The short answer: a built-up felt roof is made from bitumen sheets that differ by the position they are designed for and by what is built into them. Underlays bond and give the roof a second line of defence, cap sheets take the weather, and inside every sheet a carrier and a polymer decide how it behaves.
What is inside a sheet of roofing felt?
Three things, arranged the same way in every product: a carrier running down the middle, bitumen on both faces of it, and a surface finish. The carrier is a fleece or mat that gives the sheet its strength, the bitumen does the waterproofing, and the finish protects what lies beneath it or prepares the face to receive the next layer.
How those sheets stack into a finished roof, from the deck upward, is set out in the guide to reinforced bituminous membrane. This piece stays with the sheets themselves, because that is where the differences between one quotation and another usually hide.
What separates an underlay from a cap sheet?
The position each is made for, and therefore the job each has to do. An underlay never sees daylight once the roof is finished: it is built to bond, to bridge whatever it is laid over, and to keep water out on its own if the layer above it is ever punctured. A cap sheet is the weathering layer, made to survive sunlight, rain, frost and the occasional boot.
They are not alternatives to each other. An underlay left exposed will age quickly because nothing in it was designed for ultraviolet light, and a cap sheet used low in a build-up wastes its surface where no weather can reach it. A roof made of two of the same thing is a roof made from whatever was on the van.
Why does the carrier matter more than the surface?
Because the carrier is what deals with movement, and movement is what splits flat roofs. A polyester carrier stretches a long way before it gives up and pulls back afterwards, so it absorbs the tug of a substrate that expands in the sun and contracts overnight. A glass fibre carrier barely stretches at all: it holds its shape beautifully and will not creep, but when it is strained past its limit it breaks instead of yielding.
That trade-off decides where each belongs. Polyester goes where the surface underneath is expected to move, which on a refurbishment means almost everywhere: timber decks, old boarding, joints between insulation boards, anything with a history. Glass fibre suits the layers whose virtue is staying exactly where they are put, and it brings useful behaviour in fire.
None of this is visible from the roof. Two cap sheets can look identical underfoot, wear the same colour of granule, and behave completely differently across a joint that opens and closes every day of the year. It is the one specification line that most often explains why one roof cracked at ten years and its neighbour did not.
What do SBS and APP actually mean?
They name the polymer blended into the bitumen, and that polymer sets how the sheet behaves as the temperature swings. SBS is a rubber-type modifier: the sheet stays supple in the cold and springs back after it has been pulled. APP is a plastic-type modifier: harder, more indifferent to sustained heat and strong sunlight, and noticeably stiffer once the weather turns.
On a London roof that difference tends to favour the rubber-modified sheet. Our climate rarely delivers the prolonged heat APP was developed to shrug off, but it reliably delivers cold nights, frost and the small daily movement that comes with them, which is precisely the loading an elastic sheet absorbs best. APP also wants more heat to bond, which narrows where it can sensibly be used on a building in occupation.
What are the granules on a cap sheet doing?
Blocking ultraviolet light, first and foremost. Sunlight is what ages bitumen, driving off the oils that keep it flexible until the surface hardens and crazes, so the mineral layer is armour rather than decoration. The granules are pressed into the face while the sheet is made rather than scattered over it afterwards, which is why a sound cap sheet keeps hold of them for years.
They earn their place twice more. The grit gives a surface somebody can walk on for inspection without polishing it smooth, and the mineral layer contributes to how the roof behaves if fire reaches it from outside. Bare black patches spreading across a green or grey roof are the earliest honest warning that the armour is going, and they appear long before anything shows on a ceiling. Whether a roof at that stage can still take a new layer over the top is the question overlaying an existing felt roof exists to answer.
When are self-adhesive or mechanically fixed sheets used?
Where heat is unwelcome, or where a bond is not wanted at all. Self-adhesive sheets carry their own adhesive under a release film and stick as that film comes off, which makes them the material answer wherever an open flame has been ruled out. Mechanically fixed sheets are held by fasteners into the structure instead, which suits a substrate nothing will reliably stick to.
Both are ordinary parts of a bitumen specification rather than compromises, and most real roofs use more than one method: fixings low in the build-up, bonding above, self-adhesive around the awkward detail. What matters commercially is that the quotation says which method applies where, because the answer changes the labour, the programme and what happens at every edge.
What does a two-layer or three-layer system mean?
The number of waterproofing plies above the deck or the insulation, not the total number of layers in the roof. Two is the normal specification for a built-up bitumen covering: one underlay and one cap sheet, with the seams of each deliberately offset so no joint in the finished roof has a joint directly beneath it.
A third ply adds another full layer of waterproofing and another set of offset seams. It is specified where the covering has to be more robust than usual, where the surface underneath is uneven enough that an extra layer buys a fairer base for the cap sheet, or where a longer published guarantee depends on it. It also adds material, weight and days. Our felt work is quoted with the ply count stated for exactly that reason: two prices for the same roof can describe two different roofs.
Why does a brand name tell you less than the specification?
Because product names and grades do not translate between manufacturers. Every maker runs its own naming, its own grading and its own idea of what counts as premium, so a name from one range means nothing measurable next to a name from another. Two quotations can each look confident and specific while describing sheets that share almost nothing.
Four lines settle it, whoever made the material: what the carrier is, which polymer the bitumen is modified with, how thick each ply is, and how each one is fixed. Those four say what is going on the roof and how it will behave once it is up there. A quotation that supplies only a trade name has told you where the roofer shops, and nothing at all about your roof.