Back to Basics; Tile Backing Options
The issue of a suitable backing is even less glamorous than choosing your grout and adhesive. If it’s all going to be covered up, what’s the point of worrying about it? Of course, there is every reason to give the backing for your tiles sufficient attention. For one reason, it won’t remain covered by the tiles for long if the surface is unsuitable and it all comes crashing down!
The first considerations should be:
- Usage: Where are the tiles going to be used? Will there be moisture or extremes of heat?
- Strength: Are you going to use large, heavy tiles? Adhesive is only as good as the surface to which it is attached.
- Shape: Is it flat enough to require just a little thickening of the bed?
- Cleanliness: Is there anything loose on the backing, like dust or old paint?
Types of Tile Backing
With the above considerations in mind, you can consider three major types of backing. Structural backing is when you fit the tile directly to the building’s structure, be it brickwork or concrete etc. It is rare to find structures which are flat enough to accommodate tiles, however.
In situ finishes are materials which are applied to walls to provide a finish and appropriately flat surface. These include plaster and cement rendering. Plaster is common for most uses but sand or cement rendering is better for damp environments. Always give newly plastered walls sufficient time to dry before you begin – a minimum of four weeks is standard.
Preformed backings come in sheets and are fixed to walls. They can be plasterboard, chipboard, blockboard or countless other materials. Some are specifically produced for tile use so you should check with your supplier what’s available. They are usually stronger and unaffected by water so it’s an option worth investigating. As preformed backings are applied dry, they are good if you are in a hurry because they are immediately ready for use.
Checking the tile backing
You can check plaster or cement or sand render if it has been placed directly onto solid walls by tapping it with something small and solid. Listen for a hollow sound. A small amount of hollowing is acceptable; if it covers a large area then cut a hole in the plaster to check if it has become loose. If it has, then you’ll need to replace it. Plaster-coasted plasterboard should be obvious if it has become loose as its only a thin layer.
All wall boards should be checked for stability. Plasterboard can be fixed with nails but screws are necessary in all other cases. Check the corners of each board for crumbling and, as a final test, push the wall firmly with all your weight. There should be very little movement. Once you are satisfied, you are ready to proceed.
Cleaning the tile backing
If it’s a new surface, you should only need to wipe it to remove any dust. Painted surfaces will need to be sandpapered over to remove most of the paint (but you don’t need to get every last bit off). Focus on anything that could later come loose.
Priming the surface
Priming ensures that the backing is immune enough to stop it absorbing moisture and resin from the adhesive. Check the label of your adhesive, or better still check with your supplier before purchase, whether there are any recommendations for priming. Cement-based adhesives, for example, will generally need to a primer.
Dealing with Deviations in Flatness
First, you need to check that the walls are sufficiently flat. It will be impossible to lay the tiles correctly if this is not the case. You can use a laser level to check the wall in no time at all. The part of the wall that sticks out the most is called the “proudest” area. If this area on a plaster wall is small you may be able to cut it back. Otherwise, you need to bring the rest of the wall inline with its proudest part.
Smallish deviations can be dealt with by using a little more adhesive where necessary to straighten it up as you fix the tiles, but this is not always possible. The manufacturer of your adhesive will give information as to its maximum recommended width and you should never thicken beyond this. If you have potted hollow areas to fill then you can use adhesive to skim over it with a trowel. Remember that you don’t want a smooth finish – only a straight one.
If you have an in situ finish that is too uneven for these techniques then you may need to cut out the areas and re-plaster them – again flat, not smooth. You can buy special purpose one-coast plasters for this job.
Wall boards are less likely to be out of line. If they are, then they’ll need to be replaced.
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