A roof covering is more than a weather skin. It is a load, carried by battens, rafters, purlins and finally the walls, and every one of those timbers was sized for the covering the house started with. Lay a heavier one on top and you add weight the frame was never designed for, often with no visible sign until years later, when a ridge begins to dip or a slope bellies between its supports.
That is why the regulations care about weight. They take no interest in whether you prefer the look of slate or concrete. They ask whether the roof can carry what you propose to put on it, and they name a figure at which the answer has to be checked and approved. The figure is roughly 15 per cent, and two official documents state it in slightly different words.
It helps to say at the outset that this is a Building Regulations test. It has nothing to do with planning, however often the two get muddled. What follows sets out the two wordings, how weights are compared, what a structural check involves and what happens when the survey concludes that the frame needs reinforcing.
Two wordings of one threshold
Approved Document A, the structural guidance, is the 2004 edition with its 2010 and 2013 amendments. Paragraph 4.4 defines a significant change in roof loading as an increase of more than 15%. Paragraph 4.5 then deals with the consequence: where the structure cannot carry the extra, it should be strengthened or replaced, and the work is classified as a material alteration.
The Planning Portal phrases the same idea differently. Its page on the roof and Building Regulations lists, among the reasons you will need approval, a new material that makes the covering 15 per cent or more heavier.
Why the difference exists
One says "more than 15%" and the other "15 per cent or more". They disagree only at exactly 15, and in practice nobody can weigh a roof to that precision. Different tile lines have different weights, laps and fixings, and the finished roof also carries battens, underlay and clips on top. So we never design to land just under the line.
We treat roughly 15 per cent heavier as the point where building control and a structural check come in. If the new covering is clearly lighter or about the same weight, we show you the figures that prove it, side by side, from the makers' own data sheets, so you can see the comparison for yourself. If it is close to the line, we assume it is over, and plan the approval accordingly, because guessing wrong costs far more than asking.
The 15 per cent test belongs to Building Regulations. A heavier covering does not by itself create a planning requirement, and a planning decision does not replace the structural approval.
How the two coverings are compared
The comparison is between the finished weight of what sits on the roof today and what you propose to put there instead. That counts the covering and its fixings, together with the battens (the timber strips that tiles hang from) and the underlay beneath. Tile and slate makers publish a weight per square metre on their data sheets, so we compare the actual products rather than lean on general assumptions about slate against clay against concrete.
Assumptions let people down here more often than they expect. A thin natural slate and a deep-profile interlocking tile are very different weights, and two clay tiles can differ from one another as much as either differs from a concrete one. The accurate answer to "is slate lighter than concrete?" is that it depends on the products.
Swaps that deserve an early look
Some changes are worth checking at the very start of planning. Moving from a lightweight covering to a deep-profile one is one. Replacing a stone-slate roof is another, because the timbers beneath were built for substantial weight: Horsham District Council's conservation appraisal observes that Horsham stone roofs rely on stout timbers set at a steep pitch to bear the stone. A third is any roof that has already been given a heavier covering at some point in its life, since the frame may be carrying extra already.
The roof as it is now
The base figure is the roof as it stands, not as it left the builder. A roof with a second layer of tiles laid over the first, or with mortar bedding that has soaked up water, carries more than its original design allowed. We record what we find and compare against that, so the starting point is real and not assumed.
Lighter is checked too
Weight problems are not only about adding load. The Planning Portal's re-roofing page says a weight check is called for whenever the new tile differs markedly in weight, up or down, so a move to a much lighter covering gets a look as well.
The reason is wind. A light covering resists uplift less well. Marley, the tile maker, explains that on pitches below 30° the windward slope can face strong suction, a negative pressure that tries to lift the covering away. On the Sussex coast, where roofs take sea gales, that is a live design question and no footnote.
Fixings carry the difference
Since the 2014 revision of BS 5534, each ridge and hip tile gets a clip, screw or strap on top of its mortar bed, because the mortar cannot be left to hold it unaided. For single-lap tiles, a manufacturer such as Wienerberger asks for a mechanical fixing in every tile, laid out to suit the site's position, height above sea level and exposure. A lighter covering may need a tighter pattern of fixings to meet the same wind loads.
Close to Shoreham airport, the tile maker may want to give special advice on fixing, because aircraft can create local wind effects stronger than the standard calculation allows for. We put that question to the supplier before ordering, not after delivery.
Why this belongs in the same conversation
Moving to a lighter covering rarely calls for strengthening, but it can raise questions about how well the roof resists lift and holds the walls together. The covering, the frame and the fixings behave as one system. Change one without checking the others and you get a roof that looked fine on the day and develops a fault a few winters later, usually in the first big gale.
What a structural check involves
The Planning Portal suggests getting a structural engineer or a surveyor involved before any work begins whenever the new covering is markedly heavier or lighter. That is sound advice, and we build the survey around it. It begins in the loft.
Inside the roof space
Here we measure the rafters and the gaps between them, then the span from one support to the next. We trace the purlins and the struts that carry their weight down to internal walls, and we inspect the wall plates, the lengths of timber bedded along the wall tops to receive the rafter feet, together with the ridge. We check whether any rafter has sagged between supports, whether joints have opened, and whether old alterations have notched or cut into timbers.
We also look for rot, beetle damage and damp. A timber that has lost section is no longer the timber the original design relied on, so a roof can be sound enough for its present covering and still be unable to take a heavier one.
On the outside
From the slopes we check whether they run true or show dips, bellies or bulges that point to a structural fault. A sight line along the ridge shows whether it has sagged. The condition of the walls matters too, because the load travels through them to the ground.
What comes out of it
The outcome is a view on whether the existing frame can take the new covering as it stands or needs work. Where there is doubt, we recommend a structural engineer's calculation. It is a judgement about the structure and not a roofer's opinion, and building control will ask for it. Its fee varies with how large the roof is and how complicated the timbers are.
When the finding is that the frame needs reinforcing
Approved Document A is clear about the consequence. If the structure cannot carry the extra load, it must be strengthened or replaced, and that work counts as a material alteration. A material alteration has to satisfy the regulations in its own right, so the strengthening needs approval itself and not only the new covering above it.
What strengthening can mean
It depends on what the survey finds. Typical measures include adding support beneath a purlin, fixing new timbers alongside weak rafters, replacing decayed lengths of wall plate or tying the roof structure to the walls more securely. The details come from the engineer, not from us.
Choosing the smallest job that works
Our rule is to put forward the least work that fully cures the fault. If swapping to a heavier covering forces a structural project you had not planned, it is worth asking whether a covering of similar weight to the existing one would give you what you want for a fraction of the cost. Sometimes it does. Sometimes the look of the house, or a planning rule, makes the heavier material the only acceptable choice, and then the strengthening is part of the price.
We tell you which case you are in. The price, which arrives in writing after we have been up in the loft, is one figure. Four things move it: access, the covering, the roof's area and the condition of the timbers. The structural work is the part most likely to surprise, which is why we go into the loft before saying anything about tile colours. If the survey shows earlier repairs have already left the frame weaker than it should be, we show you the evidence in pictures and explain the findings, so you can judge for yourself rather than take our word for it.
Special cases: solar, one slope, protected homes
Some jobs bring extra load or extra rules. Solar panels add weight, and a roof already close to its limit can be tipped over it. Our guide to fitting a new roof with solar explains how the two jobs fit together, and the covering and panels are best checked in one structural look instead of two.
Replacing one side only
When one slope is replaced and the other left alone, a heavier covering on one side puts the load unevenly on the ridge and the walls. It also leaves two different finishes meeting at the ridge. If you are weighing that up, read replacing one side of a roof first.
Listed and conservation-area homes
Materials are often not a free choice on a protected home, and a listed building needs consent for non-matching materials. Where the existing roof is stone, the loading review arrives together with the consent question. Both are explained in re-roofing a listed or conservation-area home.
Terraced and semi-detached houses
A shared roof structure means the frame and the walls are shared too, so a heavier covering has to work with your neighbour's roof as well as your own, and a party wall notice can come into it.
Who approves it
Either the council's building control team or a registered building control approver gives the sign-off. For work in Adur and Worthing, the shared team sits at Worthing Town Hall. We explain the consent and handle the notification, and we tell you early whether the extra weight means the rafters need checking before any tiles go on. The wider job is described on our roof replacement page.
