Pipe flashings and back-trays
Every hole in a roof is two items, not one — and the second one is the one that goes missing off the docket. This is how penetrations are flashed and how they are counted.
A pipe penetration needs a dektite and a back-tray. The dektite is the moulded boot that seals around the pipe; the back-tray sits behind it, under the sheets, and takes the water running down the roof around the penetration before it reaches the boot. A boxed penetration — a chimney or a skylight upstand — is not one flashing but five: back, two sides, front apron and a saddle.
The dektite
A moulded rubber or silicone cone with a flexible metal base, cut to the pipe diameter, pushed down over the pipe, dressed into the profile of the sheet and sealed and fixed around the base. It is what makes the pipe itself watertight.
Two things decide which one:
- The pipe diameter. Dektites come in overlapping size ranges and the cone is cut to the pipe. Measure the pipe, do not guess it from the fitting.
- The temperature. A flue is not a waste pipe. High-temperature penetrations need the material rated for it, and the standard EPDM one is not.
The back-tray, and why it exists
Water runs down a roof. A penetration sits in that path, and everything above it drains onto it. The dektite seals the pipe; it does not stop the sheet above delivering the whole upslope catchment straight into the base of the boot.
The back-tray is a folded tray that sits behind the penetration and under the sheets, and steers that water into the pans either side. On any penetration with roof above it, it is not optional.
- Full length runs from the penetration up to the ridge, so nothing upslope can reach it. The safe choice, and the one to use on low pitch, on long runs, or where the penetration is near the top of the roof.
- Short runs a defined distance above the penetration. Enough on a steeper roof with a short run above the hole, and less steel.
Whichever you choose, it is a folded piece at a length and it goes on the order as one —
1 @ 2.4 m, rounded up to the next 0.1 m like every other flashing.
Why it is the most-forgotten item on a roofing order
Because nothing on the paperwork asks for it. The plan shows a pipe. The merchant's docket shows a dektite, because that is what you asked for. The tray is invisible in both places, and it only becomes obvious when the sheets are up and there is nowhere to put it without lifting them again.
The fix is a counting habit, not a memory: every dektite on the schedule gets a tray line written directly under it, at the moment you write the dektite, before you move on to the next penetration.
Boxed penetrations are five pieces
A boxed chimney, a boxed flue, a skylight upstand — anything square coming through the roof — is a set:
| Piece | Where | What it does |
|---|---|---|
| Back flashing | Upslope face | Takes the water arriving from above and turns it out to the sides |
| Side flashings ×2 | Left and right | Carry that water down past the box, in the pans |
| Front apron | Downslope face | Returns the water onto the roof below the box |
| Saddle | Ridge to the top of the box | Splits the flow above the box so it never arrives head-on |
Ordering one flashing for a chimney is ordering a fifth of a chimney. Write the five out every time, even when the box is small.
Where a penetration sits matters
- Not in a valley, and not close to one. A valley is already carrying the water off two planes.
- Clear of a side lap. A penetration sitting on a lap means cutting both sheets and sealing a join in the middle of a hole.
- Clear of the purlins you need to fix to, and of the ridge, where there is no room upslope for a tray.
- In a pan, not on a rib, wherever the plumbing allows it. Cutting through a rib takes away the stiffness the sheet has at exactly the point you have made a hole in it.
On a re-roof you usually inherit the positions. On a new roof it is worth a conversation with the plumber before anything is cut, because moving a pipe 300 mm on paper is free and moving it later is not.
Counting them for the order
Per penetration, the list is:
- The dektite, sized to the pipe, and rated for the temperature
- The back-tray, full or short, at a length
- Sealant, and the fixings for the base
- For a boxed penetration, five flashing pieces instead of items 1 and 2
Then count the sheets again. A penetration usually means at least one sheet is cut, and on a boxed one it can be two or three — which is a cut list change, not just an extra line on the flashing schedule.
How the rest of the flashings are counted is in roof flashings explained, and the per-piece allowance for laps and trims is in flashing wastage.
What comes up most.
Do I need a back-tray with every dektite?
On any penetration with roof above it, yes. The dektite seals the pipe; the back-tray stops the water from the whole upslope catchment arriving at the base of the boot.
It is the single most-forgotten item on a roofing order, because the plan shows a pipe and the docket shows a dektite, and nothing anywhere asks for the tray.
What is the difference between a full and a short back-tray?
A full tray runs from the penetration all the way to the ridge, so nothing upslope can reach it. A short tray runs a defined distance above the penetration.
Full is the safe choice on low pitch, on long runs above the penetration, and where you are not certain. Short uses less steel and is fine on a steeper roof with little run above the hole.
How many flashings does a boxed chimney need?
Five: a back flashing on the upslope face, a side flashing each side, a front apron below, and a saddle running from the ridge down to the top of the box.
Ordering "a chimney flashing" gets you one of the five. Write all five out on the schedule, every time.
How do I size a dektite?
By the outside diameter of the pipe. The sizes overlap, and the cone is cut to the measured pipe — so measure the pipe rather than inferring it from the fitting or the plan.
Check the temperature rating separately. A flue needs a high-temperature material; the standard one is not rated for it.
Where should a penetration not go?
Not in or beside a valley, not on a side lap, not right at the ridge where there is no room for a tray, and not through a rib if the plumbing allows a pan instead.
On a re-roof you usually inherit the positions. On a new roof, sorting it with the plumber before anything is cut costs nothing.
Does a penetration change the sheet count?
Often, yes. A pipe usually means one sheet is cut; a boxed penetration can involve two or three, and they may need to be ordered as separate lengths.
That makes a penetration a cut-list item as well as a flashing item, which is a second place it can be missed.
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