A roof covering is not just a skin. It is a load, sitting on battens, which sit on rafters, which pass the weight down into the walls. Rafters are sized for the covering they were built to carry, plus wind and snow. Change the covering to something much heavier and you change the sums.
Plenty of re-roofs involve a change of material. A homeowner wants to move from tired plain tiles to a stone-effect slate, or a landlord is offered concrete interlocking tile as the cheaper option. Each of those might be fine. Some are not, and the difference is invisible from the ground. This guide explains the 15 per cent line that building regulations draw, where it comes from, how the structural check is done and what happens when the answer is no.
If you are weighing up a new roof, our guide to the best time to re-roof sets out the timing side, and our roof replacement team can talk through materials at survey.
Where the 15 per cent rule comes from
Two official documents give the same idea in slightly different words, and it is worth knowing which says what.
The Planning Portal's guidance on re-roofing lists, among the cases needing approval, a replacement material that raises the load from the covering by 15 per cent or more. Approved Document A, the structural part of the Building Regulations, takes a similar line in paragraph 4.4, defining a significant change as an increase in loading of more than 15%.
The difference is a hair's breadth. The Planning Portal says "15 per cent or more"; Approved Document A says "more than 15%". For practical purposes both tell you to treat anything near 15 per cent as needing a check, and we would not build a plan around being a fraction under.
Is this a planning rule?
No. It is a Building Regulations rule about structural safety. Planning permission is a different question: a like-for-like re-roof generally falls outside it, with listed buildings and Article 4 areas as the exceptions. Our guide on re-roofing and Building Regulations sets out the bigger picture.
Does it only apply when the new covering is heavier?
The Planning Portal adds that weight checks also apply when the new tile is much lighter than the old one, not only heavier. A much lighter covering can change how the roof resists wind uplift and how it is fixed, so we check that direction too.
How the weight of a covering is worked out
Weight is quoted per square metre of finished roof, in kilograms. The figure comes from the manufacturer's data sheet and depends on the tile or slate, the gauge (the spacing of the battens) and the headlap, which is the overlap between courses.
For illustration, Marley states a concrete plain tile at a 100 mm gauge weighs 73.8 kg/m². A Bradstone reconstructed Cotswold-style slate laid at an 80 mm headlap is given as about 84 kg/m². Traditional stone slate roofs are described in Cotswold District Council's technical guidance as very heavy, at about 1000 kg per 10 square metres. These are product figures and traditional estimates, not a price list, and they show how different materials can be.
Can I just compare the two headline numbers?
You can start there. Going from 73.8 to 84 kg/m² is about a 14 per cent rise, which sits close to the line. But the existing roof's weight depends on what is actually on it: the tile type, any extra layers, the battens and the old underlay, and the bedding of ridges and hips. A figure from a data sheet is not a substitute for knowing what is on your roof today.
What else adds load?
Solar panels, extra insulation laid on the rafters, a second covering over the first, heavier battens and the mortar bedding for ridges and hips all add to the total. The Approved Document also suggests considering whether the covering being replaced is the original, as-built one, since earlier changes may already have increased the load.
The structural check: what is looked at
When the new covering is significantly heavier, someone has to ask whether the frame can take it. The check is not complicated in principle, but it needs to be done with the roof open or the loft inspected.
What does a surveyor look at in the roof space?
The rafters and their sizes and spacing, the span from the wall plate to the ridge or purlin, and the presence of ties, collars and struts. The condition of the timber matters as much as the size. A rafter with rot at its foot or beetle damage along its length is not carrying what it was designed to carry. We also look at the wall plates, the timbers on top of the walls that the rafters bear on, and at the purlins that give mid-span support.
Is any deflection already visible?
Yes, and it changes the answer. Approved Document A mentions overstressed members and deflection leading to ponding, and says a structure unable to sustain the extra load needs appropriate strengthening work or new roofing members. A roof line that already dips is a red flag, and our guide to sagging roof repair covers what causes it.
Who does the calculation?
A structural engineer, where the answer is not obvious. The Planning Portal's advice is to consult an engineer or surveyor before starting if the roof structure could need modifying or strengthening. The survey tells you whether we think yours does; a borderline case is never guessed.
When the timbers need strengthening
If the check shows the frame cannot carry the heavier covering, there are three ways forward: strengthen the frame, choose a lighter covering, or accept the lighter option from the start.
Strengthening is usually done timber by timber. Rafters can be reinforced by fixing a new section alongside the old (sistering), and collar or tie arrangements can be added to reduce spread. A rotted rafter foot can be cut back and spliced to a new length. Where the roof lacks intermediate support, a purlin or strut may be added to cut the span of each rafter. Approved Document A classes strengthening work or replacement of roofing members as a material alteration, so it needs to meet the Building Regulations.
Is it cheaper to choose a lighter covering?
Often, at least in terms of the structure. A change in covering that stays within the 15 per cent line means no structural work and no application for loading. The price of the covering itself changes with the material, and MyBuilder's new roof guide, updated in August 2026, gives typical roof replacement costs of £5,000 to £12,000 for a semi-detached house.
What about the walls?
Walls take the weight at the bottom, and older ones can be thin, bowed or in poor condition. We check the tops of the walls and the bearing of the wall plate. If the walls are not up to it, the job is bigger than a roof job, and we tell you plainly instead of quoting for something that will not hold.
Strengthening is a structural alteration, so approval is needed. Where consent is needed we explain it, and the building control notification comes from us where the work calls for it. Skipping it is not an option, because an unapproved structural change can surface later when the house is sold or surveyed.
Stone, slate and the historic roof problem
The heavier-covering question comes up most often with older houses, where the original covering may be stone slate, clay or natural slate, and the temptation is to replace it with something cheaper and heavier or lighter.
Stone slate roofs are some of the heaviest in the country, and the frames beneath them were built accordingly. Cotswold stone slate roofs are traditionally laid at pitches of around 47 to 55 degrees, and their weight is one reason the roof structures are substantial. If you replace stone with a lighter concrete product, the frame is safe on weight, but the effect on the character of the building may not be acceptable, and on a listed building it may need consent. The English Heritage policy quoted in Cotswold District Council's guidance says substitutes are not suitable for use on historic buildings.
What about going the other way, from light to heavy?
Replacing a thin natural slate with a thick concrete or stone-effect tile is the case that most often crosses the 15 per cent line. Slates are comparatively light, and many frames built for them are not generous. The same applies to some Victorian terraces, where original timbers are slender. Our re-roofing a terraced house guide covers the shared-roof side of this.
Do planning rules apply to a change of material?
A change of covering that affects the external appearance may not count as like-for-like, and in a conservation area, a listed building or an Article 4 area it can need permission. Raise it with your local council's planners before you order a single tile.
A re-roof with a heavier covering, step by step
On a job that changes the covering, the order of events is deliberate. Nothing is ordered until the roof has been looked at.
- The survey inspects the roof space and the outside, photographing the rafters, purlins, wall plates and any damage.
- The weight of the existing and proposed coverings is compared using data sheets and what is on site.
- If the change is close to or over the line, a structural engineer is brought in, and the result decides whether the frame is strengthened or a lighter covering chosen.
- Building control is told where approval is needed. Swindon Borough Council runs its own building control, and private approvers can also do the work.
- The covering is ordered and fitted, with fixings and battens suited to its weight and the roof's exposure.
Does scaffolding change with a heavier tile?
The scaffold is the same, but the delivery is different. Heavy tiles are lifted in packs onto the roof, and a hoist or crane may be needed on a site with poor access. Our guide to scaffolding for a re-roof explains how scaffold shapes the quote.
What if I only re-roof one side?
The same rules apply. A single heavier slope on a pair of rafters loads that side more than the other and can distort the frame. Our guide to replacing one side of a roof looks at that question.
After the survey you get one written total price and photographs of your own roof. Our workmanship guarantee runs for 10 years, in writing. To book a survey, ring 01793 626096 or message us on WhatsApp.