The short answer: nobody can name a figure without seeing the roof, and one offered over the telephone is advertising rather than pricing. What sets the cost is knowable in advance though. A new flat roof is three separate prices added together, and only one of the three is the roof itself.
What are the three prices?
Getting to the roof, dealing with what is already on it, and building the new one. Most people assume the third dominates and are surprised by how often it does not, particularly on smaller buildings where the first two can between them outweigh every material that ends up permanently in place.
Separating them is not an accounting trick. It is the reason two honest contractors can look at one roof and produce different totals without either of them being wrong: they have made different assumptions about the first two, and only the second is testable before anybody starts.
Why does getting to the roof cost so much?
Because everything that goes onto a roof and everything that comes off it has to travel, and roofs are famously not at ground level. The question is never how high the building is on its own; it is what route exists between a vehicle and the working surface, and who else has to agree to it.
A roof reached across a flat rear extension from a side return is one job. The same roof with no side access, so that materials go through the house or round a block, is a different one before a single sheet is laid. Add scaffold where the access genuinely demands it, a licence where the pavement is involved, or an agreement with a neighbour whose ground the ladder has to stand on, and the getting-there price can rival the roofing.
What does removal actually involve?
Taking the old covering off, carrying it down, and paying for it to be taken away by weight. That last part is the one that catches people out, because waste is charged for what it weighs rather than for how large the roof looked from the garden, and old flat roof material is dense.
The multiplier here is history. Where two people have already overlaid it, there are three coverings to lift instead of one, and each was somebody's economical answer at the time. That is why the removal price for two identical-looking roofs can differ by a factor nobody would guess from the surface, and why the roofs people have re-covered themselves so often cost more to strip than to rebuild.
Where there is nothing to remove at all, because the roof is being built for the first time on an extension or a dormer, this entire price disappears and the quotation gets shorter and firmer at the same time.
What if the boarding underneath has been wet?
It gets renewed, and it is the one part of the job nobody can price in advance with certainty. Boarding does not recover from years of wet; once saturated it will hold neither a fixing nor a bond, so it comes out. How much of it comes out is genuinely unknown until the covering is off and somebody is standing on the structure.
The professional answer to an unknown is not to guess at it and it is not to ignore it. It is to price it in advance as a rate, so that if the roof opens up badly the arithmetic was agreed in daylight rather than negotiated on a Friday afternoon with the building open. What that looks like in a real strip and renew is a firm price for the work everybody can see and a stated rate for the work only opening it can reveal.
How much does the insulation move it?
Further than the price of the boards alone, which is what catches people out. Depth is set by what the design requires rather than by what the budget would prefer, and every extra millimetre above the deck has to be accommodated somewhere: at the upstands, at the edge trims, at the threshold of any door that opens onto the roof.
So insulation is rarely a line item on its own. It is a decision that reaches into the detailing, and renewing enough of a covering brings thermal performance into scope in a way that patching one never does. Where the build-ups differ and why is a subject of its own; the point here is only that the depth on your specification is not a number anybody chose to be generous with.
Why does the edge cost more than the middle?
Because the middle is the fast part. Rolling out and bonding a membrane across open field is the most productive hour of any roofing day; every upstand, corner, outlet, trim and abutment is hand work that cannot be hurried and is where roofs actually leak.
This is why area alone predicts a price so badly. Set a plain rectangle beside an L-shape of identical size wrapped round a stair with three rooflights in it, and you have two entirely different weeks of work at the same square metrage. When you compare quotations, the useful question is not how many square metres each has counted but how many junctions each has priced.
- Every wall the roof turns up against
- Every pipe, vent or aerial passing through it
- Every outlet, and whether it connects to something that works
- Every change of direction, level or material at the perimeter
- Every threshold where a door opens onto the finished surface
What separates a price from a guess?
Whether the person naming it has stood on the roof. Everything above is either invisible from the ground or invisible from the surface, and a figure produced without that visit has necessarily assumed the favourable answer to every one of them, which is exactly why such figures are always attractive and rarely survive.
A quotation worth comparing states what covering is going on and in how many layers, what insulation and to what depth, how each junction will be finished, what is included in access and disposal, and what happens if the deck disappoints. A written condition report before any of that gives every contractor the same facts to price against, which is the only circumstance in which comparing totals means anything at all.