How to Flash a Roof to a Vertical Wall (and Where Roofing Cement Belongs)
Quick answer: To flash a roof to a vertical wall, weave an L-shaped step flashing piece under and over each shingle course up the wall, lap the wall's water-resistive barrier over the flashing, cover it with counter-flashing or siding, and install kick-out flashing at the bottom. Roofing cement only supplements flashing; it never replaces it.
Key takeaways
- A roof-to-wall joint needs step flashing on sloped sides, apron flashing across the bottom of a wall and counter-flashing or siding lapped over the top.
- Kick-out flashing at the lower end keeps runoff out of the wall; missing kick-outs cause hidden rot on stucco and siding homes.
- A roofing cricket diverts water around the uphill side of wide chimneys; the code threshold and exceptions depend on the adopted edition and the penetration detail.
- Roofing cement is for small seals and exposed nail heads, not for fixing a missing or failed flashing.
How do you flash a roof to a vertical wall?
You flash a roof to a vertical wall by layering metal so every piece sheds water onto the piece below it, never behind it. On the sloped side of a roof meeting a wall, that means step flashing woven with the shingles plus a cover layer on the wall.
This is an explanation of the drainage sequence for reviewing a professional scope, not a universal installation specification. Flashing dimensions, material compatibility, fastening and overlaps must follow the roof and wall manufacturers and the locally adopted code. PNNL shows how roof-wall flashing integrates with the wall drainage plane.
- Prepare the wall. Remove siding or trim enough to expose the wall sheathing a few inches above the roof line. On a re-roof, inspect existing flashing and specify which pieces need replacement.
- Integrate the membrane. Follow the selected assembly detail so roof underlayment, wall barrier and metal flashing drain outward in sequence.
- Install kick-out flashing first. At the eave end of the wall, set a kick-out diverter so its lower lip projects past the wall and directs water into the gutter.
- Start the shingle course. Lay the starter and first shingle course to the wall.
- Place one step flashing piece per course. Set it on top of the shingle, with the vertical leg against the wall and the horizontal leg on the shingle. The next shingle course covers the horizontal leg.
- Fasten to the approved detail. Keep fasteners out of exposed drainage paths and allow the specified movement between roof and wall.
- Overlap pieces. Use the specified lap for the shingle exposure and flashing dimensions so water stays on top.
- Lap the wall barrier over the flashing. The house wrap or building paper on the wall comes down over the vertical legs.
- Cover the vertical leg. Reinstall siding or trim with a clearance gap above the shingles, or install counter-flashing over the step flashing on masonry and stucco.
- Seal only where intended. A small bead of compatible sealant where counter-flashing meets masonry is fine; the metal does the waterproofing.
For a wall at the top of a roof slope (a dormer front or a second-story wall above a lower roof), use continuous apron flashing instead of steps, bent up the wall and lapped over the last shingle course.
Step flashing vs counter-flashing vs apron flashing
Each flash roofing component covers a different joint. They are designed to work together, not as alternatives.
| Flashing type | Where it goes | What it does |
|---|---|---|
| Step flashing | Sloped roof against a sidewall, chimney side or skylight side | One piece per shingle course; sheds water down the slope |
| Apron (front) flashing | Bottom of a wall or chimney that crosses the slope | Continuous piece lapped over shingles below the wall |
| Counter-flashing (cap flashing) | Over the top of step or apron flashing on the wall | Keeps water from running behind the base flashing; allows movement |
| Kick-out flashing | Lower end of a roof-to-wall joint at the eave | Diverts water into the gutter instead of down the wall |
| Back pan / cricket flashing | Uphill side of a chimney or wall | Carries water around the obstruction |
On brick and stone, counter-flashing is commonly let into a mortar joint or saw-cut reglet and sealed. On stucco, a proper detail ties into the stucco system's weather barrier, which is why stucco wall leaks often need a stucco contractor as well as a roofer.
What is a roofing cricket and when is one required?
A roofing cricket, also called a saddle, is a small peaked roof built on the uphill side of a chimney or wide wall so water splits and flows around it instead of pooling against it.
The 2024 IRC roof-assembly chapter calls for a cricket on the ridge side of a chimney or penetration wider than 30 inches measured perpendicular to the slope, with a specific unit-skylight exception. Confirm the adopted edition and the actual chimney detail. Many roofers also add crickets behind wide skylights, curbs and wall sections where debris collects. The cricket is framed, sheathed, covered in membrane and flashed or shingled to match the roof. A wall section without one often becomes a dead valley; see flashing repair and water-management details for that problem.
What is roofing cement used for, and when should you not use it?
Roofing cement is a thick asphalt-based compound (plastic cement) used to bond and seal small areas: sealing exposed nail heads, adhering shingle tabs that will not self-seal, setting flashing edges, and making small temporary patches.
It should not be used as the primary waterproofing at a wall, chimney, valley or pipe. Cement dries, shrinks and cracks under UV and heat cycles, so a "tar patch" over a failed flashing does not establish a durable repair. If a roof-to-wall joint is held together mostly by cement, inspect the flashing underneath. A missing, short, damaged or reversed detail may need flashing repair, not more cement.
Also avoid cement on surfaces it is not compatible with, such as some single-ply membranes, and do not use it under shingles in a way that prevents them from lying flat or self-sealing.
How to apply roofing cement correctly
Use the sealant or cement specified for the exact roofing product and repair detail. Surface preparation, application thickness, reinforcement and temperature limits vary; there is no universal dab size or layer thickness. A solvent-based asphalt product can be incompatible with some membranes.
Ask the contractor to identify the product, show the failed detail before covering it and explain whether the work is temporary weather protection or a permanent repair. Excess cement does not correct a reverse lap or restore a split shingle. Avoid wire-brushing sound shingles or coating their drainage surfaces. For a leak at a wall, opening the assembly enough to inspect the drainage path is more useful than adding another exposed patch.
Where on the roof should there be surface-exposed nails?
On a shingle roof, field fasteners should be covered by the course above. Some finishing details have exposed fasteners that require sealing; the exact location and method come from the product instructions. Exposed nails in the open field deserve investigation rather than automatic sealing.
Other places you may legitimately see exposed fasteners: exposed-fastener metal panels (screws with sealing washers), some flashing edges and roof vents with exposed screws that are sealed. Exposed nails in the middle of shingle fields usually mean a repair was done with face-nailing or that nails have backed out of the deck.
How should exposed nails be repaired? A roofer should identify whether the fastener belongs there, whether the deck holds it and whether it has damaged the overlying shingle. Moving a nail or applying sealant without diagnosing the assembly can leave an unresolved leak. Ask for the repair detail and completed photographs.
Are roofing nails supposed to go through the deck?
Yes, roofing nails are supposed to go through the deck or penetrate deeply into it. The IRC requires shingle nails to penetrate through the roof sheathing or at least 3/4 inch into it, so on thinner decks you will see nail tips poking through into the attic.
Nail points in the attic are normal and not a defect. What is not normal: nails that miss the deck, rusted nail tips with dark rings (a reason to investigate moisture rather than assume its cause), or nail heads driven through the shingle surface by an over-pressurized nail gun. Shingle manufacturers specify nail placement in the nailing zone, and the number of nails per shingle; high-wind installations often call for more.
Safety note and when to call a professional
Roof-to-wall flashing work happens on a slope, next to a wall, often at the second-story line. Leave rooftop access and work to professionals with an appropriate access and fall-protection plan. Do not work on wet or dusty shingles, and avoid midday summer heat when shingles soften and scuff and heat illness sets in fast.
Call a professional when:
- the leak shows up inside a wall, around a window below the roof line or as stucco staining;
- flashing must be cut into brick, stone or stucco;
- the roof is under a manufacturer warranty that requires certified installation methods;
- the damage may be part of an insurance claim and needs documented inspection;
- the joint is at a second story or a steep pitch.
Penetration leaks follow similar logic; see penetration flashing repair.
Frequently asked questions
Can you use roofing cement instead of flashing?
No. Roofing cement can seal a small gap or nail head, but it cannot replace metal flashing at a wall, chimney, skylight or pipe. Cement dries and cracks under heat and UV, and it does not shed water down the slope the way overlapping metal does. A joint held together by cement usually has missing or failed flashing.
How do you seal exposed nails on a roof?
First have a roofer determine whether the fastener belongs there, whether it is secure and whether it has damaged nearby material. The approved repair may involve replacement rather than sealant alone. Use the roof manufacturer's detail and compatible product; homeowners should not climb onto a roof to investigate exposed nails.
Should roofing nails poke through the decking?
Yes. Building code requires shingle nails to go through the roof sheathing or at least 3/4 inch into it, so nail tips showing in the attic are normal. Watch for rusted tips or frost-like rings, which point to attic moisture, and for nails that missed the deck entirely, which can let shingles blow off.
What size chimney needs a cricket?
The 2024 IRC roof-assembly provisions use a width greater than 30 inches measured perpendicular to the slope for a chimney or penetration, with a specific unit-skylight exception. Have the contractor check the locally adopted edition and the actual roof connection. A cricket may also be useful where an obstruction collects runoff or debris.
How long does roofing cement last?
Service life depends on the product, surface, exposure and application; there is no reliable universal interval. Ask whether the proposed use is temporary or part of an approved permanent detail. Inspect repairs under the product maintenance guidance and replace failed flashing rather than assuming another coat of cement restores the drainage assembly.
Get a wall-flashing leak traced properly
Leaks where a roof meets a wall often show up far from the source. Texas Professional Roofing offers a free, no-obligation roof inspection in Austin with photo documentation, and handles roof repair and flashing work across Central Texas. Call (512) 910-4110 to schedule.