A Tiled Roof Is a Drainage System, Not a Sealed One
Tiles do not form a waterproof layer and were never intended to. They overlap so that water runs down the surface and off the edge, and a certain amount of wind-driven rain always gets past them. That is by design.
What stops that water reaching your ceiling is the underlay, the membrane sitting on the battens beneath the tiles. The underlay is the actual waterproofing on a tiled roof. The tiles are the weather shield that protects it from UV and impact.
This matters because of what people are routinely sold. A contractor offers to spray a sealant or a coating over the tiles to waterproof the roof. It looks like a solution. It is not.
Why Sealing a Tiled Roof Backfires
Coating over tiles does three things, and two of them are bad.
It traps moisture in the roof. A tiled roof breathes. Water vapour from the house rises through the ceiling into the roof void and out through the tile joints. Seal those joints and the vapour has nowhere to go. It condenses on the underside of the tiles and on the timbers, and you get rot and mould in a roof that was previously dry.
It hides the real fault. Almost all tiled roof leaks come from specific places: cracked or slipped tiles, failed ridge bedding and pointing, valleys, and flashings where the roof meets a wall or chimney. A coating over the top does not repair any of those. It covers them.
It fails in a way that is expensive to reverse. A coating that cracks and peels has to be removed before proper repairs can be done, and it takes tile surface with it. Roofs get replaced for this reason.
What actually waterproofs a tiled roof: intact underlay, sound bedding and pointing at ridges and hips, properly formed valleys, correct flashings at every abutment, and tiles that are whole and sitting where they should. That is the work.
Where Tiled Roofs Actually Leak
Ridges and hips. Old sand-cement bedding cracks and the pointing falls out. Flexible pointing compound is the modern fix and it moves with the roof instead of cracking.
Valleys. Where two slopes meet, water concentrates. Valley metal corrodes, debris collects, and the tiles either side get cut short or bedded badly.
Flashings. Every point where the roof meets a wall, chimney or parapet. Mortar fillets crack. Proper flashings chased into the wall do not.
Slipped and cracked tiles. Broken tiles let water straight onto the underlay, which is fine if the underlay is sound and a leak if it is not.
Failed underlay. Older roofs used bituminous felt that becomes brittle and tears at the battens. Once the underlay has gone, the roof relies entirely on tiles being perfect, which they never are.
Cape Town Tiled Roof Problem
Winter rain here arrives on north-westerly fronts at 50 to 65kph, which drives water horizontally under tile overlaps and up the slope. A roof that copes with vertical rain leaks in a front.
That is exactly why the underlay matters more in Cape Town than in a low-wind region. Wind-driven water getting past the tiles is normal and expected. Sound underlay makes it a non-event. Failed underlay makes it a ceiling stain.
The summer south-easter does its own damage, lifting and cracking ridge tiles with weak bedding, ready for the next winter.
