Bradley Cutts, ARDEX Group training and technical support manager explains the most common methods of fixing tiles and slabs, and when to use which one
WITH the Spring patio and terrace season in full swing, it’s important to understand the two most common methods of fixing slabs and tiles – bound construction and traditional tiling.
Bound construction
We recommend using the bound construction method when fixing onto pre-prepared Type 1 MOT subbase – this involves the use of a bedding mortar mix and sometimes a priming slurry bonding coat.
Before fixing, you must ensure the ground conditions and the subbase are suitable prior to the installation of a suitable Type 1 MOT grade, laid to the correct depth of 100mm (min) and fully compacted. It’s crucial to ensure the subbase should be stable and not liable to settlement.
It’s important to note that when installed, all pavers and tiles should be free from standing or pooling water, so ensure that adequate falls are introduced to the Type 1 MOT or concrete to drain the water away to a suitable disposal point i.e. a gully, linear drain, soakaway or the edge of the terrace or patio. Direct surface water away from buildings where possible.
Where the patio is adjacent to the house, particular attention should be paid to drainage, and the patio’s height must be significantly below that of any damp proof courses, typically at least 150mm.
Once fully prepared, when using an ARDEX Group bedding mortar, we recommend the Type 1 MOT or concrete subbase is primed with 2-3mm of an ARDEX Group priming slurry bond coat. However, in BS 7533, for site category IV i.e. patios, it’s not essential to slurry the surface for all laying course materials/bedding mortar, but it will help in stabilising the MOT and increase the overall strength of the monolithic system.
A bedding mortar should then be applied wet-on-wet and then compacted down to the required height, approximately 30mm.
When using an ARDEX Group bedding mortar, consider the use of a forced-action mixer, as traditional “free-fall” or bell mixers aren’t suitable for this type of semi-dry screed mix and create clumping of the mixture.
The amount of mortar mixed and the area to be screeded should be limited so that trowelling off, finishing and tile fixing can be completed in the working time.
Where a new bay is laid against a set and hardened ARDEX Group mortar bed, it’s recommended that such daywork joints are vertical and treated with an ARDEX Group priming slurry bond, with the bedding mortar applied onto the wet slurry.
On Type 1 MOT subbases, ARDEX Group bedding mortars should be applied as required from 25-100mm thicknesses, while on concrete substrates, it can be applied as required from 10-100mm thicknesses.
To fix an external porcelain tile while the bedding mortar is still wet, apply another coat of priming slurry coat at 2-3mm on top of the bedding mortar, as well as a thin coat to the back of the tile or paver to ensure 100% coverage is achieved. This is also recommended with natural stone and slabs.
This step is critical, as the bedding mortar on its own will not naturally adhere to porcelain pavers. The priming slurry acts as the “bond” between the low-absorbent porcelain tile and the mortar bed:
With fast setting pre-mixed mortars, grouting can commence in as little as 12 hours. With a hand-mixed sand:cement mortar mix this can be anything from 24-48 hours – significantly extending project timescales.
When using traditional OPC (Ordinary Portland Cement) and sand mixes, consideration must also be given the quality of sand along with its potential water content, as well as the prolonged set times associated with standard Portland cement formulas, which can be in excess of three weeks, even in good conditions.
We recommend the use of a cementitious grout that’s suitable for external use.
Traditional external tiling to concrete
When fixing porcelain or natural stone tiles suitable for external use, we recommend the direct fixing method. This is most suited when tiling direct to an existing concrete base. This method is essentially the same as internal tiling and can be used for patios, terraces or driveways.
The caveat being the concrete, must cure for a minimum of six weeks, expansion joints may need to be incorporated, and falls will need to be included in the concrete to ensure rainwater drains away fully. An effective DPM should also be incorporated into any direct-to-earth subfloors. This is essential to protect the tiling solution from any subfloor moisture, which may cause joint discoloration and efflorescence.
However, these installs also mean that 20mm (self-supporting tiles) aren’t a requirement. Porcelain of most thickness may be installed, as long as permitted for external use by the manufacturer. Which means potential reduced tile cost with easier handling and cutting and more tile choices available too.
Before direct fixing, the concrete base must be fully cured and free from efflorescence, laitance, dirt, polished finishes and other loose materials. These should be removed using suitable mechanical methods – some exposed aggregate should be visible in the surface but not loose.
Crucially there’s no need for traditional uncoupling systems when tiling externally -unless these are specialist drainage mats with capillary passage drainage. Traditional uncoupling mats can cause damage to external tiling systems by trapping water beneath the tiles in cavities.
When levelling concrete, there’s no need for priming, simply dampen the surface with clean water (without pooling), prior to applying a smoothing compound or tile adhesive -this will hugely improve the performance of your fixing solution.
Where necessary, you can smooth the surface with an external smoothing compound, however it’s important when doing so, falls created in the concrete pour for drainage, must be maintained. For isolated repairs, external repair mortars may also be considered.
Once the concrete is suitably prepared, tiles can be fixed using a suitable external tile adhesive. Your selection may be based on weather considerations and of course may also be impacted on the tile or stone type and size too.
Some external tile adhesives can be used directly onto tamped concrete without the need to pre-smooth. This may be an option if there are time or budget constraints, but choose your adhesives correctly, as not all can be “built up”.
Back-buttering of the tiles should always be considered – particularly on heavily keyed tiles to ensure 100% adhesive contact.
Solid-bed fixing of the tile is critical (as per British Standards) so no voids remain beneath, and failure to do so can lead to costly failure, water entrapment, joint discoloration, efflorescence etc.
Once the adhesive has cured, grouting can commence with an appropriate cementitious grout which is approved for external use.
The ARDEX Group has several services to support installers including a nationwide team of training and technical support managers, plus trusted technical advisors available on the phone or on-line for our BAL, ARDEX and DUNLOP brands.
01440 714939
info@ardex.co.uk
www.ardex.co.uk
