The Brew Cabin

I have just had a new patio laid, using 300x300 slate tiles. The supplier of the slate recommended using a thick-bed adhesive on a concrete base, which I have done. They further recommended a waterproof grey tile grout, which I purchased but not yet used because of concerns expressed by my builder.

The concerns are that because of the depth of the joints(up to 10mm + thickness of tile) together with the width of the joints (approx 7mm), 2 things could happen:

1. The grout could take a long time to go off, thus making the task of cleaning up the tiles after smoothing the joints difficult and lengthy.

2. The amount of grout could cause the grout to crack under its own weight.

The builder has therefore suggested that he uses a strong dryish mortar mix instead. He acknowledges that mortar is not waterproof but believes that a couple of coats of sealant will do the trick.

Are his concerns reasonable? And is his solution appropriate?

Many thanks,

John

The grout could take forever and a day to cure, in theory, but it depends on the type. Do you have a brand name and/or manufacturer?

Even if the grout does take a while to cure, that really shouldn't affect cleaning, as the microscopically thin smear of grout on the surface of the slate will not be affected by joint width or depth. However, you have to offset that against the obvious reluctance of your builder who seems to want to find problems with this method.

As for the grout cracking - I'm not having that! Most of the waterproof grouts have a damned good flexural strength and are far less likely to crack than mortar.

Using a mortar is ok, and there are additives that can be used to make a standard cement mortar as waterproof as any tile grout, which may be a better solution than using a sealant, which has to be re-done every couple of years or so, anyway!

If your builder is dead set against using the grout, then maybe you could consider summat like Ultraflow, a rapid setting, hard-as-bloody-iron pumpable mortar from Instarmac. That'd do the job, and, because it can be applied via mastic gun, there's little or no spillage and no snots to clean!

I Laid 1000 terracotta tiles five years ago on a cement base over an old concrete patio. I used a waterproof grout - brushed dry into the spaces, then watered with a very fine spray. no problems with frost, cracking etc. Caution use a tile sealer first or you will stain your tiles.