The roof you see is a covering laid over a structure. Rafters, purlins, wall plates and the walls beneath them were sized for the weight of the original covering, plus snow and wind. Change that covering for something heavier and you change the load the whole structure has to carry. Often the timbers cope. Sometimes they do not, and the signs only show once the old slates are off.
This guide is about the loading check: the rule in Approved Document A, how the Planning Portal words it, what a structural check involves, and what strengthening looks like when the answer is no. It is not about which material to choose; for that, see re-roof materials. The hub for all of it is our roof replacement page.
The 15% loading rule in Approved Document A
Approved Document A is the part of the Building Regulations that deals with structure. In the edition incorporating the 2010 and 2013 amendments, paragraph 4.4 treats a rise in roof loading of more than 15% as a significant change. It also suggests asking whether the covering coming off is the one the house was first built with.
In practice the rule asks one question: will the new covering weigh materially more than the old one? If the answer is yes, the roof structure has to be shown to cope. Paragraph 4.5 covers the case where it cannot. If the existing structure is unable to take the extra load, for instance where timbers are overloaded or bend enough to let water pond, strengthening work follows.
What counts as the covering
The comparison is between coverings: slates or tiles, plus their battens and underlay where these change. It is not the whole roof, and it is not the weight of the rafters. A roofer works from the manufacturer's weight per square metre for each product and compares the old with the new.
Why it matters on a house, not a shed
A roof structure holds up more than itself. Add weight and you add load to rafters, to the purlin that supports them mid-slope, and to the walls and foundations at the bottom. A change that looks small on paper can show up years later as a sagging ridge or a bowed slope.
How the Planning Portal puts it
The Planning Portal's re-roofing pages for England word the trigger a little differently from Approved Document A. Approval is needed, it says, when a new material makes the covering 15 per cent or more heavier. The structural checks it describes apply when new tiles are significantly heavier or lighter than the ones they replace, and it advises an engineer or surveyor takes a look before work begins.
More than 15% or 15% or more
The two sources do not use identical words. One says more than 15%, the other 15 per cent or more. The sensible reading is that around 15% is the point where the question stops being a matter of opinion. We do not try to work out which side of the line a borderline case falls on. If the difference is anywhere near 15%, ask building control, and treat the structural check as part of the job.
Lighter is on the list too
The Planning Portal names heavier or lighter. That matters because very light coverings can change how a roof resists wind uplift, and because the weight of the old covering may have been helping to hold the structure down. Check before assuming that going lighter is always free of consequences.
If the new covering is near 15% heavier than the old, ask building control before materials are ordered, rather than once the old roof is off.
What a heavier roof means in practice
Weights vary by product, and the data sheet for the covering you are considering is the source to trust. What we can give is the direction of travel and one sourced figure.
Stone slate
Cotswold District Council's 2000 technical guidance describes stone slate roofs as very heavy, at about 1,000 kg per 10 m². Roofs built for it were framed with sturdy timbers to match. If you put stone on a roof that was framed for something lighter, such as the slope of an extension, expect the loading check to matter.
Concrete tile on a roof built for slate
Natural slate is generally lighter than concrete tile, and many older houses were framed for it. Moving to concrete tile can bring a notable increase in the weight on the rafters. Whether it crosses the 15% line depends on the products compared, which is why the comparison is made on paper, using manufacturer figures, before the quote is final.
Clay and concrete
Clay plain tiles, double-lap concrete tiles and large interlocking tiles each have different weights and different batten spacings. Do not assume that two tiles of a similar look weigh the same.
Weather loads still apply
The roof also has to carry snow and resist wind, and a heavier covering changes how those loads reach the timbers. We do not quote snow or wind figures for the valleys here, because the engineer works from the design rules, not from local rule of thumb. What we do is give the engineer accurate details of the covering, the batten spacing and the pitch, since those are what their sums depend on.
Other changes that add weight
Weight does not come only from the covering. Extra battens, thicker insulation laid over the rafters, a layer of boarding, solar panels and a new ridge system can each add to the load. When we write up the check we note all of these, not just the tile.
The rules are the same whichever direction the change goes. The key is to compare the new roof, as built, with the old one, as it was.
The structural check: what is looked at
The check begins in the loft, with the rafters, the purlins and the ridge board, then the wall plates they sit on and the walls beneath. We measure sections and spans, note any sag, and look for decay, past movement or earlier repairs. Spans matter as much as sizes: a thin rafter over a short run can be fine, and a generous one over a long run can be marginal.
Signs the structure is already working hard
A dip in a slope, a ridge line that no longer runs straight, gaps at the wall head, cracked plaster below, and rafters bowed between supports are all warnings. So are rafter feet that have rotted where moisture got in. A roof like that may be at its limit with the covering it has. Adding weight would make it worse.
Who does the calculation
For anything near the threshold, the sound route is a structural engineer or surveyor who reviews the existing timbers and confirms the load they can carry, as the Planning Portal recommends. They may specify strengthening. Building control then looks at the proposal as part of the application. Stroud District Council's building control runs through the Gloucestershire Building Control Partnership, shared with Gloucester City, while a registered building control approver offers the private route.
What goes in the file
The old and new coverings with weights, photographs of the structure, the engineer's note where there is one, and any strengthening drawing. We keep it alongside the quote, so the reason for the work is clear. Our guide to when roof timbers need replacing explains the signs of decay that matter most.
Strengthening when the answer is no
If the existing structure cannot carry the new load, you have three realistic choices: strengthen it, choose a lighter covering, or leave the covering as it is.
What strengthening can involve
The detail depends on the engineer, but common approaches include adding new timber alongside weak rafters, fixing new support to the purlins so they carry the weight into load-bearing walls, replacing decayed members, and improving the connection at the wall plate so that the roof is tied to the building. Where deflection is already visible, the aim is a roof that supports the new covering without sagging further.
Why it is easier with the roof open
Strengthening is easiest once the old covering is stripped and the timbers can be seen. That is one reason to settle the check before the strip, so that the work is allowed for, not met as a surprise at the ridge.
Who pays for what
Strengthening is extra work that a like-for-like re-roof would not need, so it only appears in the quote if the check calls for it. If the survey finds the structure sound, there is nothing to add. If it finds weak timbers, you will see the repair and its cost before you decide, and you can still choose a lighter covering instead.
Choosing not to change
For a period house, the best answer is often to put back what was there: stone replaced by stone, slate by slate. That keeps the loading the structure was built for. Our guide on repair or replace a roof sets out when keeping the existing covering, and repairing it, is the better choice.
Approval, notification and how it appears on the quote
When the Planning Portal says you need approval because the covering is 15 per cent or more heavier, it means a Building Regulations application, not planning permission. A straight swap that leaves the outside looking the same is not normally development in planning terms, but a different covering can change the look, and a listed building or a conservation area brings in the council's planning team. Check with them before you commit.
The fire performance test
The Planning Portal also says approval applies when the fire performance of the new covering is significantly different from the old. A change of material can trigger that for reasons that have nothing to do with weight.
Thermal upgrade
If the work affects more than 25% of the building envelope or more than 50% of the roof, or the renovation involves exposing the rafters, Part L may call for better insulation in the roof. The Planning Portal sets the thresholds. A whole-roof change of covering usually crosses them, so loading and insulation are often decided together.
What you will see in writing
Our written quote after the survey covers the structural check, any strengthening, scaffold, strip-out, battens and underlay, the covering, labour and waste. Where we handle the notification, you will see that too. For typical figures, MyBuilder's guide updated 21 August 2026 puts a full re-roof at £5,000 to £12,000, and our new roof cost guide explains the detail. The work carries our 10-year workmanship guarantee, in writing.