ROOF-WALL FLASHING SNAFU - CONTENTS:
Building roof flashing details at abutting sidewalls. Specifications for using single-piece metal flashing at roof-wall intersections. Step flashing details at roof-wall intersections
This article illustrates and discusses alternate or rather goofy attempts at roof-wall intersection flashing to prevent leaks & water damage and points out where leak risks may remain.
In our page top photo my grandson, Tanner Gilligan points out the white caulk that was installed as a stopgap measure where the builder has installed a bay window into a stuccoed wall without proper roof-wall flashing (red arrow).
Tanner would have also pointed out the horizontal flashing (orange arrow) had he not needed his other arm to hold on to the building.
This article series discusses best practices construction details for building exteriors, including water and air barriers, building flashing products & installation, wood siding material choices & installation, vinyl siding, stucco exteriors, building trim, exterior caulks and sealants, exterior building adhesives, and choices and application of exterior finishes on buildings: paints, stains.
At left we show a feeble attempt at flashing and counter flashing at the roof-wall intersection of a low slope roof. The installer tacked a row of asphalt shingles to the wall, caulked their top edge, and hoped for the best.
Using a single piece of metal flashing where the uppermost edge of a shed-roof abuts a vertical building sidewall works fine, though the use of exposed nails (see our photo below) may form leak or wear points in the roof.
Below we discuss the very different case of the use of single-piece versus step flashing at the abutment of the side of a sloping roof to a building sidewall.
Re-Using Step Flashing When Re-Roofing?
At re-roofing time, when the old shingles are to be torn off, it's not always so easy to re-use the original step flashing that extends up under the building siding.
The new shingle courses have to line up exactly with the original shingle course/step flashing placement, the old step flashing is often bent-up during old shingle removal, making it hard to get the new shingles to lay flat.
As we see in our step flashing re-use at re-roofing time photo (left) the installer cut the new shingles too long so they have two reasons to be buckled, lifted, and vulnerable to wind-blown rain leaks at this building wall.
More Step Flashing Snarl ups
It's easy to get confused when installing step flashing, leading to lots of building leaks. Our step flashing snafu photos below demonstrate a few typical step flashing snarl ups.
At left our photo shows some interesting work. It looks as if someone re-sided the building and installed new counter flashing over a wood strip along the roof-wall intersection.
At least we hope the brown bent-over flashing also extends up under those clapboards.
But take a closer look (click to enlarge) and you'll see that the new roof shingles are out of step with the original step flashing, so that the flashing extends less than an inch on top of each shingle course, and the head of the flashing in some locations appears not very far at all under the shingle course above.
This installation looks questionable. Check inside for leaks.
Flashing against irregular sidewalls such as this up-state New York cabin (above left) can require some thought.
We'd need to use custom-formed lead counterflashing as is done on tile roofs, or cut a reglet into the wall deep enough to bend the counterflashing and hook it into the wall to keep wind-blown rain and wall run-down rain from moving behind this step flashing. Our second photo (above right) shows leak stains on the interior of the building wall where this detail was used.
Proper Roof-Wall Abutment Flashing Examples
Below are two photographs of roof-wall step flashing, completed and effective of a slate roof against a brick wall (below left), and in-process, with step flashing against a dormer sidewall before the dormer siding has been put in place (below right).
Use of Continuous Single-piece Solid Metal Flashing vs. Step Flashing at Roof-Wall Abutments - Steep Slope Roofs?
Our photo (left) shows the bottom end of a single-piece of (damaged) sloped-roof-wall flashing. In our OPINION it's not likely that this flashing installation will long resist any significant quantity of water at the lower roof edge, and both blowing wind and any backup due to gutters, ice, or snow will exacerbate the leak risk.
NRCA (Berg) points out that while continuous metal flashing is used at wall junctures in certain steep-slope roof installations, special flashing installation details are required that are different from a step-flashing sealed roof, and even when installed according to specifications, this approach can leave the roof vulnerable to leaks or moisture damage from wind-driven rain.
If single piece flashing is to be used, according to Mr. Berg, [paraphrasing]
The flashing needs to be installed before installing the roof shingles
The metal flashing is formed with a hook edge and cleated on 12" centers
The flashing extends up the wall at least 4"
The flashing extends onto the roof a minimum of 2 inches - 4 inches is preferable in more severe climate areas.
Flashing joints are lapped 6" in the direction of water flow
Building siding material and roofing felt on the building vertical wall may serve as counterflashing [this is true also with the step flashing method if installed at original construction]
The shingles are sealed to the metal flashing with plastic cement to attempt to reduce wind-driven rain penetration, however as this sealing may separate from the material the single-piece flashing approach again becomes vulnerable to leakage
Berg continues that "Because the hook edge and cleats tend to raise the shingles above the flashing, the detail is somewhat vulnerable to wind-driven rain and from moisture trapped in debris that may accumulate in lower areas of the flashing".
OPINION: DF: inspecting thousands of residential properties we have encountered quite a few attempts to use a single piece of metal flashing instead of step flashing at roof-wall abutments, virtually never installed according even to the not-entirely-reliable NRCA recommendations above.
And in our experience, when a single piece of flashing extends only a few inches under the shingles, especially on a long roof slope, and worse on a roof that happens to slope slightly towards rather than away from the abutting vertical building wall, the accumulated roof drainage water near the bottom end of the roof-wall intersection will overwhelm the width of the flashing and, because it is not directed back out on top of successive shingle courses, it leaks into the building or building wall.
In sum, single piece roof-wall flashing is a bad idea in the hands of typical residential roofers and re-roofers, and this approach has been found by home inspectors to be unreliable in practice.
This article is an addendum to FLASHING ROOF WALL DETAILS [live link just below] adapted from BEST ROOFING PRACTICES.
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Steve Bliss's Building Advisor at buildingadvisor.com helps homeowners & contractors plan & complete successful building & remodeling projects: buying land, site work, building design, cost estimating, materials & components, & project management through complete construction. Email: firstname.lastname@example.org
Steven Bliss served as editorial director and co-publisher of The Journal of Light Construction for 16 years and previously as building technology editor for Progressive Builder and Solar Age magazines. He worked in the building trades as a carpenter and design/build contractor for more than ten years and holds a masters degree from the Harvard Graduate School of Education.
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