How the two differ in kind
These are two genuinely different ways to make a roof watertight, and the difference starts with what turns up on the pavement. EPDM is a cured synthetic rubber sheet, thin, black, flexible, and manufactured wide — wide enough that the roof of an ordinary single-storey extension can be covered by one piece with nothing joined across the middle of it. It is unrolled dry, allowed to relax, then bonded to the deck with a cold adhesive, and its joints are made with butyl splice tape and primer rather than heat, because a thermoset rubber cannot be melted back together the way a thermoplastic can. Torch-applied modified bitumen is asphalt reinforced with a polyester or glass mat and modified with polymer to keep it flexible, supplied in narrow rolls, and installed as two plies: a base sheet and a granule-surfaced cap. The roll is unwound ahead of a propane torch that melts its underside until the bitumen flows, and the sheet is bedded into its own molten asphalt as it goes.
Everything downstream follows from how much joint each of those methods creates, and it is not a small difference. A membrane's seam length is not a property of the product, it is a property of the product meeting YOUR roof: EPDM's field lap length behaves as a step, either zero because the roof fits inside one sheet, or one lap for every sheet width the roof runs past — and which of those you get is decided by the width your supplier actually stocks, not by the material, which is exactly what the roll coverage figure in the EPDM tool encodes. Set that figure to a narrow strip roll and the tool will hand you a stack of rolls and the laps that go with them; set it to the wide sheet the roof is really getting and it will not. Modified bitumen has no such switch: its lap length is close to arithmetic — roof area divided by the effective width of a roll, and then doubled, because the base ply generates the same tally the cap does. Tens or hundreds of hand-made metres against a handful, or against none. That asymmetry is what the two calculators on this page inherit: the EPDM tool counts rolls against bare roof area and states plainly that overlap and flashing are not in the figure, while the modified bitumen tool nets the lap out of its coverage figure first and then adds a scrap allowance on top, because the torch consumes material along every overlap it makes, embedding granules and scorching the sheet as it goes. Read that second tool for exactly what it is, though: it sizes the CAP alone. The base ply is a second count of its own, on the built-up base sheet calculator, and skipping it is how a two-ply roof gets ordered half short.
Then the deck votes, and on an extension it usually votes loudest. Most house extensions have a timber deck, sit against the wall of an occupied building, and often carry a parapet or an abutment with a cavity behind it — which is the exact geometry that makes an open flame a governed activity rather than a technique. Torching is hot work under NFPA 51B and its equivalents: a permit, a charged extinguisher, a fire watch that continues after the flame is out, and a set of manufacturer and insurer restrictions on what may sit under the torch, because bitumen is never melted directly onto bare timber or onto foam insulation. None of that applies to a cold-bonded rubber sheet, which is why so many small extension roofs end up as EPDM regardless of how the two systems compare on paper. So the honest order of questions is this. What is the deck, and is anything combustible within reach of the flame or hidden behind the abutment. Will anyone walk on this roof, put plant on it, or drop things on it. Does it fit inside one sheet. Is anything ever going through it later — a rooflight, a flue, a second extension. And who is actually doing the work, because one of these is a cold, quiet, pausable job and the other is a skilled trade with a licence and a fire watch attached.
The factors that actually differ
| EPDM rubber membrane | Torch-applied modified bitumen | |
|---|---|---|
| How much joint the roof ends up with | A step, not a rate. The sheet is manufactured wide, so a modest extension is frequently covered with no lap at all in the field and the only made joints on the roof are the corners, the upstands and the outlet. Seam length does not grow with area at all — it jumps only when the roof outgrows the width you can buy. | Close to proportional. Narrow rolls mean lap length tracks the area divided by effective roll width — and then doubles, because the base ply makes the same tally again before the cap starts. A bigger roof buys strictly more hand-made joint. |
| How a joint is made, and how it is proved | Cold chemistry. The lap is cleaned, primed and taped, and the bond develops over hours rather than announcing itself. There is nothing to watch happening, so the discipline is in the preparation — contamination and a rushed primer flash-off are what fail, months later. | Visible thermodynamics. The torch takes the compound to flow and the tell is bitumen bleeding out along the edge of the lap in a continuous bead. Too little heat is a joint that never bonded; too much scorches the sheet and burns off the reinforcement. The evidence is on the roof while you work. |
| Layers between the room and the weather | One. A single ply is the whole waterproofing system, so a puncture is a leak, and the sheet's thickness is the entire margin. The compensation is that there are almost no places for one to start. | Two, deliberately. The cap protects the base and the base backs up the cap, so a pinhole, a lifted granule patch or a small mechanical injury meets a second waterproof layer rather than the deck. Redundancy is the reason this system persists. |
| Open flame, and what may sit beneath it | None anywhere in the process. No permit, no fire watch, no restriction on working against a cavity wall or over a timber deck, and no exclusion period before the building can be reoccupied — which on an extension attached to an occupied house removes an entire category of constraint. | Hot work, with everything that carries: a permit, extinguishers, and a fire watch continuing after the torch is off, to the duration the permit and the insurer set. Manufacturers require a mechanically fastened base sheet or a suitable cover board between the flame and any combustible deck, and torching directly onto foam insulation is never permitted. |
| The window you can work in | Temperature is the binding constraint. Adhesives carry a minimum application temperature and contact adhesive has a flash-off window that heat and humidity both move, so a cold snap stops this job outright where it would not stop the other. When it does stop, it stops safely — at any row, with no cold joint created and nothing half-bonded left overnight. | Wind is the binding constraint. The flame is indifferent to low air temperature and the roll is bedded instantly, so cold weather that would shut the A side down barely registers here — but gusty conditions turn flame control into a safety question rather than a quality one, and that is a call made on the morning, not in the forecast. (A dry deck is not a differentiator: both systems require one.) |
| Living on the surface | A thin rubber skin. Dropped tools, ladder feet, grit ground underfoot and hail all reach it, so any route that gets regular traffic wants dedicated walkway protection designed in rather than added after the first puncture. | A mineral-surfaced wearing course. The granules exist to take ultraviolet and abrasion, so the roof tolerates the aerial installer, the window cleaner and the boiler service without a purpose-built path — the practical reason it stays the default where anything sits on the roof. |
| What it is allowed to touch | Fussy about petroleum. Oils, solvents, greases and asphalt itself swell and degrade cured rubber, so it may not be laid in direct contact with an old bituminous roof and must not sit downstream of a kitchen extract discharging grease — a separation layer or cover board is a requirement, not a refinement. | Asphalt meeting asphalt. It is chemically at home on an existing bituminous roof, which makes it the straightforward recover over an old built-up or felt roof, and it shrugs off the same oils that attack rubber. |
| Cutting into it afterwards | Cold, small and local. A new penetration or a repair is clean, prime, patch — a kit, an afternoon, no permit, no notification, and it can be done on a building nobody has to leave. The rubber accepts a patch decades after it was laid. | Hot work again, every time. A torched patch means the permit, the fire watch and the trade come back for a small job, which is why cold-applied mastics and self-adhered patches do so much of the repair work in practice. Matching granule colour on a weathered cap is the cosmetic half of the problem. |
| How the cost is shaped | Material-light, with the labour concentrated at the edges. Thin sheet and adhesive against a job whose skilled hours sit almost entirely in upstands, corners, outlets and terminations — so a small, cut-up roof with many penetrations punishes it disproportionately, while a large plain rectangle is where it is at its most efficient. | Material-heavy and labour-heavy, both tracking area. Two plies of dense sheet, propane, and welding hours that grow with the lap count the roof geometry forces. It scales predictably rather than steeply, and it is quoted the same way by every roofer who does it — which in many markets is all of them. |
Which one, and when
Choose epdm rubber membrane when…
- The deck is timber and the roof abuts an occupied house. Removing the flame removes the permit, the fire watch, the insurer conversation and the risk of putting heat near a cavity — and on an extension that is usually the whole argument, decided before durability is even reached.
- The roof fits inside one sheet. A small extension, a dormer, a porch or a garage covered with no lap in the field is a class of failure eliminated rather than managed, and no other system on this page can offer that.
- You or a small crew are laying it yourselves. The field work is cold, quiet, pausable at any point and reversible up until the adhesive goes down, which is a genuinely different proposition from hiring a licensed torching trade.
- Something is going through this roof later. A rooflight, a flue, a soil vent or a future extension tying in — each of those is a cold patch on rubber and a repeat hot work event on a torched roof.
Choose torch-applied modified bitumen when…
- The roof will be used or abused. Plant, an access route, a service walkway, or simply a surface people stand on and drag things across — a granulated cap takes that as its ordinary condition, where a single ply needs protection designed in.
- Redundancy is worth more to you than a low seam count. Over a room that must not get wet, two plies is the cheapest second line of defence available, and a small injury to the cap meets a base sheet rather than a deck.
- You are recovering an existing bituminous roof. Asphalt on asphalt is compatible and straightforward, while rubber over the same substrate needs a separation layer before it may touch anything.
- The deck is concrete and the detailing is exposed. On a non-combustible deck with room to work, the strongest objection to torching largely evaporates, and what remains is a tough, redundant, well-understood roof.
- Torch-down is what your local trade actually does. A system installed weekly by crews who will warrant it beats a system chosen on paper and detailed by somebody learning it on your roof.
Now run your own numbers
This page holds no prices on purpose — a national average is wrong for almost every real project. Quantify both options with your dimensions and your local quotes.
Frequently asked questions
- Which one costs more?
- The two costs are shaped too differently for a single answer, and this site will not manufacture one. EPDM is material-light — a thin sheet and its adhesive — with almost all of its skilled labour concentrated at the perimeter, the upstands, the corners and the outlet, so its cost tracks how CUT-UP the roof is far more than how large it is. Torch-applied modified bitumen is heavy on both counts and both track area: two plies of dense sheet, propane, and welding hours that rise with the lap count the roof geometry forces on you. Three things move the answer more than the arithmetic. Whether the deck permits torching at all, because a required base sheet or cover board is real money added to one side only. What your local trade installs routinely, since a system nobody near you fits is priced as a specialty. And the cost of changing the roof later, which lands years after the quote and never appears on it — a cold patch against a repeat hot work event. Run both takeoffs on your own dimensions and price them locally.
- Can a torch-applied roof go straight onto a plywood or timber deck?
- No, and this is the single fact most likely to decide an extension roof. Bitumen is never melted directly onto bare timber: manufacturers and insurers require a mechanically fastened base sheet, or a suitable non-combustible cover board, to stand between the flame and any combustible deck, and the same prohibition applies with more force to foam insulation, which the torch would simply destroy. On top of that sits the hot work regime itself — a permit, extinguishers on the roof, and a fire watch that continues after the torch is out for a period the permit and your insurer set, not the roofer. Parapets, wall abutments and anything with a concealed cavity behind it are where roof fires actually start, because heat travels into a void nobody can see and smoulders long after the crew has packed up. None of this makes torching wrong; it makes it a governed operation with a real cost and a real risk attached. Ask any torch quote what goes between the flame and your deck, and who holds the fire watch.
- Is EPDM genuinely a DIY job?
- On a small, simple, rectangular roof it is one of the few waterproofing systems an unqualified person can honestly attempt, and the reason is the seam: if the roof fits inside a single sheet, the hardest thing in flat roofing has been designed out rather than executed. The field work is unrolling, relaxing, folding back, spreading a cold adhesive and rolling the sheet down. What defeats people is never the field — it is the edges. Upstands against the house wall, external and internal corners, the trim at the drip edge, and the outlet are where every flat roof leaks, and every one of them is a hand-formed detail that has to be right the first time. Add the working window: the deck must be dry, the temperature above the adhesive's minimum, and the flash-off judged rather than guessed. If the roof has a chimney, a rooflight, two changes of level and a cavity abutment, the fact that no flame is involved does not make it a beginner's roof. Be honest about which roof you have before you order the sheet.
- APP or SBS — does the type of modified bitumen matter?
- Yes, and on a cold site the choice inside this option can matter more than the choice between the two systems on this page. The two families are named for what is mixed into the asphalt. APP is modified with a plastic and is the classic torch grade: it holds up well to heat and ultraviolet and flows cleanly under the flame, but it stiffens as temperatures fall and is the less forgiving of the two in a hard winter. SBS is modified with a rubber and keeps its flexibility much further down the scale, which is why it dominates in cold climates and on roofs that move — and it is available not only as a torch grade but hot-mopped, cold-adhesive and self-adhered as well. If your winters are severe, ask which family is being quoted before you compare warranties, because the polymer decides how the roof behaves at the temperature where flat roofs split.
- Is there a modified bitumen system without the torch?
- There is, and it deserves to be on the shortlist whenever the flame is the objection rather than the material. Self-adhered and cold-applied SBS systems keep everything this comparison credits to the B side — two plies, a granulated wearing surface, the redundancy, the compatibility with an existing bituminous roof — while removing the permit, the fire watch and the restriction on combustible decks. Torch-free grades are also hot-air welded on larger commercial work, which is hot but not open flame. The honest caveats are that self-adhered sheets have their own minimum application temperatures and want firm roller pressure across every lap to develop the bond, that the lap count does not shrink at all, and that they are frequently a smaller part of a local roofer's ordinary practice than torching is. If the deck rules out a flame but you want the two-ply roof, ask for this rather than treating the decision as settled.
- Why does one calculator add a scrap allowance and the other does not?
- Because the asymmetry is real, and it is this comparison in miniature — though the modified bitumen tool does it in two moves, and the distinction is worth getting right. The lap itself is NOT the scrap percentage: it comes out first, in the coverage figure, which asks for the area a roll covers after its overlaps rather than the area of the roll. The scrap percentage then sits on top of that net figure and is a granule-loss allowance, because the torch embeds and displaces surface granules along every seam it makes and can scorch material at the edge of the lap. Both of those are functions of a quantity the roof geometry forces on you, so both belong in the arithmetic. One more thing about that tool: it sizes the cap sheet alone, and the base ply underneath it is a separate count. The EPDM tool counts rolls against bare roof area and says openly that overlap and flashing are not in the figure, because on a small roof there may be no field overlap in existence to allow for. That does not mean the EPDM number is complete: its waste lives at the PERIMETER, in the sheet turned up walls and trimmed around upstands, corners and outlets, and that is an allowance you have to add yourself from your own detail drawing. Take the honest lesson from the pair rather than the numbers — on one of these roofs the waste is spread across the field and predictable, and on the other it is concentrated at the edges and specific to your roof.