Roof to wall flashing is the detail that closes the joint where a low slope roof runs into a vertical wall. It is built from two parts: a base flashing that carries the roof membrane up the face of the wall and terminates it above the water, and a counterflashing, almost always metal, that comes out of the wall above that termination and sheds water onto the roof. Neither part works alone.
That joint is where most commercial roofs leak. The field of a membrane is a large flat sheet installed by one crew in one operation, and it rarely fails first. The wall detail is handmade, it sits at the boundary between the roofing trade and whoever built the wall, and it has to survive two materials expanding and contracting at different rates for thirty years. Flashings in general account for a majority of the leak calls on a building of any age, and the wall detail is the hardest of them to get right.
The two parts, and why one without the other fails
The base flashing is roofing. It is the membrane itself, or a compatible sheet welded or torched to it, turned up the wall and fastened near its top edge. Its job is to get the waterproofing above the level that water can reach.
The counterflashing is wall work. It is a formed metal section that leaves the wall assembly, comes down over the top of the base flashing and stops with a free edge. Water running down the wall arrives on the outside of that metal and is thrown clear.
| Part | Belongs to | Fails when |
|---|---|---|
| Cant strip | The roof assembly | Omitted, so the membrane turns a sharp corner |
| Base flashing | The roof | Too low, unfastened at the top, or adhered to a dusty wall |
| Counterflashing | The wall | Omitted, face fixed, or sealed instead of lapped |
| The lap between them | Both | Reversed, so water is directed behind the base flashing |
The lap is the detail. The counterflashing must overlap the top of the base flashing by enough that wind driven water cannot be pushed up past it, and the two must not be sealed together, because the roof has to be able to come off one day without the wall coming with it.
Upstand height, and what decides it
There is no single right height, and a bid that does not mention it is hiding a decision. What the height has to clear is the question:
- Standing water. Whatever depth collects at that wall before it drains or evaporates.
- Snow and ice, where the climate puts it, since a drift against a wall sits for weeks.
- Backed up drainage. If the nearest drain blocks, the water level rises to the overflow, and the flashing should still be above it.
- The splash line, meaning water hitting the roof and bouncing against the wall during heavy rain.
Every membrane manufacturer publishes a minimum in its detail manual, and that minimum is the condition of the warranty rather than a suggestion. Ask which published detail the bid follows and whether the measured height on your roof meets it. Where it does not, the options are raising the membrane, lowering the roof surface at the wall or accepting a detail outside the warranty, and the owner should be the one choosing.
The cant strip, and what it stops
A membrane turned up at a right angle is a membrane with all of its stress in one line. Add thermal movement, foot traffic and a crew’s knees to that line and it eventually splits.
A cant strip fills the corner at an angle so the turn happens in two gentle folds instead of one hard one. On built up and modified bitumen roofs it is not optional in practice, because the sheets are stiff and a cold sharp fold is a crack waiting to happen. On a reinforced single ply it is still what keeps the membrane supported rather than bridging a void.
The cheap alternative is a bead of sealant in the corner and a membrane pushed against it, which looks identical on the day and is the first thing to open. If you are watching the work, the cant goes in before the base flashing, and after the base flashing is on you cannot tell whether it is there.
How the counterflashing is held to the wall
Four arrangements, in rough order of how long they last.
- Through wall flashing. The metal passes right through the wall and turns down on the face. Water inside the wall is intercepted and thrown out. It has to be built with the wall, so it is a new construction detail.
- A reglet cut into masonry. A groove is cut, the metal’s folded edge is set into it, and the groove is wedged and sealed. The wedge holds the metal; the sealant keeps water out of the groove.
- Metal lapped behind the cladding. On metal panel, siding or rainscreen walls, the counterflashing slides up behind the finish and lets the wall’s own drainage plane do the work. Clean when the sequence allows it.
- A surface mounted reglet. A manufactured section fixed to the face of the wall with a sealant joint along its top edge. It works, and it is a maintenance item for as long as the roof lasts, because the sealant is the only barrier.
What does not belong anywhere on this list is a sheet of metal face fixed through its middle with screws and washers, or a termination bar left exposed with a bead along the top and nothing over it. Both put the whole detail on sealant, and sealant is the shortest lived material on the roof. The types of flashing a roof needs covers the other locations where that same mistake shows up.
Three wall types, three different problems
Masonry and concrete. The most workable case, because you can cut a reglet into it and the wall itself is water resistant. The risk is a wall that absorbs water above the flashing and releases it below, which no roof detail can intercept. Look at the condition of the mortar joints and the coping above before blaming the roof.
Stucco and EIFS. The hardest case. The finish is usually run down to the roof and stopped, which leaves the roofer with nothing to lap under. Done properly, the wall trade installs flashing out of the wall before the finish goes on. Done as it usually is, the detail is a termination bar and a sealant bead, and the sheathing behind the finish gets wet for years without anybody seeing it.
Metal panel and rainscreen. Mechanically straightforward, since the counterflashing can go behind the panel, but the sequence has to be right and the two trades have to be on site in the right order. Metal roof flashing deals with the related case where the roof itself is panel rather than membrane.
Where these details go wrong
- The counterflashing was never installed, and a termination bar is holding the roof’s reputation up on a bead of sealant.
- The old counterflashing was left in place and the new base flashing was terminated below it rather than behind it, so the lap is reversed.
- The base flashing was adhered to a dusty or painted wall and has let go at the top, invisible behind the metal.
- The detail is correct and the wall above it is not, so water enters above the flashing and the roofer keeps being called back.
- The flashing is high enough for normal rain and lower than the level the roof reaches when the drain blocks.
What to ask before anybody prices it
- Which published detail from the membrane manufacturer does this follow, by number.
- What is the finished upstand height at each wall, and does it meet that detail.
- Is there a cant strip, and in what material.
- How is the counterflashing engaged with the wall, and is the existing metal reused or replaced.
- Who is responsible for the wall above the flashing, and what happens if it needs work.
A bidder who answers all five has been on the roof with a tape. A bidder who answers none has priced the field and will treat every wall as a variation.
Where to go next
If the perimeter of your roof is open rather than running into a wall, roof edge flashing is the equivalent detail and fails in its own way. If you are putting a specification together, the wall details belong in it before the membrane is chosen, because the height available at the wall sometimes rules a system out.
Common questions
What is a roof to wall flashing made up of?
Two separate parts. The base flashing carries the roof membrane up the face of the wall and terminates it above the water. The counterflashing is metal, fixed into or behind the wall, and it covers the top of that termination so nothing runs behind it.
- Base flashing belongs to the roof and moves with it
- Counterflashing belongs to the wall and sheds onto the roof
- The lap between them is the whole point of the detail
- Sealant is a backup to that lap, never the lap itself
- A single piece of metal doing both jobs cannot allow movement
The split matters because the roof and the wall move differently and on different schedules. A roof is replaced several times in the life of a building and a wall is not, so the detail has to come apart and go back together without rebuilding the wall. Where one piece of metal has been made to do both jobs, the next reroof either damages the wall or leaves the old termination buried under the new membrane, which is how a wall leak outlives three roofs.
Does a roof to wall detail need a cant strip?
On most membrane roofs, yes. A cant strip is an angled fillet set in the corner between the deck and the wall so the membrane turns up at a slope instead of a right angle. It takes the strain out of the fold.
- A sharp inside corner concentrates stress in one line
- Bitumen sheets crack along that line as they age
- Single ply membranes bridge the corner and tear at the bridge
- The cant also keeps debris from packing into the angle
- Fully adhered systems on a rigid corner still benefit from one
Materials differ in how much they need it. Built up and modified bitumen assemblies are the clearest case, because the sheet is stiff and a cold right angle is where it fails. A reinforced single ply handles the turn better but still wants support behind it. What never works is a membrane stretched across an open corner with a void behind, because foot traffic and ice both push it into the void until the reinforcement gives. Ask whether the detail drawing shows one, and in what material.
What goes wrong where a roof meets a stucco or EIFS wall?
The counterflashing usually gets left out. Stucco and EIFS are applied finishes, so the trade that installs them tends to run the finish down to the roof surface and stop, leaving the membrane termination with nothing over it but a bead of sealant.
- Water enters the finish above the roof and runs down inside it
- A termination bar alone relies on sealant for the roof's whole life
- EIFS needs a drainage plane and a flashing that turns water out of it
- Cutting a reglet into stucco later disturbs the finish
- The repair is wall work, which is why it gets deferred
The fix on a new building is sequence: the wall trade installs a flashing that comes out of the wall assembly before the finish is applied, and the roofer laps up behind it. On an existing building the honest answer is that the detail has to be opened, which is a wall job with a roofing component rather than the reverse. Owners get quoted a sealant bead instead, every year, and the leak that was never fixed slowly consumes the sheathing behind the finish.
Why does the leak show up inside the wall rather than at the roof?
Because water that gets behind a counterflashing is already inside the wall when it passes the roof line. It then travels down the cavity or through the masonry and appears wherever the first horizontal element stops it, which can be a floor or two below.
- The stain has no useful relationship to the entry point
- Masonry walls absorb at the top and release lower down
- A wet ceiling near a wall is a flashing symptom more often than a membrane one
- Water tests should start above the suspect detail, not on it
- Infrared on the roof will not see water inside a wall
This is the main reason wall leaks get misdiagnosed as membrane failures and bought as patches. A leak that appears after wind driven rain, stops in still weather and never shows up during ponding is telling you it comes from the wall above. The test that settles it is a controlled water test run in stages, wetting the roof first, then the base of the wall, then the detail, then the wall above it, with somebody watching inside between stages.
What should a specification say about the roof to wall detail?
It should name the detail by its number in the manufacturer's manual, state what the counterflashing is made of and how it is fixed to the wall, and say who installs each part. Those three things settle most arguments before they start.
- A named detail makes the warranty claim possible later
- Metal type, gauge and finish belong in writing
- How the metal engages the wall decides whether it lasts
- Allow for wall repair as a unit rate, not an exclusion
- Say whether existing counterflashing is reused or replaced
Reuse is where bids diverge quietly. Lifting old counterflashing, cleaning the reglet and setting the metal back is slower than running a termination bar and a bead of sealant, so the cheaper bid often assumes the second without saying so. Ask each bidder to state, in a sentence, what happens to the existing wall metal. The answer tells you whether you are comparing the same scope, and it costs nothing to ask before the price is fixed.