Stone cobbles are one of the most beautiful materials to use in paving. Done properly they have a European elegance and robustness which is hard to match.
Cobbles come in several types of material, including granite, porphyry and even concrete. They can be laid on a sand base if they are at least 60mm thick and grouted with gravel, sand, tar or mortar. For the best results though they should be glued onto a concrete slab and this article discusses this procedure.
Cobbles can be purchased either as individual stones or already glued in a pattern to a mesh backing. The mesh backing allows quicker laying although they are not always precise and, if glued to a steep concrete surface, the mesh inhibits good glue contact between the stone and the concrete and is not recommended for vehicle traffic. On level surfaces there is no problem with using the mats.
Once stones are glued, grouting can begin the following day. Grout can be a simple sand/cement mix with an oxide added for colour if desired or a premixed coloured grout. The premixed grout is more expensive but less labour-intensive and is more consistent in colour and hardness.
The Problem:
Traditionally grout is spread into the 10mm gaps using a rubber squeegee or by hand and cleaned with a sponge and water when the grout has partially cured. Over a large area such as a driveway this is a very slow and tedious process. Granite cobbles have a rough surface which is difficult to wipe clean and easily absorbs cement, which can leave stains. These can later be cleaned with hydrochloric acid but this requires considerable time and runs the risk of staining the grout.
Alternatively, the stone can be sealed before grouting with a good quality penetrating stone sealant. This will prevent staining. However, since the cobbles need to be clean and dry before sealing, on large job this is not always practical.
The Solution:
Faced with these problems, I was convinced that if I could use an extrusion method such as a large grouting gun to force mortar into the gaps I could cut the time on this part of the project significantly.
Some investigation revealed ‘The Pointmaster’, a very simple device consisting of a PVC cylinder with a replaceable, stainless steel nozzle and a hand plunger. The Pointmaster was designed primarily for pointing old brick work but it proved to be a magic tool for grouting cobble.
Using premixed mortar, fill the cylinder. Taking care to not spill any, place the nozzle in a gap and slowly squeeze grout out. It takes some practise (and a strong back) to get a smooth flow.
If you fill gaps to near the top then let the mortar set for about 3 hours. It can be smoothed down as low as you want to allow the stone to stand higher. For smoothing I use bolts of various sizes or even sticks. It’s a simple and rather satisfying process and produces and excellent result.
Tips:
1.Don’t try to smooth the mortar too early or it will be slow and messy. Neither should you leave it set until the next day or it will be very difficult to work. What works best is if you grout in the morning, leave it to set and smooth in late afternoon.
2.While they are noisy and annoying, a leaf blower is the best thing for clearing gaps in the stone before grouting and for removing debris when the grout is smoothed down.
Happy grouting!