The short answer
Ridge tiles on modern concrete or clay roofs are normally bedded in a sand and cement mortar, often coloured to match. Older roofs, soft clay tiles and listed buildings call for a lime-based mortar instead, which is more flexible and lets moisture out. Whatever the mix, mortar alone should not hold the tiles, and mechanical fixings are expected as well.
The ordinary mix for a modern roof
On a roof covered in concrete interlocking tiles, or modern clay tiles, the ridge is usually bedded in a mortar made from builder's sand and cement. The sand should be a clean, sharp type that packs well, and the cement binds it. The proportions are set by the roofer for the exposure and the tile, rather than a figure that fits every job, and the mix must be workable enough to bed the tile and firm enough to hold its shape.
Colour matters. Plain grey mortar against dark tiles looks like a scar, so the mix is often tinted to sit in with the tile, and a sample dried on a spare tile shows the true shade before the whole ridge is laid. Some roofers use a ready-mixed bedding product with a bonding additive, which keeps the colour and the strength consistent from one batch to the next.
The mortar sits in the apex of the roof, where it takes rain, wind and frost, and where it cannot easily dry out. A too-wet mix will shrink and crack. A too-dry mix will not bond. Getting the consistency right at the point of laying is more skilled than it looks.
Why older roofs need lime
An older roof is a different case. Traditional clay tiles, stone slates and the soft brick or stone beneath them were built to cope with moisture by letting it move through and out. A hard, strong cement mortar works against that. It does not flex, and it holds water in the tile instead of passing it on. Over time, the tile or the stone nearby can crack, flake or decay faster than it otherwise would.
Lime mortar is softer, more flexible and more permeable. It lets the roof breathe and accepts small movements without cracking. For that reason, conservation guidance and the Bath Preservation Trust favour lime for historic work. In its comments on planning applications the Bath Preservation Trust has argued for lime on historic Bath buildings, because a hard cement mortar can trap moisture against old stone.
That is a casework example, not published policy, but it shows how the preference is applied in Bath. Our page on lime mortar ridge covers the practical side, and the question of whether a listed roof can take anything other than lime is in dry ridge and listed buildings.
Strength is not the aim
One of the commonest mistakes on a ridge is to assume stronger means better. A very hard mortar looks durable, but if it is harder than the tile it holds, the weaker material takes the strain. On a soft clay tile, a hard bed can crack the tile at its edge or pull its surface away when the roof moves.
The aim is a bed that is compatible with the tiles, a little weaker than them, and able to move. It should also be dense enough to shed water, because a crumbly bed lets rain through to the timbers below.
The mortar also needs the right conditions to cure. New bedding laid in hard frost, driving rain or hot, drying sun will not set properly. That is why ridge work waits for suitable weather, and why a job done in a rush in poor conditions often starts to crack within a year or two.
Mortar by itself is not enough
Whatever the mix, the rules of good practice have changed on how ridge tiles should be held. BS 5534:2014 recognises that mortar by itself cannot be trusted to secure ridge and hip tiles, and asks for mechanical fixing wherever a tile or fitting is bedded. In practice that means a clip, screw or strap into the timber beneath, on top of the mortar bed.
Mortar keeps a role in the build. It fills the gap, takes the weight evenly, keeps water out and gives the ridge a finished line. But it is no longer the thing keeping the tile on the roof in a gale. That has moved to the fixings.
A dry-fix ridge goes a step further and removes the bed entirely, using clips and a vented roll, with the test specification BS 8612:2018. Our note on dry verge cost looks at the same question at the gable edge.
Choosing the right mix for your roof
How old the roof is, and what covers it, comes first. A 1990s estate with concrete tiles can take a sand and cement bed. A Victorian terrace with clay tiles, or a stone-slate roof on an older house, should be assessed before anyone mixes anything, and a listed building almost always needs lime.
The second is what is there already. If a ridge was bedded in lime and has lasted well, it is sensible to match it. If a cement bed has cracked the tiles, the repair should not repeat the mistake.
Ask any roofer you use what mortar they plan to use and why. The answer should mention the tile, the age of the building and the fixings. A roofer who says one mix suits every roof has not looked at yours. When we survey, we note the tile, the existing bed and any listing, and the quote states the mix. The re-bed process is described step by step elsewhere, and the rest of this work sits under ridge and verge repairs.
