Sealing roof tiles - when it pays and how to do it

There is no obligation, and whether it pays off depends on the state of the roof and how much it gets overgrown. A roof tile sealer penetrates the material and limits water uptake. A roof that absorbs less dries faster after rain, is easier to clean and picks up dirt more slowly. Moss and algae need moisture, so a dry tile is a less welcoming place for them. A sealer will not change the conditions, though: a north-facing slope in shade dries slowly and growth will keep coming back to it, just less often.

You gain the most on a sound roof that darkens after rain and gathers green growth. You can check the protection by watching the water: as long as droplets bead on the surface, the layer is working. Once water soaks in and the tiles stay dark for a long time, it is time to seal again.

A concrete roof tile is concrete: absorbent and containing lime, which water can carry to the surface as a white bloom. Here a sealer limits both water uptake and the risk of efflorescence. A clay tile is fired clay. Unglazed, it absorbs water much like concrete does, while a glazed or engobed tile has a surface closed at the factory and will take noticeably less product. A drop of water tells you which one you have: if it soaks in, the tile will benefit from sealing.

One product covers both. The VD-1 roof tile sealer is made for clay and concrete tiles. It is a water-based water repellent that penetrates the material instead of forming a coating, has a water vapour diffusion resistance of Sd = 0.0 m and leaves the look of the tile unchanged. What a water base means in practice is explained in our guide water-based or solvent-based sealer.

If the same product should also cover the facade and the paving, go for ImperGuard Ultra 4 in 1: it makes roofs, render, brick, terracotta, slate and concrete water-repellent, and one litre covers about 8 m². A shingle roof is a different job - bitumen coverings take the EI-25 renovator. You will find every product for roofs and walls in the Facades and roofs category.

A shaded slope almost always carries algae or moss, and a sealer applied over them will lock them in. For washing, use the C-11D roof tile cleaner - an alkaline concentrate made only for clay and concrete tiles. It removes algae and moss growth, soot, grease, honeydew and leaf marks.

You dilute it to match the soiling: 30-50 ml per litre of water for light growth, 80-100 ml/l for moderate and 200-300 ml/l for heavy. Apply it with a hand sprayer, foam sprayer, brush or mop, leave it for 15-20 minutes and rinse with clean water. Apply and scrub the product by hand and use pressure only for rinsing - never aim the jet up under the tile overlaps, or you will drive water beneath the roof covering. Wash on a cool surface, in the morning or evening, never in full sun.

Moss rooted in joints or in damaged spots on a tile should come off mechanically before washing. Where green growth comes from and how to slow its return is covered in our guide to moss and green growth, and the products are gathered on the moss and algae page.

You seal a roof that has been washed, rinsed and has dried out completely, because water in the pores takes the place of the product. The full checklist is in our guide to preparing the substrate before sealing. VD-1 is ready to use, so you do not dilute it. Apply it with a soft roller, a brush or by spraying, in one generous, evenly spread coat, until the tile is saturated.

Plan the job for settled weather: protect the freshly sealed surface from rain and direct sun, because it becomes rain-resistant after 5-6 hours. How much you use depends on how absorbent the tiles are. Seal one square metre first, measure what went on, and multiply by the roof area with a small margin. How to get from a test square metre to the number of containers is shown in our guide how much sealer do I need.

Working on a roof is a risk in its own right. Wet, overgrown tiles are slippery, so use fall protection, keep off a wet slope, and wear gloves and safety glasses. On a steep roof, leave the washing and sealing to a roofer or a firm with the right equipment. Cover plants below the eaves and rinse them with water when you finish.

A sealer protects a sound roof, but it does not repair one. It will not hold together a cracked, crumbling or slipped tile - that tile has to be replaced. Nor will it stop a leak: failing flashings, valleys and ridge tiles need a roofer before any sealing. And it will not bring back the colour of a faded tile, because VD-1 is colourless and leaves the look of the roof unchanged. On an old, cracked roof, chemistry is no substitute for replacement.