Time and again, the same questions are raised by pebble mosaic beginners, so here are answers to a few of them.
1. HOW DO I PREPARE THE SITE?
A solid base must be achieved. Depending on the compaction of the site and the type of traffic that might use it, there can be plenty of work to do or very little. Topsoil should always be removed, and the site evacuated down to the level of solid subsoil. Then, depending on whether the mosaic is to be subjected to foot-traffic alone, light vehicles or even heavy vehicles (a fire engine perhaps?), the base beneath the mosaic must be designed to withstand that type of load. Bases for both in-situ and pre-cast mosaics are the same.
The edges of the mosaic are particularly vulnerable to damage, and must be well protected. A stout edging of bricks, settings or stone flags should be set on a concrete foundation.
Remember that heavy duty specification is necessary for vehicle use. For in-situ work, the edging may be required to serve as the final level for tamping down the pebbles, so it should be carefully set to the right fall so that rainwater is shed from the mosaic surface. If wooden battens are being used to establish the final level and fall, then they are placed in position with pegs or mortar and checked with a spirit level.
2. HOW MANY PEBBLES DO I NEED
To calculate the pebbles required, the easiest method is to make a measured trial in sand and count the number of pebbles used. Such trials are always useful, not only to calculate the number of stones, but to check if you really like the pattern you have in mind. Also, you may find that you haven’t enough of the special stones required, in which case you can re-design before you start.
3. HOW DO I MAKE BEST USE OF THE PEBBLES AVAILABLE
There will always be a lot of variation in the size and the shape of the pebbles. Bought-in ones will have been graded, using a screen of a particular mesh-size; a typical example for in-situ work would be 2-3 in (5-7.5cm). If you are lucky enough to be able to collect you own, you can aim for uniformity, but you’ll need a lot of persistence!
Take time to sort the pebbles thoroughly. Discard any really bad shapes: they are more trouble than they are worth. Keep to one side the ‘short’ ones that you’ll need for the ends of the rows. Mostly you’ll be sorting according to ‘fatness’ so that you can get uniformity of a width for each row. And, as you start another row, use the different lengths of the pebbles to off-set the gaps created in the previous row between the pebbles. Keeping a brickwork type ‘bond’ both looks good and gives the greatest strength to your work.
4. WHICH WAY UP SHOULD A PEBBLE BE?
Every pebble has a ‘best’ edge. For ‘long’ pebbles this means the longest and flattest. An edge with a protruding point will show les surface when the mosaic is finished, and will not look so good.
As you go along, examine each pebble and decide which is the ‘best’ edge for showing on the top surface, bearing in mind the following tips;
Make sure that its deep enough. A good rule of thumb is that every large pebble should be 5 cm or more in depth.
Large long pebbles can often be placed either way, like the one in the picture above left which is deep enough on both sides.
Try to place the pebbles so they touch the sides of the previous row and touch each other end to end. The tighter the pebbles are packed together the stronger the mosaic will be.
A small tool like a pointed paint-scraper will be most useful for digging and loosening the base mix. Dig, place the pebble and then firm the mix around its base.
5. WHY DO PEBBLES FALL OVER?
When the pebbles are tamped down there is a tendency for the deepest pebbles to lean over because of the resistance of the firmed mix beneath them.
Remember the ‘dig, place, firm’ routine.
Place the pebbles so that they stand only just above the finished level.
Tamp down every row, making sure that all the pebbles go down vertically rather than tipping over. Any tendency to lean over should be corrected immediately.
6. WHAT ABOUT THE GAPS?
When the section is completed, it’s best to make all the pebbles firm by filling up the gaps and bringing the top surface of the base mix to ¾ in (20-25mm) below the tops of the pebbles. Sprinkle carefully; you don’t want to have gritty particles showing through the final surface of the top mix.
7. HOW MUCH WATER SHOULD BE USED?
Plenty! As soon as you begin to spray the mix with water, the cement begins the chemical reaction which eventually results in hardening, so make sure that the water has penetrated the full depth of the base mix. This first spraying also helps to clean cement dust from the pebbles and wash down any loose particles. Water each day’s work as you go along and cover it with polyethylene sheet. The next day it can be walked on and used as a level for adjacent sections if necessary.
8. HOW MUCH TOP MIX SHOULD I USE?
Before you begin with you top mix, remove the polyethylene and make sure that the pebbles are completely dry. Then there is a choice to be made;
Less top mix you use, the better the pebbles will show their shape;
Use more top mix and the surface will be less liable to trap leaves and dirt, and will stay cleaner and easier to sweep.
9. WHAT ABOUT THE FINAL SPRAYING OF THE TOP MIX?
No watering cans now! This operation needs care and only a small amount of water. A pressure-sprayer (like a pump-up garden bug sprayer) will produce a suitable fine jet of mist. Make sure that the sprayer is pumped to a high pressure. Make a test burst on the ground and then spray the mosaic evenly until the surface just floods with water. Then stop the surface is smoothly bound, and you’re done.