Ask The Expert 

My customer is complaining that his grout has gone black.  Why does this happen and what is the solution to the problem?

The black patches on grout are caused by a type of fungus which thrives in warm humid conditions.  The best long term solution is for your customer to increase ventilation so there is less moisture left to settle, although using a good quality grout containing a fungicide will also help prevent the problem occurring.

Once you've got it, however, the black mould can be removed from the grout using a dilute solution of bleach or a specialist fungicidal containing mould killer to make sure it doesn't come back.

Even without mould, grout will discolour over time.  Old grout can either be raked out and replaced or can be improved using a grout reviver.
 

Do I need to use an epoxy grout in a kitchen?

Epoxy grouts provide a high strength, chemically resistant grout joint with a hard smooth surface which is impervious to stains and mildew.  They are ideal for areas that tend to get wet or where aggressive chemicals are used, which makes them perfect for use in areas where hygiene is critical, such as hospitals or commercial kitchens.

Epoxy grouts can be used in a domestic situation but are more expensive than the cement-based variety and can be tricky to use as they cure very quickly and set up hard.  You should generally consult your tile supplier to check whether an individual installation requires epoxy joints.
 

Can I tile on timber floorboards?

Generally floorboards will need to be covered with external grade plywood (18mm is ideal) which needs to be firmly screwed down at maximum centres of 300mm.  Tongue and groove floorboards can also provide a suitable substrate, provided they are will fixed, glued and rigid prior to tiling.  However well fixed, a wooded substrate will still be subject to minor movement so a flexible tile adhesive should always be used.

These are available as both single part, polymer-modified powder products, two part rubber crumb and liquid polymer products or ready mixed flexible tile adhesives.  In situations where significant deflection is unavoidable, tile adhesives containing rubber crumb should always be used.

Grout should also be able to accommodate some movement, so it is essential to use a polymer modified grout or standard grout  together with a liquid admixture.
 

How do I get a smooth line of sealant around a bath?

Use either a silicone or acrylic sealant: silicone provides the best long term performance as it is more durable and resistant to bacteria which can produce the black patches seen around baths and showers.

Using the applicator provided with the sealant, apply the material into the joint between the batch and the wall in a single continuous line starting at one end and using even pressure throughout.  The trick is to avoid over application of the sealant as it can be difficult to remove cleanly when the joint is smoothed off.

As soon as the sealant is applied (do it quickly, the sealant will be difficult to work when it becomes tacky), smooth the surface.  Use either the applicator provided or a suitable tool such as a piece of round plastic rod, the end of a metal knife or spoon, and draw it along the sealant bed to form a smooth concave profile.  You'll get the best finish if the tool is regularly wetted in soapy water.

More detailed answers to these and any other questions relating to tile adhesives or grouts can be obtained by calling Biscem's Technical Helpline on 01785 272727 or email us at bisceminfo@Bostik.com.