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Exterior Siding Flashing Details: Head Flashing, Sill Pans, Z-Flashing, Kick-Outs, Transitions and the Photographs to Take Before the Wall Is Closed
Exterior Finishes
Flashing Is Where Siding Jobs Fail: Twelve Places on a House, One Rule for All of Them, and the Pictures That Prove It Was Done
Siding does not leak in the middle of a wall. It leaks where something interrupts it: a window, a door, a roof that dies into the wall, a deck ledger, a change from stone to siding, a light fixture. Every one of those needs a piece of metal or membrane that catches the water and hands it back outside, lapped so the water runs over the next piece and never behind it. That is the whole of flashing, and the OBC says so in 9.27.3 in more words. On a re-side it costs $85 an opening for the sill pan, the tapes and the head flashing. The wall is open for one week; after that nobody can see whether it was done.
The one rule
Water runs downhill, and every layer on the wall has to be lapped so that it does. The wrap below a window goes under the sill pan; the sill pan goes under the wrap at the sides; the jamb tapes go over the window flange; the head flashing goes over the top flange and under the wrap above it. Each piece sheds onto the piece below. Roofers call it shingle fashion and the code calls it lapped in the direction of water flow. If you can find a single edge on the wall where water running down would run behind the next layer, that is where the stain on the drywall will be, and it may take five years to show up.
The second half of the rule: flashing has to go out as far as the face of the cladding, with a drip, and it has to have end dams, which are the little upturned ends that stop water running off the end of the head flashing and down behind the trim. Head flashing with no end dams is a trough that empties into the jamb. I see it on half the houses I look at and it is a five-minute fold with tin snips.
The twelve places and what belongs there
| Location | What goes there | What goes wrong |
|---|---|---|
| Window and door sill | A sill pan: self-adhering membrane or a formed pan, turned up at the back and sides, lapped over the wrap below | No pan at all, or a pan taped at its bottom edge so water cannot get out. The window sits in a bathtub. |
| Window and door jambs | Flashing tape over the nailing flange, lapped over the pan, under the head | Tape applied to a dusty flange in November; it lets go by spring. |
| Window and door head | A metal head flashing (drip cap) over the top flange or brickmould, sloped out, with end dams, tucked under the wrap above and taped at the top edge only | Caulk instead of metal. Flashing taped on all four sides. Flashing stopped short of the trim. |
| Horizontal trim band or belly board | Z-flashing over the top of the trim, out past its face, up behind the siding above | Trim butted to siding with a bead of caulk. The caulk fails in three years; the trim rots in six. |
| Stone or brick below siding | Z-flashing over the top of the masonry, behind the siding, with a drip clear of the stone | Siding run down behind the stone cap with no metal; water runs into the top of the stone. |
| Roof meeting a wall along a slope | Step flashing, one piece per shingle course, up behind the wrap, siding held 2 in. off the roof | One long piece of continuous flashing, or siding cut to touch the shingles so it wicks. Wall and chimney flashing leaks. |
| Where a roof edge ends against a wall | Kick-out flashing: a diverter that throws the water from the last step flashing out into the eavestrough instead of down the wall | Missing on most houses built before 2005. The single commonest cause of rot in a wall, and the cheapest piece of metal on this list. |
| Deck ledger | Flashing over the ledger, tucked under the wrap, and a drip below it; or stand the deck off the wall | Ledger bolted through the siding, water trapped behind it, rim joist gone in ten years. |
| Bottom of the wall | Siding kept 200 mm above grade (6 in. is the fibre cement minimum), the wrap lapped over the foundation flashing or termination, a starter strip that drains | Siding buried behind the garden, or run down over the foundation and sealed so the wall cannot drain. |
| Stucco at the bottom of the wall | A weep screed, the perforated metal strip that lets water out of the base coat | Stucco run into the ground or sealed to the slab. Moisture behind stucco. |
| Brick veneer at the bottom and over openings | Through-wall flashing to the outside face with weep holes every 800 mm, over the ledge and over every lintel | Flashing stopped inside the brick, weep holes mortared shut. Weep holes and flashing. |
| Penetrations: hose bibs, vents, lights, the meter | A mounting block flashed like a small window, head flashing over it, tape at the sides, open at the bottom | Caulked to the siding. Caulk is a temporary flashing with a three-year warranty. |
The flashing kit on a re-side, $85 per opening, covers the sill pan, the jamb tapes and the head flashing with end dams. Kick-outs, step flashing and ledger flashing are small on their own and usually rolled into the siding rate; ask, because “usually” is not “always”.
The transitions that catch everybody
Siding to stone. The stone on the bottom four feet of a front elevation is the most popular detail in Ontario and the one most often built without metal at the top. The stone needs a sloped cap or a Z-flashing over it, lapped under the wrap behind the siding, so that water running down the siding goes out over the stone face and not into the lath. Without it the top course of stone is a sponge with a view of the sheathing. The stone problems page has what happens next.
Siding to a roof. Where a lower roof meets a wall, the siding stops 2 in. above the shingles, the step flashing runs up behind the wrap, and at the bottom end of that roof a kick-out turns the water out. A kick-out costs twenty dollars. The sheathing and framing it saves at that corner cost $6.50 a square foot and $400 to $900 per window below it. If your house has a roof that dies into a wall above a window, go outside and look for the kick-out now.
Siding to a different siding. Board and batten above lap siding, with a band board between: Z-flashing over the band, drip clear of the lap below. Vertical siding has to end on something that sheds; a vertical panel sitting on a horizontal trim with caulk is a channel into the wall.
What to photograph before the siding goes on
- Every window and door, opened wall, with the sill pan visible and its back dam, before the jamb tape covers it. One photo per opening. Twenty-two openings, twenty-two photos, five minutes.
- The head flashing at each opening from below, showing the end dams and the wrap lapped over its top edge. If the wrap is taped over the bottom of the flashing, that is the photo you will want later.
- Every kick-out where a roof ends on a wall, with the eavestrough in frame so it is plain the water lands in it.
- The deck ledger with its flashing before the deck board goes back on.
- The bottom of each wall with a tape from grade to the starter strip.
- Every transition, stone to siding, band to siding, roof to wall, with the Z-flashing showing its drip.
- The wrap laps at one corner and one seam, to show which way they run.
Those photographs are what settle an argument with an installer two years on, and they are what a manufacturer’s inspector asks for first when a fibre cement claim comes in. The full before, during and after list is on the exterior cladding inspection checklist, and if the wall is already closed and there is a stain inside, water behind siding is the page for that.
Questions people ask
What flashing is required for siding in Ontario?
OBC 9.27.3 requires flashing at every opening and at every place the cladding meets something else: window and door heads and sills, roof-to-wall junctions, horizontal trim, changes of material, and the bottom of the wall. It has to be lapped in the direction of water flow and run to the sheathing membrane behind the siding.
Do windows need a sill pan?
Yes. The sill is where every window eventually leaks, and the pan is what catches it and sends it back out under the siding. A formed or membrane pan with a back dam, lapped over the wrap below the window, never taped at its bottom edge. It is part of the $85 flashing kit on a re-side.
What is kick-out flashing?
A small diverter at the bottom end of the step flashing where a roof edge meets a wall. It turns the water from the roof out into the eavestrough instead of letting it run down behind the siding. Missing on most houses built before about 2005 and responsible for a large share of rotten walls.
Can caulking replace flashing?
No. Caulk is a filler with a working life of a few years in Ontario sun and frost. Flashing is a lapped piece of metal or membrane that works with no sealant at all. Where a quote says “sealed with caulk” at a head or a transition, there is no flashing.
What does flashing cost on a siding job?
$85 per window or door on a re-side for the pan, tapes and head flashing. Z-flashing, step flashing, kick-outs and ledger flashing are small items normally in the siding rate; make sure they are listed, because they are the first thing a low quote leaves out.
Count the openings, price the flashing
The exterior cladding cost calculator counts your windows and doors and puts the flashing kit beside the siding so it cannot get left out. If you have quotes already and none of them mention a sill pan, the $99 review will tell you which contractor is planning to caulk it.
Sources and further reading
- Ontario’s Building Code, Section 9.27 Cladding: the first and second planes of protection, sheathing membrane, flashing at openings and transitions, and the clearances from grade and roofs that every siding and stucco job has to meet.
- James Hardie installation instructions: the clearances, fastening, flashing and primed-cut rules a fibre-cement warranty depends on; the installer is held to this sheet.
- Vinyl Siding Institute: the installation manual every vinyl warranty points to: loose nailing, expansion gaps, laps and the J-channel details.
