Dave Rowley, ARDEX group training and technical support manager, explains what you need to know when tiling porcelain over concrete.
The last few years have seen a massive increase in the popularity of external tiling installations using a range of materials, but primarily external self-supported porcelain (typically 20mm porcelain), slabs, pavers and natural stone.
There are two common installation methods – monolithic fixing (i.e. traditional paving onto a mortar bed) and traditional external tiling (direct fixing onto a concrete base).
Before the Spring patio and terrace season gets into full swing, here we outline the key considerations with both methods of fixing.
Monolithic fixing
When fixing onto pre-prepared Type 1 MOT sub-base or concrete where a build-up of more than 20mm is needed than we would recommend using the traditional monolithic fixing method.
This traditional wet-on-wet fixing method is favoured most by landscapers and paving installers and involves the use of a semi-dry bedding mortar mix and a priming slurry bonding coat on the back of the tiles.
Before fixing, you must ensure that the subbase is of a suitable Type 1 MOT grade, laid to the correct depth and fully compacted.
The subbase should be stable and not liable to settlement.
It is important to note that 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 or a soakaway at the edge of the terrace of patio. Wherever possible, surface water should be directed away from any buildings.
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.
Once fully prepared, we recommend that the Type 1 MOT or concrete subbase is primed with 2-3mm of a suitable priming slurry bond coat.
A bedding mortar should then be applied wet-on-wet and then compacted down to the required height.
The bedding mortar – whether it’s a pre-mixed product or suitable sand:cement mix – should be mixed to a semi-dry consistency. Normal “free-fall” mixers aren’t normally suitable for this type of semi-dry screed mix, so consider the use of a forced-action mixer.
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 within the working time.
Where a new bay is laid against a set and hardened mortar bed, it is recommended that such daywork joints are vertical and treated with a priming slurry bond, with the bedding mortar applied onto the wet slurry.
On Type 1 MOT sub-bases, bedding mortar should be applied as required from min. 25mm to max. 100mm thicknesses, while on concrete substrates, a bedding mortar can be applied as required from min. 10mm to max. 100mm thicknesses.
To fix an external porcelain tile, paver, slab or natural stone, 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 step is critical, as bedding mortar on its own will not naturally adhere to porcelain pavers. The priming slurry acts as the “bond-bridge” between the low-absorbent porcelain tile and the mortar bed.
Once in place, the tile or paver can be adjusted and then firmly bedded with rubber mallet.
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 sharp 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.
We would recommend the use of a cementitious grout that is suitable for external use.
However, consider the use of a hard-wearing, chemically resistant and durable epoxy grout system for driveways and eating areas for example.
Traditional external tiling to concrete
This method is most suited when fixing porcelain, pavers, slabs or natural stone (suitable for external use) direct to an existing concrete base.
This direct fixing method is essentially the same as internal tiling and can be used for patios, terraces or driveways.
The caveat being that the concrete must cure for a minimum of 6 weeks, expansion joints may need to be incorporated, and falls will need to be added 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 tiles from any subfloor moisture.
However, these installs also mean that 20mm (self-supporting tiles) are not a requirement. Porcelain of any 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.
Before direct fixing, the concrete base must be fully cured and free from efflorescence, laitance, dirt, polished concrete and other loose materials. These should be removed using suitable mechanical methods or even removed using high pressure jet washing. Some exposed aggregate should be visible in the surface but not loose.
No need for priming, simply dampen the surface with water, taking care not to leave any pools, before applying your tiling solution.
Where necessary, you can smooth the surface with an external levelling/smoothing compound. It is important to use any compound in accordance with the manufacturers instruction which can usually be found on the packaging. For isolated repairs, external repair mortars may also be considered.
If a levelling/smoothing compound is used, then protect this from adverse climatic conditions, e.g. rapid drying air flow, direct strong sunlight, rain, frost etc., until hardened.
Once the concrete is suitably prepared, external tiles, slabs, pavers or natural stone 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 with 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) and failure to do so, can lead to costly failure, efflorescence etc.
Once the adhesive has cured, grouting can commence with an appropriate cementitious grout which is approved for external use.
Again, consider whether an epoxy grout maybe more suitable – for example food preparation areas or areas of high traffic such as driveways.
The ARDEX Group has services to support installers including a nationwide team of training and technical support managers, plus technical advisors available on the phone or on-line for our brands.
01440 714939
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