The reliable way to stop leaks where an addition roof meets an existing house is to build a continuous, shingle-style drainage transition. Water must move from the roof underlayment to wall flashing, then over the wall’s weather-resistive barrier (WRB) and cladding, with a kick-out diverter sending concentrated runoff into the gutter. On an asphalt-shingle roof, that normally means corrosion-resistant step flashing at least 4 inches up the wall and 4 inches across the roof, plus kick-out flashing at the lower eave.
Do not cover the junction until you have opened it far enough to find concealed rot and confirmed how the old housewrap, roof underlayment, siding and gutter are connected. A new piece of metal over an undiscovered wet wall can hide damage rather than stop it.
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Why this roof-to-wall junction leaks
An addition creates a new roof plane that drains against the original wall. Rainwater accelerates down the addition roof, concentrates at the sidewall and then reaches the lower eave in a narrow, high-volume stream. Any gap, reverse lap or unsealed penetration can send that water behind the siding and into sheathing, framing, insulation and interior finishes.
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The junction is therefore a drainage-plane transition, not merely a seam to be caulked. Every layer above must overlap the layer below so gravity carries water outward. The 2024 International Residential Code (IRC) requires flashing at wall-and-roof intersections, at changes in roof slope or direction, and around roof openings. The locally adopted edition and amendments control your permit and inspection, so treat the IRC dimensions below as a baseline to verify with your building department.
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What a correct detail contains
| Part | Job | Key requirement |
|---|---|---|
| Roof underlayment | Secondary water protection below the roof covering | Run it to the wall and lap it up the vertical wall at least 8 inches, following U.S. Department of Energy guidance. |
| Step flashing | Protects the sidewall where each shingle course meets the wall | For asphalt shingles, each piece is at least 4 inches high against the wall and 4 inches wide over the roof, interwoven with the shingles. |
| Continuous flashing | Creates a continuous upstand for systems that do not use individual shingle courses | Use the detail specified for a metal roof, rubber membrane or other roof covering; do not substitute a shingle detail without the roof manufacturer’s approval. |
| WRB or housewrap | Drains water that gets behind the cladding | The upper WRB edge laps over the upper edge of flashing; lower layers remain underneath so water exits outward. |
| Kick-out flashing | Turns concentrated water away from the wall at the lower eave | Discharge must enter the gutter or otherwise clear the wall; size it for expected stormwater flow. |
| Cladding clearance | Prevents siding or trim from wicking roof water | Maintain the cladding manufacturer’s clearance. A Building America asphalt-shingle retrofit example shows a 1.5-inch reveal; another guideline specifies at least 1 inch. |
Building Science Corporation summarizes the governing principle: “Flashings must always direct water out over the cladding or drainage plane without openings that would allow water to be directed back into the wall.”
Inspect the existing junction before doing repair work
A leak may appear at the ceiling or trim several feet from its entry point. Start at the exterior junction and expose enough of the assembly to follow the water path.
- Map the symptom. Note where staining, peeling paint, mold odor or damp insulation appears inside. Photograph the exterior, gutter and roof edge before removing materials.
- Remove only what is necessary to see the layers. Carefully take off siding, trim and a controlled area of roofing at the sidewall and lower eave. Preserve pieces that show the original lap direction.
- Inspect sheathing and framing. Probe for softness, delamination, darkened fastener holes, insect channels and crushed or wet insulation. Check the overhang and fascia where runoff may have escaped the gutter.
- Trace the drainage plane. Identify the existing housewrap or building paper, roof underlayment, old metal flashing, siding backer and any sealant. Look for laps that face uphill, holes from fasteners and membranes that terminate behind the wrong layer.
- Dry and repair damage first. Replace deteriorated sheathing, framing or insulation and correct the moisture source. Do not install new flashing over wet or structurally unsound materials.
If the wall cavity is wet, allow it to dry and verify that no plumbing, ductwork or window flashing is contributing to the stain. A roof repair that ignores another water path will appear to fail even when the roof-to-wall detail is correct.
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1. Prepare a sound, clean substrate
Remove loose nails, failed sealant and damaged membrane. Plane or replace uneven sheathing so the new flashing lies flat. Keep the work area dry while membranes and underlayment are being installed; follow each product’s temperature and surface requirements.
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2. Install the transition membrane
Apply a fully adhered air-control or flashing membrane from the existing roof sheathing onto the wall sheathing. Roll out wrinkles and press the adhesive firmly around corners. The membrane is a transition layer, not a substitute for metal step flashing. At the wall, carry roof underlayment at least 8 inches vertically, as DOE guidance recommends, unless a listed system requires more.
3. Run roof underlayment to the wall
Lay the roof underlayment so its side edge reaches the wall and its laps shed toward the eave. Follow the underlayment manufacturer’s overlap and fastening pattern. At low-slope areas or in locations exposed to ice dams, use the specified self-adhered membrane rather than assuming ordinary felt is adequate.
4. Install step flashing with the shingle courses
Use corrosion-resistant pieces sized at least 4 inches high on the wall and 4 inches wide over the roof for the asphalt-shingle detail. Place the first piece at the eave, then install one piece at every shingle course. Each shingle should cover the horizontal leg of its flashing while the next flashing piece covers the upper edge of the shingle below. This alternating sequence prevents a direct vertical water path.
Fasten flashing according to the roofing and flashing manufacturers’ instructions. Avoid placing fasteners where they can create a water path through the exposed horizontal leg. Do not rely on a bead of caulk as the primary connection between pieces; sealant is a supplement that eventually needs maintenance.
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5. Integrate the wall WRB
Cut the existing housewrap carefully rather than stripping the entire wall. Fold the upper flap out of the way, install the flashing and membrane, then lap the WRB back over the upper flashing edge. Tape or seal the approved seams with compatible products. The lower WRB layer stays beneath the flashing so any water behind the siding drains onto the metal and out, never behind it.
6. Install kick-out flashing at the lower eave
Where the addition roof terminates against the original wall, install a kick-out diverter that turns the roof’s concentrated flow into the gutter. Prefabricated units may be seamless molded plastic or welded-seam metal. The diverter must be large enough for the expected stormwater volume, securely integrated with the WRB and shaped so water cannot run behind the end of the step-flashing line.
Coordinate the kick-out with the gutter end cap and downspout. A diverter that stops short of the gutter can throw water onto siding; one that is undersized can overflow during a storm. Do not cut away the gutter end or siding until the full discharge path has been planned.
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Replace the roof covering in the same shingle-course sequence used for the flashing. Keep siding and trim above the roof surface by the clearance required for the cladding and roofing system. The Building America example for asphalt shingles shows a 1.5-inch reveal, while another guideline gives at least 1 inch; use the greater manufacturer or locally required clearance when they differ.
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Do not bury the flashing behind new trim, and do not block the kick-out outlet with caulk or siding. Finish exposed edges with compatible trim details that preserve drainage.
8. Verify the finished drainage path
Before closing the wall, inspect every lap from the top down. Water should always encounter the next lower layer and exit over the cladding. Confirm that the kick-out discharges into the gutter, the gutter has adequate slope and the downspout is connected. Photograph the concealed layers for future maintenance and inspection records.
Choosing the right flashing for the roof covering
| Roof covering | Typical wall detail | What to verify |
|---|---|---|
| Asphalt shingles | Individual step flashing interwoven with each shingle course | Minimum 4-inch wall leg and 4-inch roof leg for the IRC-based detail; corrosion resistance; correct shingle overlap. |
| Standing-seam or other metal roofing | Continuous or system-specific sidewall flashing | Panel movement allowance, hemmed edges, sealants and closures specified by the metal-roof manufacturer. |
| Rubber or other membrane roof | Continuous flashing and membrane-compatible termination | Adhesive compatibility, termination bars or bonding details, corner treatment and manufacturer-approved penetrations. |
Do not mix metals casually. Copper, galvanized steel, aluminum and coated products can react when they contact each other in the presence of moisture. Match the flashing’s corrosion resistance to the roof, fasteners, cladding and local exposure, and follow the product instructions for compatible sealants and tapes.
Can you fix the leak without removing all the siding?
Often, yes—but only if the existing WRB and flashing can be exposed and re-lapped correctly. A contractor may remove a limited siding panel, trim board and several shingle courses, then reinstall them after the membrane, step flashing and kick-out are integrated. This approach preserves sound cladding while opening the area that actually controls water.
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Full siding removal becomes more likely when the WRB is missing, brittle, discontinuous, trapped behind old flashing or installed in reverse order over a large wall area. It is also prudent when sheathing or framing damage extends beyond the small opening. A patch that leaves the upper WRB behind the flashing cannot create a dependable drainage path, no matter how much sealant is applied.
Common failure modes and how to correct them
| Symptom or shortcut | Why it fails | Correction |
|---|---|---|
| Caulk-only joint at the wall | Sealant weathers, cracks and does not provide a shingle-style drainage path. | Open the assembly and install the required membrane, step or continuous flashing and WRB laps. |
| One long strip of metal behind asphalt shingles | It can trap water, conflict with shingle movement and leave vertical seams or fasteners exposed. | Use individual step pieces interwoven with each shingle course unless the roofing system specifically calls for another detail. |
| Step flashing stops at the eave | Concentrated runoff pours down the wall at the end of the roof. | Add a properly sized kick-out connected to the gutter and WRB. |
| Housewrap terminates behind the flashing | Water behind the siding can drain into the wall instead of over the flashing. | Lap the upper WRB flap over the flashing and seal compatible seams. |
| Siding touches the roof | Capillary uptake, debris and ice can bridge the drainage gap. | Restore the required clearance, using the cladding and roofing manufacturer’s dimensions. |
| New flashing over rotten wood | Fasteners lose holding power and wet material continues to deteriorate. | Replace damaged sheathing, framing and insulation before rebuilding the detail. |
| Kick-out too small or aimed away from the gutter | Stormwater overflows or runs behind the siding at the discharge point. | Resize or replace the diverter and coordinate it with the gutter end and downspout. |
How to compare contractor proposals
A low price can reflect a narrow patch that leaves the drainage plane untouched. Ask each contractor to describe the exposed layers, the repair boundary and the sequence used to reconnect them.
| Proposal item | Questions to ask |
|---|---|
| Flashing method | Will the asphalt-shingle sidewall use individual step flashing? If the roof is metal or membrane, which manufacturer detail governs? |
| Kick-out geometry | Is the diverter prefabricated or site-made, what material is it, and how will its capacity and outlet be matched to the gutter? |
| Membrane and WRB laps | How many inches will the underlayment and transition membrane extend up the wall, and how will the upper WRB lap over the flashing? |
| Material compatibility | Are flashing, fasteners, tapes, sealants, roof covering and cladding compatible and corrosion-resistant for this exposure? |
| Clearances and code | What siding-to-roof clearance will remain, and which locally adopted code and product instructions will be used? |
| Concealed damage | Is inspection and replacement of rotten sheathing, framing or insulation included, or billed as an allowance? |
| Closeout evidence | Will the contractor provide photographs of the membrane, WRB laps, step flashing and kick-out before the wall is closed? |
Maintenance after the repair
- Clean leaves and roof grit from the gutter and the kick-out outlet so water can discharge freely.
- Inspect the lower wall and gutter end after heavy rain for splash marks, overflowing water or dampness.
- Look for cracked sealant, lifted shingles, displaced trim and corrosion during routine roof inspections.
- Keep tree branches and stored materials from abrading the roof edge or blocking the gutter.
- Investigate any new interior stain promptly; repeated wetting can damage framing even when the original finish appears intact.
When professional work and permits are warranted
Use a qualified roofing or building-envelope contractor when the junction involves structural rot, a second-story wall, a low-slope membrane, complicated metal roofing, inadequate gutters or an occupied wall with suspected mold. The contractor should be able to explain the water path, identify the locally adopted code and provide a written scope for concealed repairs.
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1Repair Windows errors before they cause bigger problems2Scan for outdated or missing drivers - takes under a minute3Clear out junk files and repair common Windows errorsPermit requirements vary by jurisdiction and by whether roofing, siding, structural framing or drainage is altered. Ask the building department before opening the wall, and arrange the required inspection before concealed layers are covered.
The practical standard to remember
At an addition-to-house roof junction, the durable solution is a connected system: sound sheathing, a transition membrane, underlayment carried up the wall, correctly interwoven sidewall flashing, an outward-lapped WRB, adequate cladding clearance and a kick-out that empties into the gutter. If any one of those layers reverses the drainage direction or ends short, water can still enter the wall. Open the assembly far enough to inspect it, repair hidden damage, rebuild the laps from the roof upward and verify the result with the locally adopted code and each product manufacturer’s instructions.
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