Slate stone has been mined for hundreds of years and blue-grey slate adorns the roofs of thousands of houses in the UK. In recent years the choice and quality of slate stone available has improved, largely due to imports of colourful Indian slate embossed with fossils. Whatever its origin, slate makes a wonderful paving material.
What Type of Slate Paving?
Your choice of paving will depend on the effect that you are trying to achieve.
Smooth slate stone slabs will give a more even finish underfoot. You can also buy uncut slate blocks that give a rougher finish. This will tend to flake relatively easily with use and may be prone to puddling.
Symmetrical rectangular paving using the same size pavers will give a formal feel. It is probably more usually laid by installing rectangular pavers of different sizes. This still has an ordered appearance.
Alternatively you can lay slate ‘crazy paving’. This is where you lay irregularly shaped slabs. This type of paving stone is cheaper than others and can look extremely effective in cottage gardens.
If you are planning to pave with slate stone slabs in or around your garden, then you must prepare a solid foundation. There are several options available.
Dig out the area to be paved to a depth of around 225mm plus an allowance for the slabs.
Remove any unwanted tree roots and bridge around roots that get in the way if they are from trees or other plants that you wish to keep.
Hammer wooden stakes into the ground around the periphery of the area to be paved. Install further stakes every metre or so within the area to be paved. Tie string to these stakes at the height you would like the surface of the paving to reach. Use a spirit level to ensure that the string lines are even.
If you need to lay cables or drains beneath the paving, do it now, ensuring that you use armoured cable or protective conduit.
Backfill with 100mm of hardcore and compact with a compacting plate.
Lay 100mm of concrete (1:2:4) over this and allow to cure.
Lay a bed of approximately 25mm mortar (1:4) and position the slabs in this. Lay the bed for just one slab at a time, tamping it down with a rubber mallet or the handle of your lump hammer, so that it is level with the string line. If the slab sinks too much, raise it and lay some more mortar beneath it. Put a very slight gradient on the slab to encourage rain water to run off it when it is set in position.
For large expanses you may need to incorporate underground drains or soakaways. A soakaway is an underground area comprised of rubble or hardcore. You can set a grate at surface level with a drain beneath it leading down to a soak away.
Use an angle grinder with a stone cutting blade to cut your slate slabs to the dimensions you need.
Once the slate is set in position, use a mortar mix (1:3) to joint the gaps. You can tint the mortar to match the colour of the slate if you so wish.
Install a gulley around the edge of paving to carry excess rain or hose water off into underground drains and soak away pits.
If you feel that your paving is going to receive less traffic, and consequently does not need to be quite so secure, then you can lay a lighter foundation. All you need to do is lay and compact 100mm hardcore as before.
Next, spot bed your paving slabs using five spots of mortar at the corners and centre of each slab. You can lay your slate stone slabs directly onto the hardcore and level as above.
Joint as described above.
A third option is to lay your slabs directly onto a sand bed 40mm thick. Once again they should be jointed and grouted as above.
Wet or dry mortar for jointing?
When jointing your paving you will find it much easier to use a dry mix. Simply sweep the mortar into the joints and allow to settle. Repeat this procedure until it has reached the desired level. Lightly water or allow the rain to set the mortar.
If you are skilled at jointing, you may prefer to use a pointing trowel and a wet mix. Compact the mortar between the pavers using a stick as you go. Always ensure that any mortar mix is wiped off stone surfaces before it goes off or it will be much more difficult to remove later.
When laying random stone paving using different sized rectangular slabs, be careful not to leave small gaps to fill. The odd really small paver will destroy the overall effect as well as being easier to loosen with heavy traffic.
If you wish to lay pockets of plants between pavers plan this at the foundation stage. You will need to allow for soil instead of the mortar bed. Once the mortar has set it will be extremely difficult to remove.
If there are manhole covers leading to underground drains or services in the area you wish to pave, then these can be easily incorporated and disguised. It is possible to get recessed covers that you can cut and lay a piece of slate in so that you wouldn’t even know it was there.
If you feel that the gaps between your slabs are too large, when it comes to jointing between them you can use a rounded stick to scratch a small channel through the centre of the mortar. This has the effect of drawing the eyes away from the gap and into the recess so that the gap seems less than it is. It also has a decorative effect.