BSI 5385
Some thin-bed cement-based adhesives have been specially formulated for use on suitably primed gypsum plaster. As with all plaster backgrounds success is very dependent on the plaster being sufficiently strong and dry throughout.
4.2.1.5 Application of adhesive and tiles: thin-bed
4.2.1.5.1 Notched trowelling method
For situations where dry conditions will prevail after tiling is completed, the notched trowelling method should be used. The final bed thickness of the thin-bed cement-based adhesive should not exceed 3 mm. If it is used at a thickness greater than this, excessive stresses may develop possibly resulting in cracking of the tiles and/or adhesion failure.
The adhesive should be applied to the surface as a floated coat with a trowel, pressing the adhesive into the surface, to give a bed thickness of about 3 mm, which should then be combed through with a notched trowel of the type recommended by the adhesive manufacturer. This will give a series of ribs into which the dry tiles should be pressed with a twisting or sliding action. This operation has to be carried out correctly to ensure that the adhesive wets the back of the tile and achieves an area of contact of at least 50 %, spread evenly over the back of the tile.
It is essential that the tiles should be fixed before surface drying of the ribbed adhesive bed prevents the adhesive wetting the back of the tiles. The period of time during which tiles can be adequately bedded after spreading the adhesive is the open time, this will vary according to the prevailing atmospheric conditions and is usually about 20 min. It is important not to spread more adhesive on the wall than can be covered with tiles within the open time of the adhesive.
A suitably designed notched trowel should be used for combing the adhesive. The important point is that whichever type of trowel is used, it should be such that adequate contact between tile and adhesive is assured. The amount of adhesive used and the height of the ribs obtained are governed by the angle at which the trowel is held against the surface. The amount of contact is also dependent on the twisting or sliding of the tiles as they are pressed onto the ribs of adhesive.
It is good practice to remove a tile occasionally as fixing proceeds to check that adequate contact and wetting is being maintained with the adhesive.
4.2.1.5.2 Buttering method
This method may be necessary for occasional awkward tiling positions, e.g. around openings and restricted areas where a notched trowel cannot be used. Where this technique has to be adopted the adhesive should be spread evenly over the whole of the back of each dry tile with a trowel. The bed thickness should be slightly greater than the final thickness required so that when each tile is pressed or tapped firmly into position the correct thickness is achieved. The thickness should not be greater than the maximum recommended by the manufacturer of the adhesive. Care should be taken to ensure that as far as possible no voids are left behind the tiles.
4.2.1.5.3 Notched trowelling and buttering method
This method combines 4.2.1.5.1 and 4.2.1.5.2 and should be used for fixing large tiles (e.g. 700 cm2 and above) and tiles with ribbed, deep keyed or heavy buttoned back profiles. A thin coating of adhesive buttered over the backs should fill the deep keys before placing the tiles in position on the combed adhesive bed. There should be no significant increase in the bed thickness.
NOTE This bedding method aims to achieve a solid bed but, in practice, it is inevitable that there will be a number of small voids.
4.2.1.6 Application of adhesive and tiles (thick-bed)
Where surfaces are not sufficiently flat and true to permit thin-bed fixing, some irregularity in the background may be accommodated by applying some cement-based adhesives as a thick-bed.
NOTE Thick-beds should not be used to make good deficiencies in the surfaces of backgrounds left by preceding trades.
In practice the amount of irregularity that can be taken up will be limited by several factors. Thus, for each adhesive suitable for thick-bed application, the manufacturer will stipulate the maximum thickness at which it should be used and the final bed thickness should not be greater than the maximum thickness. A limit to the thickness of adhesive used may also be imposed by the nature of the background and the circumstances of use. For example a thick bed of adhesive will tend to dry out slowly on a dense or impervious surface, and the bed thickness may have to be limited to prevent possible slumping as tiles are being fixed.
Usually thick-bed adhesives are used at an average bed thickness of about 6 mm, applying a thicker coating by the notched trowelling and buttering method (4.2.1.5.3). Some adhesives may be applied using the notched trowelling method described in 4.2.1.5.1. With other thick-bed adhesives, however, a notched trowel should not be used except perhaps to provide light ribbing only of the surface of the adhesive to facilitate subsequent application of tiles. The manufacturer’s instructions should be observed in every instance.
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