What mortar mix is used for ridge tiles?

1What the bed under a ridge tile has to do

Run your eye along the apex of a pitched roof and you will see a line of shaped tiles capping the join between the two slopes. Beneath each one is a bed of mortar, and that bed has three jobs. It glues the ridge tile to the courses on either side, it fills the void so wind cannot whistle underneath, and it takes up the small differences in height between one tile and the next.

It is a bedding material, not a structural one. On a sheltered inland roof a good bed can last a long time. On this coast the bed is attacked twice over: rain and frost soak into the surface, and gales keep tugging at the tile above. Strong mortar that shrinks away from the tile cracks open. Weak mortar, or mortar mixed with too much sand, turns to powder. Choosing a mix is about balancing those two failures, which is why a bag of random mortar from the shed is a gamble.

2Sand, cement and a plasticiser on newer roofs

On concrete tiles and many roofs laid in the last several decades, the usual bedding is clean building sand and cement. The sand needs enough sharpness to bind, but must not be so coarse that it will not pack down under a curved tile. A proper plasticiser goes in to make the mix workable and improve how it copes with frost. Washing-up liquid, a trade habit that will not die, can leave pockets of air and a weaker bed, so it has no place in the mix.

Colour matters more than people expect. A pale grey bed under dark red clay ridge tiles looks like a patch, so mortar is tinted to sit with the roof. We keep the joint tight and finished back from the tile edge so rain runs off it cleanly.

Exact proportions depend on the sand, the tile and the day. A mix that looks fine when laid can still fail in the first hard winter. If you want to see what that failure looks like from inside the loft, our page on a leaking ridge walks through the signs.

3Lime mortar for old clay and stone roofs

Victorian terraces, village cottages and anything built before cement became common were bedded in lime. Lime mortar is softer and more porous, and it can follow small movements in old timbers without snapping. Cement laid onto old clay creates a bed that is stronger than the tile around it. When the roof flexes, the tile edge gives way before the mortar does, and the damage shows as a chipped or split ridge tile.

That is why a lime-based mix is often the better match for older roofs, particularly where the building is listed or sits in a conservation area. Changing the materials on a listed building can require listed building consent, so ask the council's planners before you change anything, or the National Park Authority where the house lies within the Park.

Lime sets slowly and dislikes frost and wet. A lime ridge is a job for a mild, dry spell, and we look at timing on the page about laying ridge mortar in cold weather.

4Weather on the day matters as much as the recipe

Even the right mix fails if it goes on in the wrong conditions. Fresh mortar needs to stay damp enough to cure without being washed out, and it should not freeze before it has set. A heavy shower on a new bed can leave a pitted, weak surface, and a hard frost can break it up from within.

That is why a careful roofer watches the forecast, covers a finished run if the sky turns, and does not rush a bed on a cold morning. Sand that has been left open to the rain also behaves differently from sand kept covered, and the mix is adjusted on site, not worked from a fixed ratio on paper. If you are told a ridge can be done in any weather, treat that with caution.

5Why the mix is now only half the job

Marley's guidance on BS 5534 explains that after the 2014 revision, mortar tensile strength was no longer treated as enough to keep ridge and hip tiles in place, and that bedded fittings should have mechanical fixings too. It adds that building to the standard is not demanded by law. It is the benchmark a careful contractor works to, and the one an insurer or surveyor will measure a poor job against.

For you, the result is a ridge with two layers of security. Clips or screws take the load into the roof structure, and the mortar seals the joint and finishes the line. If the bed cracks in future winters, the ridge stays on and the fix becomes a simple re-point rather than a rebuild. The question of what the rules now allow is answered in whether mortar bedding is still allowed.

Cost moves with the length of the ridge and how awkward it is to reach. For typical UK ranges, see ridge tile repair cost, and for the whole service start at ridge tile repairs.