it which we'd hoped to cover in slate in the future). And a mortar bed would make raising the slate difficult.

Reading between the lines, I think you're saying that this slate isn't really suitable for paving work. Since we haven't bought any of it, maybe it's time to consider some other form of paving.....

Thx for the advice

You can lay on a mortar bed without actually bonding the paving unit to the mortar. If you use a semi-dry mix, it will give you the rigidity you need to support the slate, but, when you come to lift them at some later date, they'll come away clean (ish).

If the slate is machined tiles with a ridged base, that really does suggest that they were intended for flooring, and therefore would be laid on a mortar screed over a concrete base, as discussed earlier.