Can You Replace Just One Side of a Roof?

When a single slope makes sense

Roofs do not age evenly. The slope facing south-west takes the rain and sun, while the north slope stays damp and mossy but is shielded from the worst of the weather. A south-facing slope can have tiles that have lost their glaze, nails that have rusted and underlay that has gone brittle, while the other side has years left. Sometimes the cause is an old repair: one slope was re-covered after storm damage while the other kept its original tiles.

In those cases a one-slope re-cover is a fair option. It also works for a roof over an extension or a dormer, where the work is limited to a distinct plane. The test is whether the two sides are genuinely at different stages and whether the remaining side will last long enough to justify not doing it now.

Why the saving is not 50%

Scaffolding goes up whatever the size of the job, and on a pair of semis it often has to cover a whole elevation. Skip, waste removal, delivery and the first and last day of setting up cost much the same for one slope as for two. Labour and materials do come down in proportion to the area, but the fixed costs stay. If you come back in a few years to do the second slope, you pay the fixed costs again. For that reason, a full re-roof sometimes makes more sense financially, where the second side is within a few years of needing it. Our page on scaffolding for a roof replacement explains the cost.

The junction at the ridge and hips

This is where half-roof jobs succeed or go wrong. The old and new coverings meet at the ridge, and the ridge tiles sit across both. If the old slope is in tired condition, the ridge bedding on that side is probably crumbling too. The ridge is normally removed and refixed across the whole length, with mechanical fixings, which BS 5534:2014 expects. A new covering is also likely to sit slightly differently in height from the old, since underlay, battens and tile thickness vary. A good roofer deals with that at the ridge and at hips and valleys so the water has a continuous path.

A match in appearance is the other issue. New tiles seldom match weathered ones for colour, so the new slope will look different for a few years. If the new slope faces the street, it will be seen. That is a decision for you, and on a house in a conservation area, check with your council's conservation officer before changing the look of a slope visible from the road.

The building regulations thresholds

Re-covering part of a roof does not normally need a building control application if the work affects less than 25% of the building envelope and less than 50% of the roof. One slope of a simple pitched roof is usually half the roof, which puts it close to the line. If the work goes over, approval is needed and the insulation of the whole roof may have to be upgraded. Our page on building regulations for roof replacement sets out the rules, and your building control body can confirm how they apply to your house.

Checking the side you are keeping

One point to check from inside: if the old slope is staying, we look at it from the loft. Daylight at the tile laps, stained rafters, damp felt or sagging battens will tell us whether it really has years left. We photograph those findings so you can see them. A half-roof job on top of a failing second slope is poor value, and we would rather say so than quote for it. The underlay on the old slope is the usual giveaway: if it is brittle or torn where it meets the new work, water will find the seam. Gutters and fascias along the eaves of the kept slope are worth a glance as well, since they were fitted at the same time as the tiles and may be tired too.

If the survey shows the second slope is sound, we quote for one slope as a fixed written price, with the ridge and junction included and described. Ten years of workmanship cover is put in writing with it. You can read the wider comparison between partial and full jobs on our page about re-roofing versus roof replacement, and the complete service on our roof replacement page.