Roof flashings explained
Every line on a roof is a flashing, and every flashing is a piece at a length. This is what each type does, where it goes, and how to count them so the order is right.
A flashing is the folded steel that closes a join in a roof. Six cover the lines: ridge along the top, hip down each external corner, valley up each internal one, barge along a gable edge, apron where a roof meets a wall, and change-of-pitch where the slope breaks. Penetrations get their own set. Count them as pieces at lengths, never as a total in metres.
The six line flashings
Draw a roof and every line you drew is a flashing, except the gutter line. Name them the same way every time and nothing gets missed:
| Flashing | Where it goes | What it is doing |
|---|---|---|
| Ridge | The horizontal line at the top, where two slopes meet | Caps the open ends of the sheets on both sides and keeps water and wind-driven rain out of the gap |
| Hip | The sloping line down an external corner, where two slopes meet outwards | Same job as a ridge, but on a diagonal — and every sheet arriving at it is cut on the rake |
| Valley | The sloping line up an internal corner, where two slopes meet inwards | Carries water. It is the only flashing that is a drain, which is why it sits under the sheets rather than over them |
| Barge | The raking edge of a gable | Closes the side of the roof, holds the sheet edge down, and finishes the line you actually see from the street |
| Apron | Where a roof runs into a wall | Turns water back out onto the roof instead of down the wall junction. Needs a proper upstand behind the cladding |
| Change-of-pitch | Where one slope breaks into another on the same run | Bridges two different angles in one fold — a verandah off a main roof, or a bullnose |
Two of those need a note. A valley is the one people under-order, because it is the longest single piece on most cut-up roofs and it is easy to forget it runs the full rake and then some. And a change-of-pitch is the one people forget exists at all until the sheets are up.
The penetration flashings
Anything coming through the roof needs flashing, and this is where orders go short.
- Dektite — the moulded boot that seals around a round pipe: flue, vent, waste pipe. Sized by the pipe diameter.
- Back-tray — a folded tray that sits behind the penetration, under the sheets, and takes the water around it. Every pipe penetration needs both. The tray is the item that is left off the docket and discovered on the roof.
- Boxed penetration — a chimney, a skylight upstand, a boxed flue. That is five pieces, not one: a back flashing, two sides, a front apron, and a saddle running from the ridge down to the top of the box.
If you take nothing else from this page: a back-tray for every dektite, and five pieces for every box.
Girth and folds — what the price is per
A flashing is priced off two things: its girth, which is the total developed width of the flat sheet before it is folded, and the number of folds or bends in it. Both come from the profile drawing, not from the finished appearance — a barge that shows 100 mm on the face might have a 300 mm girth once the return and the upstand are counted.
The merchant folds and cuts to your schedule, so the schedule needs girth, folds, length and quantity for each type. A flashing described only by name and metres is not an order.
Count pieces, not metres
This is the whole point, and it is the thing that most often costs money.
Write flashings as 1 @ 6.8 m, 1 @ 3.7 m, 2 @ 4.2 m
— quantities at lengths. Not 42 m of barge
. Forty-two metres of barge could be six
pieces or fifteen, and the two orders cost different money and arrive as different steel.
Then two counting rules:
- A shared edge is one flashing. Where two roofs meet, that ridge or that apron belongs to the join, not to each roof. Counting it on both is the standard way a take-off comes out over.
- Round up, per piece. Allow roughly 0.3 to 0.5 m on a run for laps and end trims, rounded up to the next 0.1 m, and allow it on each piece. A flat percentage on a total runs generous on a simple roof and short on a cut-up one, which is exactly the wrong way round.
Flashings and pitch
Anything that slopes is longer than it looks on a plan. Hips, valleys, barges and change-of-pitch flashings all need the pitch factor; ridges and gutters do not, because they are level.
Hips and valleys need more care again, because they run diagonally as well as up: at 25° a rafter is 10.3 per cent longer than its plan length, but a hip on a square corner is only 5.3 per cent longer than its plan length, since the diagonal already carries most of the distance. Applying the rafter factor to a hip over-orders it. There is a table of both in roof pitch explained.
Where flashings meet other trades
Two items sit next to flashings and are worth naming in the scope so nobody assumes:
- Fascia and soffit. The fascia is the board the gutter fixes to; the soffit is the lining underneath. Neither is a flashing, and neither is included unless you say so.
- Ventilation. Ridge vents and eave intake manage condensation under steel. If the roof needs venting, it is a scope line and often a flashing line as well.
What comes up most.
What is the difference between a hip and a valley?
They are the same line seen from opposite sides. A hip is where two slopes meet pointing outwards, so it sheds water away from the join. A valley is where they meet pointing inwards, so it collects water and has to carry it.
That difference is why a valley goes under the sheets and a hip goes over them, and why a valley is the flashing you never skimp on.
Do I need a back-tray behind every dektite?
Yes. The dektite seals the pipe; the back-tray takes the water that runs down the roof and steers it around the penetration before it reaches the boot.
It is the single most-forgotten item on a roofing order, because the drawing shows the pipe and the docket shows the dektite, and the tray is only obvious once you are on the roof with the sheets up.
How much extra should I allow on a flashing run?
Roughly 0.3 to 0.5 m per run for laps and end trims, rounded up to the next 0.1 m — allowed per piece, not as a percentage of a total.
A percentage gives a simple roof more than it needs and a cut-up roof less, because a cut-up roof has more joins per metre. That is the opposite of what you want.
What is a change-of-pitch flashing?
A flashing folded to bridge two different angles where one slope breaks into another on the same run — a verandah coming off a main roof, or a bullnose.
It is specified by the two pitches it joins, so it cannot be ordered as a standard fold. Measure both pitches on site.
Why is my flashing quote so much more than the metre rate suggested?
Because the metre rate depends on girth and folds. A wide flashing with five bends is a different product from a narrow one with two, even though both are sold by the lineal metre.
Check the girth on the profile drawing before comparing rates. A flashing that shows 100 mm on the face can easily have a 300 mm girth.
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Try it on one of your own roofs.
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