Cobbles Set Into Mortar
Cobbles, or rounded, smooth stones, either river washed or extracted during gravel and shingle operations (as distinct from processed materials e.g. tumbled marble or porphyry setts) should be laid into a bed of mortar, either over a base of MOT Type 1 as per driveways and patios, or over a concrete foundation if used to support vehicles.
For most domestic projects, cobbles are usually laid into/onto a wet mortar, and difficulty is experienced when attempting to insert another stone and create a level surface, as the cobble displaces the wet sand and cement causing the bed to ripple and rise, and the result is an unsightly mess of mortar and cobble at various depths. Push one cobble into the bed, and the neighbour pops up!
Even attempting to use equal sized cobbles on a level bed of mortar will not be without problems, as the stones will not be securely held ‘captive’ by the mortar unless secured to a sufficient depth (60% and deeper).
The preferred method, which has proven successful over many projects, is to use a sharp sand and cement mix, laid semi-dry as a laying bed, to a minimum depth of 100mm (more according to the proposed site and use). The strength and formula will vary from region to region due to the nature of the sand available, but generally recommend 1 part OP cement to 3 parts sharp sand. Generally, sharp sand should consist of a mixture of grains measuring from dust/0mm to 3mm to produce a bed capable of compaction.
Dependent on usage, a bed of between 100mm and 120mm deep should be sufficient to secure the cobbles (usually ranging from 60 – 80mm in size). If working on sloping ground, or where occasional vehicular traffic e.g. cars or garden tractors may require access, it is advisable to increase the depth of the laying bed, as wheels or heels may exert lateral pressure on the secured cobbles.
It is important to ensure that the project has firm edging in place (see Edge Restraints) to prevent lateral movement or ‘wash-outs’ of the cobbles. Ensure adequate water drain-off to avoid puddles, which may become filled with detritus and muddy water. Water should flow freely across the mortar surface, otherwise dirt may become trapped between the cobbles.
The cobbles should be laid in the following manner:
Having laid and gently compacted the laying bed of mortar, ensuring enough material to enable your chosen levels to be formed, gently scrape out a socket of the approximate size to receive the cobble to hand, either with a gloved hand or a garden trowel. Have plenty of cobbles to hand nearby, and place a suitable number into a plastic or rubber bucket of clean water. Remove each cobble, one at a time, shake off excess water and gently place it into the cement/sharp sand socket, and press it into the bed. The water clinging to the cobble will create a micro-mortar pocket, allowing the cobble to be set into position without disturbing or lifting its neighbour.
As each stone is fitted, neighbour hard up against neighbour to ensure a tight fit, constantly check levels and falls using string lines or straight edges, trying to achieve a finish as smooth and even as possible across the tops of the cobbles.
Once you have completed an area of a couple of metres or so, gently tap down a board or plank, laid from restrained edge to restrained edge, giving a smooth, tactile surface to the work. Continue working, always checking levels and falls, ensuring the cobbles are packed as closely together as possible as works progress, then allow a suitable length of time to permit the laying bed to harden.
As with all paving works, at the end of each day, ensure the work is finished in a neat and tidy manner, to avoid having to chip away at yesterday’s laying bed.
The cobbles are pointed with a mixture of one part OP cement, three parts soft builder’s sand and one part sharp sand, very carefully batched to ensure uniform strength and colour, adding water to produce a ‘runny custard’ consistency. (This is a traditional method. Some may prefer to use premixed slurries in place of the sand/cement mix). The sharp sand provides an attractive ‘gritty’ appearance to the slurry mortar, preferable to a smooth grey paste.
Damp down the area to be pointed (don’t flood) and pour the slurry mix into the cobbles, and work the mixture into the interstices ensuring that no air is trapped. Use a small nylon kitchen washing up style brush to work the slurry into the joints.
Immediately wipe off any excess slurry. The depth of pointing mix will vary according to your requirements. For vehicular traffic, leave the slurry fairly high up around the cobble. For a more decorative appearance, you may wish to leave more stone showing, but the majority of the cobble (60% or more) will be held captive by the mortar, both from the original bed and slurry.
Keep on cleaning the surface, using brushes, from soft to stiff bristle yard brooms as the mortar hardens, until the cobbles are materially free from slurry. Any slight ‘bloom’ will wash off with subsequent washing, brushing, footfall and rain. If you want to accelerate the cleaning process, scrub the area with kiln dried sand and a stiff broom. Do not use any acids.
All cleaning operations will depend on the temperature at the time. Try to avoid too much water – including rainfall – onto the newly laid finished surface, otherwise the mortar may be compromised visually and structurally. The slower the slurry dries out, the better the finish.
