Question answered
Gaps under the ridge tiles: is that normal?
The short answer
A gap under the ridge tiles can be normal. Dry-fix and ventilated ridge systems leave a controlled opening, covered by a roll or comb, so the roof space can breathe. A gap in a bedded ridge where the mortar has cracked, shrunk or dropped out is a fault, and it lets in wind-driven rain. A gap that is uneven, dark, crumbling or growing is the one to worry about.
A gap beneath the ridge is sometimes part of the design and sometimes a failure. The difference is whether it is even, covered and deliberate, or ragged, open and getting wider.
Why some openings are deliberate
Where the two slopes of a roof meet at the top, the tiles on each side stop short of the very apex, so there is always a space under the ridge tiles. What fills it decides whether the ridge works. Mortar is one answer. A dry-fix system is the other, with a roll of flexible material or a ventilated strip tucked under the ridge tile and held by clips.
On a dry or ventilated system, the opening is intended. A cold roof needs air moving through it to carry away damp, and BS 5250 sets out the principles. Marley's guidance gives eaves ventilation figures and says some roofs also need a continuous 5 mm opening at high level, for example where the pitch is over 35 degrees. A ventilated ridge detail supplies that, so a neat, regular and covered gap is the design doing its job.
We cover this in more depth on the dry hip system page, which uses the same principle at the hips.
Telling a designed gap from a failed one
Look with binoculars from the pavement or garden, and stay off the roof. A designed opening is regular along its whole length. You will see a uniform strip of vent material or a consistent shadow beneath the tile edge, and the ridge tiles sit level on top without any rocking or tilting.
A failed bed looks very different. The mortar is cracked in pieces, shrunk back from the tile, grown over with moss or missing in patches. The ridge line may wave, with one tile lifted at an end or a visible step between neighbours. Pale crumbs on the roof below or in the gutter often come from a bed that is falling out.
There is a middle case. A roof that was bedded years ago and then patched in places will show sound bed in one stretch and hollow bed in the next. That is a roof where the loft check tells you more than the view from outside, and our page on a leaking ridge shows the marks to look for.
What a real gap lets in
A fault at the ridge sits at the most exposed point of the building. Rain driven by a gale can be forced through an open joint onto the underlay and the timbers beneath. The opening also gives wind a way in, and a ridge tile with nothing but a broken bed holding it can lift or slide.
The damage usually shows in the loft rather than on the ceiling: dark tide marks along the ridge board and the tops of the rafters, and damp insulation. Crumbled mortar blocks gutters. Birds and insects will use an open gap too, so a roof can look as if it has a nesting problem when the real fault is the bed. Both bats and nesting birds have legal protection, so if you notice signs of either, get advice before work begins.
It rarely announces itself. A small gap can carry on for years and then let in a season's worth of damp in one storm, which is why a periodic look from the ground is worth it.
Gaps at the ends and where the ridge meets a hip
Ridge gaps are not only found along the straight run. The ends of a ridge, where it meets a gable, and the junction where the ridge meets the hips on a hipped roof, are the places a bedded joint is hardest to get right. The tiles are cut, the mortar is thick in some spots and thin in others, and water finds the thin ones first.
If you can see daylight, a dark void or loose mortar at one of those junctions, it is worth having it looked at even if the rest of the ridge appears sound. A dry system has its own end caps and trims, and a poorly fitted one can leave an open end that looks like a fault but is really an unfinished detail. Either way the end of a ridge is a spot to look at first, because it takes the wind from two directions.
Closing the gap so it stays closed
The repair follows the cause. If a bedded ridge has failed, the sound answer is to lift the tiles, rake out the old bed, and re-lay with fresh mortar and mechanical fixings. Since the 2014 revision of BS 5534 the mortar has been treated as a seal, not the fixing. Our page on whether mortar bedding is still allowed explains the wording.
If the roof suits it, a dry system can take the place of the bed. On slate, the answer needs more care; see dry ridge on slate. Filling the gap with foam or sealant is not a repair: it traps water, blocks any ventilation and leaves loose tiles loose.
We trace the gap from inside the loft before proposing anything, then send you photographs of the ridge with the written quote. The hub for the whole service is ridge tile repairs.
Ridge gaps: two more questions
Is daylight at the ridge from inside the loft a bad sign?
On a dry or ventilated ridge, a little light along the line can be part of the opening. On a bedded ridge, light through the line usually means the mortar has cracked or dropped out. Take a photograph and have it looked at.
Can I fill a gap under the ridge myself with expanding foam?
It is best not to. Foam traps moisture, blocks the air path and hides the fault while the tiles stay loose. A proper repair re-beds or re-fixes the ridge so it holds.
Tell us about your roof
Describe what is happening and we will call you back to arrange a visit.
- Photographs of what we find
- A written quote with one total price
- Or call 01903 299063
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