Flashing wastage, done per piece
A flat percentage on a flashing total is the wrong shape for the problem. Here is the arithmetic that shows why, and what to do instead.
Allow flashing wastage per piece, not as a percentage. Roughly 0.3 to 0.5 m on each run for laps and end trims, rounded up to the next 0.1 m. A flat percentage is only ever correct for one average piece length — at 0.4 m a piece, a 10 per cent allowance is right when your pieces average 4 m, generous above that and short below it. Cut-up roofs have short pieces, so a percentage fails exactly where the risk is.
The arithmetic behind the rule
An allowance per piece, expressed as a percentage of the total, is just this:
allowance % = allowance per piece ÷ average piece length
Nothing else is in it. The percentage that is correct for a roof is decided entirely by how long its flashing pieces average — which is another way of saying it is decided by how cut-up the roof is. At a 0.4 m allowance:
| Average piece length | 0.4 m per piece is | What kind of roof |
|---|---|---|
| 6.0 m | 6.7% | Long straight runs — a simple gable |
| 5.0 m | 8.0% | Straightforward residential |
| 4.0 m | 10.0% | The only roof a flat 10% is right for |
| 3.0 m | 13.3% | Hips, valleys, a few penetrations |
| 2.5 m | 16.0% | Cut-up: dormers, steps, verandahs |
| 2.0 m | 20.0% | Very cut-up, or lots of short apron runs |
So a flat ten per cent is not a safe middle. It is a bet that every roof you quote has 4 m flashings on it.
Two roofs, same rule, different answers
Take a plain 12 m × 8 m gable at 25°. The rake is 4.00 m in plan, so 4.41 m on the slope. Six flashings: two lengths of ridge and four barges.
| Per roof | Simple gable | Cut-up hip-and-valley |
|---|---|---|
| Flashing pieces | 6 | 22 |
| Total flashing | 29.65 m | 62.00 m |
| Average piece | 4.94 m | 2.82 m |
| At 0.4 m a piece | 2.4 m — 8.1% | 8.8 m — 14.2% |
| A flat 10% would give | 2.97 m — 0.6 m too much | 6.20 m — 2.6 m short |
Read the last row. The flat percentage hands free material to the easy roof and takes it off the hard one. On the cut-up roof that 2.6 m is two or three short pieces you have to go back for, on the job that was already going to be tight.
What the allowance is actually for
It is not waste in the sense of off-cuts. It is three real things, and all three happen per piece:
- Laps. Where two lengths of the same flashing meet, they overlap. Two pieces of ridge covering 12 m are not 6 m each.
- End trims and stop-ends. Cutting to fit at the ends of a run, and folding or notching where a flashing meets another one.
- Rounding. Every piece is rounded up to the next 0.1 m for the order, which adds an average of 0.05 m a piece on top. Small, but it compounds the same way — by piece count, not by metres.
None of those scales with how much flashing there is. All of them scale with how many separate pieces there are.
How to write it up
- List every flashing as a quantity at a length —
2 @ 6.4 m,4 @ 4.9 m— not as a total. - Add the allowance to each piece before rounding, not to the total afterwards.
- Round each piece up to the next 0.1 m.
- Group identical lengths for the merchant.
The order that comes out of that is one the merchant can fold straight from. A total in metres is not an order — forty-two metres of barge could be six pieces or fifteen, and they are different steel at a different price.
Sheets are the opposite case
All of the above is about flashings. Long-run sheets are cut to length, so their waste really is small and really is roughly proportional: two to five per cent covers the off-cut at the end of a run and the odd sheet ruined in handling.
The reason people reach for a flat percentage on flashings is that it works on sheets. It works there because sheets are ordered as long single lengths and flashings are not.
When to allow more
- Complicated junctions. A valley running into an apron, or three flashings meeting at a corner, eats more than a plain lap.
- Change-of-pitch and bullnose. Folded to two specific angles, so a mistake is a whole new piece rather than a trim.
- Anything you are measuring off a plan you have not stood on. Round up harder, and check the critical pieces before they are folded.
The full survey of what each flashing is and where it goes is in roof flashings explained.
What comes up most.
What percentage should I allow for flashing wastage?
The honest answer is that a percentage is the wrong unit. Allow 0.3 to 0.5 m per piece for laps and end trims, rounded up to the next 0.1 m.
If you must have a number: the allowance as a percentage equals your per-piece allowance divided by your average piece length. At 0.4 m a piece, that is 6.7 per cent on 6 m pieces and 20 per cent on 2 m pieces. There is no single figure that covers both.
Why does a flat percentage go wrong on cut-up roofs?
Because the things the allowance pays for — laps, end trims, rounding — happen once per piece, and a cut-up roof has far more pieces per metre than a simple one.
A 62 m flashing schedule made of 22 pieces needs 8.8 m at 0.4 m each. A flat ten per cent gives it 6.2 m. That is 2.6 m short on the roof that could least afford it.
Does the same rule apply to sheets?
No. Long-run steel sheets are cut to length, so their waste is genuinely small and roughly proportional to the quantity — two to five per cent covers most roofs.
That is why the percentage habit exists. It works on sheets, and people carry it across to flashings where it does not.
Should I add the allowance before or after rounding?
Before. Add the lap and trim allowance to the measured length, then round the result up to the next 0.1 m.
Rounding first and then adding gives you the same total by accident on some pieces and leaves you short on others, and it makes the schedule harder to check.
How do I count pieces I have not decided how to lap yet?
Count the runs first — every separate line on the roof is at least one piece. Then split any run longer than the merchant will fold in one length, and count the split as an extra piece with its own allowance.
That is the number the allowance multiplies. If you do not know the fold limit, ask before you price it rather than after.
Next
Try it on one of your own roofs.
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