External tiling considerations

Lewis Lupton discusses the most common methods of installation exterior tiles

THE last few years have seen an increase in the popularity of external tiling installations using a range of materials, but primarily external self-supported porcelain, slabs, pavers and natural stone.

There are two common installation methods – monolithic fixing (i.e. traditional paving onto a mortar bed with slurry system) and traditional external tiling (direct fixing onto a concrete bed).

The most common method used by professional tile fixers, is direct fixing onto concrete subfloors.
This method is most suited when fixing porcelain, pavers, slabs or natural stone direct to an existing concrete base.

Fixing to concrete has additional benefitsbenefits – tiles don’t have to be of the 20mm variety, therefore easier handling, cutting and finishing with a larger range of tiles available, lending itself to the indoor/outdoor look.

Fixing tiles to concrete externally is essentially the same as internal tiling and can be used for patios, terraces or driveways. The caveat being that expansion joints should be incorporated more frequently into the concrete, as well as appropriate falls, to ensure rainwater drains away fully. An effective DPM material should also be incorporated into any direct-to-earth subfloors – this is essential to protect the tile installation from any rising ground moisture.

Before fixing, ensure working in an appropriate weather window, the concrete must be fully cured (typically six weeks min) and free from laitance, dirt, and other loose materials. Any surface contamination should be removed using suitable mechanical methods or possibly jet washing. Some exposed aggregate should be visible in the surface but not loose.

Once prepared, and before applying tiles adhesives or smoothing and levelling compounds thoroughly dampen the substrate with water, taking care not to leave any pools. The use of acrylic primers isn’t required.

If necessary, it may be more cost effective to smooth a rough or uneven the surface, with an appropriate external levelling compound. Some external smoothing and levelling compounds may be mixed whereby the water content can be reduced, if a fall or gradient in the substrate must be maintained or followed.

Once the smoothing and levelling compound has cured, external tiles, porcelain slabs, cement pavers or natural stone can be fixed, using a suitable tile adhesive.

Some lateral movement may occur in external tiling projects owing to climatic change and temperature fluctuation, so an uncoupling solution may be preferred.

Traditional plastic, cavity uncoupling membranes may not always provide the best solution, as they can entrap moisture in the adhesive layer, causing grout discoloration and potential system issues as well as a significant increase the installation cost. However, there are suitable drainage membrane products in the industry, that offers an uncoupling function too, which can provide excellent performance.

If an uncoupling solution is required, consider a rubber-crumb tile adhesive with anti-fracture properties. This adhesive works as a tile adhesive and uncoupling solution in one and is water and frost resistant.

Consider rapid-setting or semi-rapid setting pourable tile adhesive for particularly large format tiles, natural stone slabs, where quick access is required or where the predicted weather window is short.

Pourable thick-bed tile adhesives give you the option to go to thicker bed-depths (some up to 25mm), perfect for uncalibrated stone and uneven or tamped concrete.

Back-buttering of the tiles should always be carried out – particularly on heavily keyed tiles to ensure 100% contact. This technique may not always be necessary when installing with pourable thick bed adhesives, however, it’s a great idea to check coverage/contact with the first tile installed, before settling on your chosen tile fixing method.

A solid bed of adhesive is always required and ensures peace of mind for the installation as well as guard from system failure, grout discoloration, water pooling (beneath the tile) and protect the tile when point loaded. Please note, spot fixing isn’t recognised as tile installation technique and is therefore deemed incorrect within British standards.

Once the adhesive has cured, grout application can commence with an appropriate grout that’s suitable for external use.

Coloured cementitious grouts with high durability and abrasion resistance, are great for external areas. For textured tiles and slabs consider applying a temporary protective sealer before grouting to avoid potential issues with staining.

Also consider the use of an admixture in an appropriate cement-based grout, for increased durability, flexibility and improved stain resistance.

Cementitious grouts require a maintenance programme. Always use cleaning products suitable for the tile/stone type and grout. Avoid using harsh chemicals or brick acids which might weaken the grout surface and/or damage the appearance of the tile or stone.

Epoxy grouts are waterproof chemical resistant and may require a reduced maintenance regime – but epoxy grouts are perfect for external areas in general.