The Brew Cabin

Hi Pavers,

I am a DIYer but a complete novice at paving with stone. I live in a wildland area in Northern California that is very vulnerable to fire and want to use slate as part of a defensible space.

I have about 10 tons of random slate that a local quarry made available free for the hauling. It contains chunks as large as 6 inches in diameter and 2 inches thick. All sizes are present down to fines. I am trying to figure out how to make the best use of this material as ground cover.

For example, I want to cover a level walking path along a 12 ft wide path I cut 30 years ago into a 40% slope. Grass is difficult to grow because of Oak canopy. So I currently have mud in the winter and dust in the summer. The area is not used very often except for our dog to run. A 30" diameter cedar tree is on the top edge of the lower bank close to my house and is in good health. I plan to stack slate 30" high to retain the lower and upper banks of the cut. The soil is silt with a high clay content. I want to make sure the tree root system (which comes to the surface in spots) is not damaged. I embedded some 2" thick slate which is about a foot wide and 2 feet long down the middle of the path a few years ago and they are quite stable.

What can I do (size, spacing, grout type) when placing additional slate to allow the roots to breath, yet fix the slate in place so it does not move when leaves are blown off it? I want to fill the spaces between chunks to reduce dust or mud but avoid slate on top of slate which could result in a slip hazard or allow slate to be knocked out of position when clearing Oak leaves with a blower. I also would like to know what techniques I should consider to 'glue' the slate down.

The slate is in a 5 foot high pile and am finding it hard to dig out the larger pieces by hand as I go down the pile because the smaller pieces get in the way as they inevitably fall back in place. Would it pay to build a screening rake to pull behind a small pickup or ATV to make the screening process more efficient? I have suitable metal and a welder.

Thanks for a great forum!

I'm not familiar with Californian slate - maybe one of our occasional US Contractors could comment, so what follows is based on experience with British slate.

If your fill material has a maximum size of 150mm (6"), the normal minimum depth required to form a stable sub-base or fill layer is twice the max size, so that's 300mm (12"). This relies on their being a good distribution of smaller sizes that will fill the voids between the larger chunks and so create a stable layer with good interlock.

If you were planning to use the larger pieces as flagstones, then given that you need to ensure survival of the tree roots, a non-bound, cement-free bed would be the way to go. In Britain/Ireland, we'd probably use a layer of the stone/slate dust to bed the larger pieces, and then use more of the dust to fill the joints. This is going to give a rough'n'ready surface that will move and may settle to a degree that makes repairs necessary, but, as there's nowt set in concrete, that shouldn't be too difficult.

However, and this is a recurring problem when discussing pavement construction with N.Americans, if there's a potential for frost heave, I'm not sure just how suitable this type of structure would be.

As for the screening rake idea, I'm not sure how this would work. Maybe it's something more familiar to you in the US than to us over here, but if you explained the process in more detail, it might make a bit more sense to me.