Frequently asked questions 

Below is a list of questions we are asked every day on the Technical Helpline.
   

Can I tile to old timber floorboards? If so which how do I prepare them and what tile adhesive and grout should I use?

I wish to stick tiles to a balcony constructed of timber and covered with roofing felt and sealed with asphalt.  Is this possible?

My grout has gone black- why does this happen and how can I correct the problem ?

I have a kitchen floor to tile that has a ceramic tile floor already down.  It is proposed we leave the existing tiles and tile straight on top with a ceramic tile. What tile adhesive would you advise?

I am proposing to fix a light colour natural stone on a screeded floor in my hallway, and have been informed that staining or darkening of the final appearance could occur if I do not use the correct tile adhesive. Is there a certain type of tile adhesive I need to use?

I have a walk in shower/ wet room to be tiled which is constructed of plywood tiling.  Is there a waterproofing product I can use before?

We wish to lay tiles directly onto an existing vinyl covering. How do you suggest we prepare the floor?

What is epoxy grout and where is it used ?

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

I am tiling my kitchen worktop and will need a grout that will not stain or harbour bacteria, which of your grouts do you recommend?

I have a bath panel constructed of thick MDF board. Which of your tile adhesives and grouts would you recommend?

I have a floating floor in my conservatory I wish to tile.  Is this possible and do I need special tile adhesives and grouts?

Do you produce tile adhesives and grouts that can be used with underfloor heating?

Which of your tile adhesives would you recommend for use in a swimming pool with a concrete shell?

I have moved into a new house and the walls of the bathroom have been painted.  I now wish to tile these wall in the shower area. What preparation and tile adhesives do you recommend?

What is so special about porcelain and fully vitrified tiles and do I need special tile adhesives to fix them?

Which of your tile adhesive and grout products can be used externally?

Do you manufacture a screed that can be used as a wearing course as I have a garage where the concrete needs leveling and I wish to park my car on it.

I propose to fix brick slips to an external render. Do you manufacture an adhesive for this type installation?

I have a floor that is very uneven and some natural stone of variable thickness. Do you manufacture a thick bed tile adhesive that I can lay the tiles into?

Can I tile to old timber floorboards?  If so which how do I prepare them and which adhesive and grout should I use? 

Wood in the form of floor boards and plywood, to chipboard and other wood composites can provide a suitable substrate on to which tiles can be affixed. However the nature of the wood substrate will affect not only the necessary preparation but also the type of adhesive that can be used and the methods by which the adhesives can be applied.

Rigidly fixed external grade plywood provides an ideal substrate on which to apply tiles.  Plywood is usually used to cover a poorer substrate such as old floorboards and hence normally requires fixing to that substrate.  Exterior grade, 18 mm plywood screwed down at maximum centres of 300 mm is usually sufficiently rigid to receive tiles. Tongue and groove floorboards can provide a suitable substrate on which to apply tiles, providing it is well fixed, glued and rigid prior to tiling. 

Where possible, Chipboard or similar composite boards should be overlayed with exterior grade WBP plywood screwed down at 300mm intervals.  In these instances a thinner plywood layer than the usual 18 mm can be used. Once boards have been fixed it is necessary that they are stabilised to the ambient humidity and conditions, otherwise warping can occur.

If wooden substrates are not rigidly fixed, any movement can cause a failure to occur in the tiling. Ceramic tiles and even ‘flexible’ floor adhesives are brittle in comparison to wood.  Deflection in a floor can then cause stresses to build up that are stronger than the adhesion of the adhesive to the tile or wooden substrate, causing cracking or debonding..

Since all wood is sensitive to moisture, great care is needed to prevent water ingress to the wood.  Even if water resistant composites are used, an external source of water such as leaking pipes or appliances can cause the composite to soak up water and expand.  If the boards are allowed to expand this will result in a structural change of the substrate. Edges of boards and the underside of plywood sheets or composite boards should be sealed using a proprietary sealing/bonding agent. Alternatively waterproof the complete installation with Bis-Watertite Tanking System.
 
Adhesives such as Biscem Bis-Flex or Timberfix are recommended .  Polymer modified grouts such as Vitri-Grouts, Timber-Fix or Wide-Joint Grout incorporating Flexibond or Biscem Flexible Grout Additive must used in this situation.
 

I wish to stick tiles to a balcony constructed of timber and covered with roofing felt and sealed with asphalt. Is this possible? 

External grade roof asphalt should never be used to receive tiling.  Designed for uses on flat roofs and some balconies, roofing grade asphalt is intended to be soft to deform with changes in temperature.   As it does not supply a rigid base it will be unsatisfactory.  In such situations consideration should be given to covering the surface with a cement sand screed which will be suitable to receive tiles. Very often balconies that are to be tiled are not built as such during the original construction.  Balconies can often be built onto garage roofs or other overhangs which have an existing surface.  These surfaces were more than likely designed as ceiling to the room below rather than as floors.  As ceilings are generally self supporting rather than having a requirement to support traffic, they tend to not be rigid, and are thus directly unsuitable to receive tiling.

If they are existing external surfaces they may also be finished in external roofing grade asphalt which is also unsuitable to receive tiling.

As a rule of thumb, unless they are of concrete construction  balconies are not suitable for tiling.  The tiles chosen are often large format dark tiles which will exhibit large thermal movements which will exaggerate any possible damaging effects.

If balconies are to be tiled they should be made rigid by covering with material such as a cement sand screed or a suitable thickness Tile Backer Board such as Dukkaboard (available from Trimline on 0208 778 9000.  If  the installation is of a wooden construction it may be necessary for the joists to be rebraced to bring them up to the quality of flooring grade supports.

Falls should be formed in the substrate to ensure puddling of water does not occur.  These falls should never be formed in the adhesive layer, but always at the design stage in the substrate.

If the weight of an unbonded or 'self-supporting' cement sand screed is considered too great, the use of a Lytag screed (lightweight aggregate) should be considered.

Recommended adhesives & grouts: Biscem Bis-Flex, Vitri-flow or  Vitri-Rapid,  Thin & Thick incorporating Flexibond. Grouting: Speed-E-poxy or Wide-Joint grout incorporating Biscem Flexible Grout Additive.
  

My grout has gone black- why does this happen and how can I correct the problem ? 

The appearance of black patches commonly found on grouts is a type of fungus that can be found growing on a multitude of surfaces where moisture and temperature levels are high. If tiles are on an outside wall they will often be a target for condensation and condensation brings black mould which mainly attacks the grouting between the tiles.  It can appear however on most types of fabric, wood, cement, linoleum, paint and other materials.

The most common conditions for the development of mildew and black mould include, poor air circulation, warm humid temperatures and persistent dampness. The nutrients that provide the breeding ground for this mould arise from natural bacteria in the air and grease, soap or body oils deposited on shower walls and around sinks, together with food residues on worktops. These can accumulate particularly on grout lines where the surface generally has a relatively coarse texture compared to adjacent glazed tiles or fittings. 

The best solution to controlling the problem is to ventilate bathroom and shower areas after use, so that there is less moisture left to settle. Leaving the windows open, when it is sensible to do so, drying surfaces after use and keeping the room warmer will all help. 
 
The use of a good quality grout will also help to minimise or delay the onset of any mould growth as they often contain fungicides.

Black mould can be removed by the use of a dilute solution of bleach or application of a specialist fungicidal containing mould killer, to ensure that the mould does not grow again.

Even if it does not have mould, white grout in particular will discolour in time. One remedy is to rake it all out and replace it with new grout.  But another solution is to use a grout reviver which is simply brushed along the grout lines, slightly overlapping the tiles.  When the grout reviver has dried, wet the area, leave for a few minutes, then wipe the reviver from the tile surface with the sponge leaving it in the grout lines.

The use of a good quality silicone sealant around a bath or shower tray for example, can often help prevent mould from developing in this area.
 

I have a kitchen floor to tile that has a ceramic tile floor already down. It is proposed we leave the existing tiles and tile straight on  top with a ceramic tile. What adhesive would you advise? 

This surface, if sound and clean, will normally present a good base for re-tiling.  The main consideration here is to ensure the surface to be tiled is perfectly clean and dry.  The previous usage of the floor should be checked and particular attention paid to cleaning, making sure all grease and grime, traces of polishes any loose materials are removed.  It may be found with existing quarry tiles that the previous usage has ingrained sufficient grease or polish that the only way to obtain a sound clean surface on which to apply the new tiles is by physically removing the surface. Remove any loose or hollow sounding tiles and make good the area with a suitable sand /cement mortar.

We would advise a “slurry coat” is used to prime the floor.  This is, to apply Biscem Flexibond at a dilution rate of 1 to 3 parts clean water and mixed with four parts Biscem Fast-set/Vitri-Flex and applied with a brush directly onto the substrate tile. Once it has dried this slurry coat will give an excellent key on which to tile.

Biscem Vitri-Flex, Vitri-Rapid, Vitri-Flow, Fast Set, Thin & Thick Bed  are all suitable adhesives. 
 

I am proposing to fix a light colour natural stone on a screeded floor in my hallway, and have been informed that staining or darkening of the final appearance could  occur if I do not use the correct adhesive.  Is there a certain type of adhesive I need to use? 

This can be true  so when fixing light toned natural stones such as marble, limestones and travertine a white adhesive such Vitri-Flow White, Vitri-Flex White, Vitri-Rapid White, Bis-Xtra White, Fast-Set White or Thin & Thick Bed White should always be used.
 

I have a walk in shower/ wet room to be tiled which is constructed of plywood tiling. Is there a waterproofing product I can use before.

Yes, as plywood is sensitive to water care should be made to prevent water ingress.  In this situation the installation must be waterproofed with Bis-Watertite Tanking System. We would then advise Biscem Bis-Xtra polymer modified swimming pool grade adhesive used to be used. For grouting Biscem Speed-E-poxy or Vitri-Grouts are most suitable.
 

We wish to lay  tiles directly onto an existing vinyl covering. How do you suggest we prepare the floor? 

The main consideration with vinyl is to ensure the surface is perfectly dry and clean. Particular attention should be paid to cleaning, ensuring grease, grime and any parting agents are removed. Remove any damaged, loose or hollow sounding tiles/sheets and make sure they are firmly bonded to a solid base. Make good the effected areas with  Biscem LF25 Latex Screed or Bis-Flex.  Priming is not required when using polymer modified adhesives, otherwise   Biscem Flexibond at a dilution rate of 1 : 5 - 6 parts clean water should be used and applied using a mop or brush.  Depending on the type of tile to be installed any Biscem cementitious adhesive is suitable.
 

What is epoxy grout and where is it used ? 

Epoxy grout uses resins and hardeners to obtain a high strength and chemically resistant, grout joint. After  cured, epoxy grout hardens into a hard, smooth surface that is impervious to stains and mildew. Therefore, it is ideal for areas that tend to get wet or are susceptible to spills or where aggressive chemicals are involved. They are therefore more often used in commercial situations such as hospitals, kitchens, shower areas and similar environments, where wear resistance and hygiene are important. In domestic situations they do find applications in kitchens and particularly shower/bathrooms where a smooth impervious grout will provide a very durable and mould resistant surface. It should be noted that some slight yellowing can occur over time with epoxy grouts.  

Because of advancements in technology, epoxy grout has become almost as easy to use as cement based grouts.  But, because epoxies generally cure much faster than cement based grouts and set up hard, it is critical to remove any excess material from grout joints and the tiles while grouting.  You should consult with your tile supplier to determine if your installation and use parameters require epoxy grout joints.

Epoxy-based grouts are more expensive than the cement-based variety, but the benefits outweigh the cost in certain applications
 

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

The most effective method of sealing around a bath is to use either a silicone or acrylic sealant. The latter being water based are easier to work with, but for long term performance a good quality silicone sealant is more durable and resistant to bacteria which produce the typical black patches seen around baths and showers.

Both type of sealant are available in standard mastic cartridges and in aluminium aerosol containers.  Often these products are supplied with special nozzles or other devices for smoothing off the joint sealant.

For best results apply the material into the joint between the bath and wall in a single continuous line starting at one end and using even pressure throughout. This will help to produce a uniform bead of sealant. The trick is to avoid over application of the sealant as this can be difficult to remove cleanly when the joint is smoothed off. As a general rule the sealant should extend about 5mm from the corner onto both surfaces.

Using either the applicator provided or a suitable alternative tool, such as a piece of round plastic rod, the end of a knife or spoon, draw this along the sealant bead to form a smooth concave profile along the joint.  With silicone in particular this operation will be helped by regularly wetting the tool with soapy water and completing the job immediately following application. Once the silicone starts to dry slightly and become tacky it is difficult to work.  With water based sealants a damp cloth can be used to finish off the joint. 
 

I am tiling my kitchen  worktop and will need a grout that will not stain or harbour bacteria, which of your grouts do you recommend?

I have a bath panel constructed of thick MDF board. Which of your adhesives and grouts would you recommend?

I have a floating floor in my conservatory I wish to tile. Is this possible and do I need special adhesives and grouts?

It is but there are many considerations. It is important that the deflection common to boarded floors is reduced to a minimum.  Ceramic tiles are brittle as too, generally, are floor adhesives.  Any deflection in the floor could cause stresses to build up that are stronger than the adhesion of the adhesive to the tile or wooden substrate.  All boards must be screwed down at maximum centres of 300 mm which is  usually sufficiently rigid to receive tiles.
 Seal the reverse side and edges with Biscem Flexibond at a dilution rate of 1 : 5 - 6 parts clean water also leave provision for ventilation behind the boards to prevent warping from atmospheric changes.

Where possible  overlay with exterior grade WBP plywood screwed down at 300mm intervals.  In these instances a thinner plywood layer than the usual 18 mm may be suitable. Once boards have been fixed it is necessary that they are stabilised to the ambient humidity and conditions otherwise warping can occur.

Even if water resistant composites are used, an external source of water such as leaking pipes or appliances can cause the composite to soak up water and expand.  If the boards are allowed to expand this will result in a structural change of the substrate.  No adhesive will absorb such a permanent change.  This is often seen by cracking in the tiles that follow the outline of the boards used. Chipboard and other Wood Composites are sensitive to water and in wet areas care should be made to prevent water ingress.  In this situation we would recommend tanking out the installation  with Bis-Watertite Tanking System.

As an adhesive Biscem Bis-Flex and Timber-fix can be considered.  However, it should be remembered that these products are designed to accept lateral, horizontal, movements in floorboards.  If there is significant vertical deflection the adhesive may remain adhered but the deflection may be sufficient to raise stresses large enough to crack the brittle tile surface.

Timber-Fix or Wide-Joint Grouts incorporating Biscem Flexible Grout Additive must be used.     
 

Do you produce adhesives and grouts that can be used with underfloor heating? 

Yes. Biscem manufactures single part flexible cementitious adhesives and grouts that are compatible with most under tile heating systems on the market.  Such products in the range include Vitri-Flow, Vitri-Flex, Vitri-Rapid, Bis-Xtra, Fast-Set or Thin & Thick Bed incorporating Flexibond admixture. Grouts are Vitri-Grout or Wide-Joint Grout incorporating Flexibond Admixture.

Due to the complexity of this subject please call the Biscem Helpline on 01924 362081 for advice on which adhesives and grouts are suitable for the type of heating system to be installed.
 

Which of your adhesives would you recommend for use in a swimming pool with a concrete shell? 

We manufacture two adhesives specifically formulated for swimming pools. These products are Biscem Bis-Xtra, a polymer modified, single part flexible adhesive and Biscem Thin & Thick Bed swimming pool grade cementitious adhesive, both available in White & Grey.

Owing to the complexity of this subject please call the Biscem Technical Helpline on 01924 362081.

Note: It is wise in all cases to approach the relevant body (SPATA) regarding the pool shell for full details on mix details for concrete and render, and queries regarding pool integrity.   (Tel 01264 356210)
 

I have moved into a new house and the walls of the bathroom have been painted. I now wish to tile these wall in the shower area. What preparation and adhesives do you recommend? 

The adhesive properties of paint are designed to be sufficient to hold the weight of the paint to the substrate.  Paints and particularly emulsion paints will not give sufficient bond strength to support the weight of tile and adhesive. 

Check that the paint is in a sound condition.  This can be done by applying strips of masking tape to the surface and leaving for up to 10 minutes before tearing them off. If the paint comes away with the tape then the paint is not sound enough to tile therefore the surface should be mechanically abraded to remove the paint prior to the fixing of the tiles. 

Chemical paint strippers should not be used as the cleaning agent itself can affect the bond of the adhesive to the cleaned substrate.  Once the paint has been removed the substrate should be treated accordingly.

Paste adhesives such as Power Showerproof, Biscem Showerproof or Mega-Grip are the preferred adhesives for tiling onto painted walls.

Any grout may be used to suit the situation depending on the joint width.
 

What is so special about porcelain and fully vitrified tiles and do I need special adhesives to fix them? 

Fully Vitrified and porcelain stone ware is manufactured to offer state of the art flooring and without question a floor covering that provides durability and superior impact resistance.  With extensive modifications in the manufacturing process in recent years these tiles have become more advanced and their demand has increased immensely. 

Porcelain tiles are fired at 1300ºC but complete vitrification of the tile body occurs at 1200ºC and therefore we are fixing in real terms, an impervious tile and for this reason standard cement based adhesives do not provide the adhesion and bond strength to grab and secure the back of the tile.

To do this we need polymer modified adhesives and Biscem have introduced  a range of products incorporating Vitri Technology, an additive included at the production stage to enhance performance and bond strength to alleviate the problems traditionally associated in fixing fully vitrified tiles and porcelain stoneware.

When using porcelain or vitrified tiles, Vitri-Rapid, Thin & Thick Bed, Vitri-Flex, or Vitri-Flow should be used.  This will enhance the bond strength at the adhesive/tile interface.
 

Which of your adhesive and grout products can be used externally?

Biscem products specifically designed for external use include Vitri-Rapid, Vitri-Flow, Thin & Thick Bed and Bis-Flex adhesives. Grouts include
Vitri-Grout, Wide-Joint Grout, Bisgrout Wall and Speed-E-poxy grout. 
   

Do you manufacture a screed that can be used as a wearing course as I have a garage where the concrete needs leveling and I wish to park my car on it.  

LF01 Single Part screed may be used as a wearing course as long as a suitable epoxy or acrylic paint system is applied. LF01 is fast setting and drying, being ready to walk on after 2 hours and being able to be fitted on after no more than 12-24 hours.  All surfaces to receive LF01 should be adequately primed with LFP5 or Neobase to reduce pinholing and to ensure adequate flow of the material.  Application thickness 3mm to 25mm.
 

I propose to fix brick slips to an external render. Do you manufacture an adhesive for this type installation ?

Biscem Bis-Xtra cement-based adhesive is ideal for fixing brick slips to a sand/cement render. The render must be at least 3 weeks old, longer if conditions are cold or damp prior to the bedding of the brick slips. Fixing and Grouting should not be applied when the material, substrate or ambient temperature is below 5°C.  Doing so will significantly retard the setting times.  At temperatures in excess of 30°C it is likely that setting times will be accelerated to such a level that the material becomes impossible to use.
 

I have a floor that is very uneven and some natural stone of variable thickness. Do you manufacture a thick bed adhesive that I can lay the tiles into?

Biscem Vitri-Flow, a fast setting, cementitious adhesive suitable for adhering ceramic floor tiles, terrazzo tiles and slabs, natural and artificial stone to a variety of dry substrates such as dense concrete and cement sand screed.  Vitri-Flow is ideal for fast track applications as it is sufficiently hard that tiles  can be grouted after only 2 hours.  Vitri-Flow remains flowable for approximately 5 minutes after which time it stiffens up and can be used as a normal adhesive.  It is specially designed to allow bed thicknesses of up to
25mm to be formed and hence is ideal for natural tiles with variable thicknesses. Vitri-Flow requires only the addition of clean water.