The short answer
Yes, you can replace one slope of a pitched roof. It makes sense when only that side has failed, usually the one facing the weather. Building Regulations approval is usually avoided while the work stays under 50 per cent of the roof and 25 per cent of the building envelope. Matching the covering and the ridge line takes care.
When one slope makes sense
Roof slopes do not age evenly. One side may take more weather, more sun or more water from a blocked valley, and show failing fixings and slipped pieces while the other remains sound. A single leak at a chimney can likewise damage only the slope below it.
In those cases a one-slope re-roof saves money and disruption. The scaffold is smaller, the strip is half the size, and you stay in a house that is largely untouched. The decision rests on the survey, not a guess from the ground, because a roof's weakest parts are underneath. We look at both sides from outside and from the loft, with photographs, before recommending one slope or both.
A one-sided job is a poor fit when the other slope is the same age and condition and will need doing within a few years. Scaffold then has to go up twice, and the extra cost of doing both now is often smaller than it first looks. Our comparison of repairing or replacing a roof helps with that call.
Ask whether the slope you keep has the same fixings, underlay and batten spacing as the one you replace. Mixing a modern slope with an original one is fine, provided the join is detailed correctly.
Where the 25 per cent and 50 per cent limits come in
Planning Portal sets the line at 25 per cent of the building envelope and 50 per cent of the roof. Keep under both, together, and building control is not normally needed.
A simple two-slope roof has two equal slopes, and replacing one is about 50 per cent. That sits right at the boundary, so a gable roof with equal sides is the case that needs a careful look. A hipped roof, a roof with a rear outshot, or a main roof with a lean-to gives different proportions, and a slope that is clearly under half is easier to place on the safe side.
Cross the limits and an insulation upgrade can follow, because Approved Document L asks for the entire roof element to be upgraded once over 50% of its surface is renovated. Our explainer on the 25% and 50% rules goes through it. Ask building control, either the council's or a registered building control approver, before you settle the plan.
If your roof is close to the line, your building control body can say which side of it you fall. Get that answer before the scaffold goes up, because changing the scope halfway is awkward.
Matching the new slope to the old
The harder part is appearance. A new slope of the same material will not look like its neighbour for some years. Clay tiles weather, slate fades, and stone slates gather lichen. On a visible slope, a patch of bright new covering can stand out from the road.
There are ways to reduce it. Reclaimed slate, tile or stone can match an older slope better than new, though supply varies. Cotswold District Council's guidance cautions against stripping other old buildings for second-hand stone, and the English Heritage policy it reproduces limits reclaimed slates to the building they came from.
Which side is replaced matters too. If the failing slope faces the road, the mismatch is on show. If it faces the garden, it matters less. The ridge, verges and any valleys at the join must be tied together properly, or water will find the seam.
Weathering can be helped on a stone or clay roof by choosing a covering with natural variation of colour, and by blending in a few reclaimed pieces where supply allows.
Where the two slopes meet
The ridge is the detail that makes a one-sided job harder than it looks. Ridge tiles sit across both slopes, so replacing one side means disturbing the ridge. Marley explains that BS 5534:2014 no longer lets mortar carry ridge and hip tiles alone, so a rebuilt ridge gets clips or screws as well.
Valleys, hips and verges at the edge of the new slope have to be sealed against the old work with lead or the system's own flashings. Battens and underlay on the new side must lap correctly with the old. You can read more in our note on battens and underlay.
Temporary cover matters during a half-roof, because one side is open while the other is fine. Our answer on rain during a re-roof explains how the open slope is made weathertight.
What goes in the quote
The quote arrives in writing after the survey and covers the whole job. For a one-slope job we quote for the ridge and the junctions too, so you can set the partial route against a full re-roof.
National figures are only a guide. MyBuilder's 2026 guide gives a full re-roof at £5,000 to £12,000, with gable roofs at £5,000 to £8,000. A single slope is not half of that automatically, because scaffold, ridge work and set-up costs stay fixed. Ten years of written workmanship guarantee cover our own work, and the waste leaves site with us. To arrange a survey, start at roof replacement in Stroud.
A partial job suits owners who want to spread the cost, provided the second slope is checked and a plan for it is agreed. We note its condition on the survey either way.