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To flash a new shingle-covered garage roof where it meets a wall, install corrosion-resistant step flashing along the full joint, weaving one piece into each shingle course. Connect the flashing’s wall leg to the wall’s water-resistive barrier (WRB), lap the upper WRB over it, and fit a kick-out diverter at the low end so roof runoff flows toward the gutter instead of down the wall. The exact dimensions and materials depend on the roof, wall, cladding, product instructions, and locally adopted code.
This is a general water-shedding approach, not a dimensioned detail for every assembly. Plan the roof, membrane, flashing, WRB, siding, and gutter as connected layers before they are concealed. The U.S. Department of Energy’s step and kick-out flashing guide and its guide to existing roof-wall intersections describe the principles and sequence; follow the current instructions for the actual products on your project.
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Before work begins, identify the roof covering, wall construction, cladding, and water-control layers. Those details determine which flashing system is suitable and how its laps and attachments should be made. For a conventional asphalt-shingle roof meeting a vertical wall, the standard approach is step flashing with a kick-out at the eave end. Metal and rubber membrane roofs may require continuous flashing instead, so do not assume the shingle detail applies to them.
Gather the materials specified for the assembly: compatible roof underlayment, a compatible transition membrane where the roof and wall sheathing meet, corrosion-resistant step flashing, a compatible kick-out diverter, and the wall WRB. Check the roofing, membrane, flashing, and cladding manufacturers’ instructions for compatibility, required laps, fastening, and siding clearance. The available guidance does not establish a universal flashing size or a universally suitable metal for every roof and wall.
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Also plan access to the connection before it is covered. On new construction, coordinate the roof and wall sheathing, WRB, roof overhang, siding, and gutter. If tying into an existing wall, remove enough siding to expose the WRB and inspect the sheathing and framing. Repair concealed rot or other damage before closing the assembly; if damage is extensive or the detail is unfamiliar, consult a qualified roofer or exterior-envelope contractor.
Do not treat a bead of exposed sealant as a substitute for the flashing assembly. The durable principle in the cited guidance is to integrate the drainage layers so water is directed outward and down: roof covering over the roof leg, wall WRB over the vertical leg, and the low-end diverter directing concentrated runoff away from the wall.
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How do you install step flashing at a garage roof-wall joint?
For a shingle roof, install the flashing in sequence with the shingles, starting at the low end. Each piece belongs at a shingle course, and each successive layer must overlap in a water-shedding direction. The full joint—not just the visible leak-prone corner—needs a continuous sequence of properly integrated pieces.
- Expose and prepare the connection. For new construction, coordinate sheathing and drainage layers before siding or roofing conceals them. For a retrofit, remove enough siding to expose the WRB and inspect the roof and wall sheathing and framing. Repair damaged materials before proceeding. The DOE’s existing-home roof-wall guide emphasizes inspecting the concealed connection and preparing it before installing new layers.
- Connect the roof and wall water-control layers. Install a transition membrane compatible with both substrates and the materials it will contact, carrying it from the roof sheathing onto the wall sheathing. The DOE guide illustrates a retrofit arrangement in which the membrane extends above the future flashing; it also describes roof underlayment running up the sidewall. Treat those as details in that guide, not as universal dimensions or a substitute for the product instructions and project-specific design.
- Begin at the eave with a kick-out diverter. At the low end of the wall-roof intersection, fit a kick-out so runoff leaves the wall and is directed toward the gutter. It addresses a key weakness of a simple roof-wall junction: water that has traveled down the wall flashing needs a route away from the wall at the end of the run. Integrate the diverter with the first step flashing and shingles according to the relevant product instructions.
- Install a step-flashing piece with each shingle course. Place corrosion-resistant flashing along the entire roof-wall intersection as the shingle courses are laid. The pieces overlap in shingle fashion: water from a higher course should pass onto the flashing below rather than behind it. The roof leg is lapped by the roof covering; the vertical wall leg must be integrated with the wall’s drainage plane. The DOE’s step-and-kick-out guide explains this shingle-course sequence.
- Complete the wall drainage layer over the flashing. Lap the WRB over the upper edge of the vertical flashing leg, following the WRB manufacturer’s instructions. This overlap helps direct water that reaches the wall’s drainage plane outward over the flashing, instead of allowing it to run behind the metal. Keep the assembly’s laps consistent from the upper wall down to the roof and toward the eave.
- Finish the siding and gutter end without blocking drainage. Install siding with the clearance required by the cladding and roofing products. Keep the lower end of the roof-wall intersection clear enough for water to drain toward the gutter; do not let the wall or finish materials obstruct the outlet of the kick-out. Confirm the final arrangement against the actual cladding, roofing, and diverter instructions.
The Building America Solution Center’s illustrated shingle-roof procedure describes a one-piece, non-corrosive kick-out diverter and says not to fasten that diverter to the sidewall in that specific detail. That instruction is not universal: follow the selected diverter’s installation instructions rather than transferring the guide’s fastening detail to a different product.
How should the flashing layers overlap?
The overlaps should make water move out and down, never behind the roof covering or wall drainage plane. Think of the joint as two connected drainage paths: roof water is carried down the shingle courses and over the step flashing, while water behind the siding is carried by the WRB over the flashing’s wall leg.
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- At the roof: the roof covering laps over the roof leg of each step-flashing piece, so water sheds onto the flashing and continues down the roof.
- Along the wall: the WRB laps over the upper edge of the vertical flashing leg, directing wall drainage outward rather than behind the flashing.
- Between courses: step-flashing pieces overlap in shingle fashion as they progress up the slope, with the lower piece positioned to receive water from the one above.
- At the bottom: the kick-out directs runoff away from the wall and toward the gutter. The outlet must remain clear enough for that water to escape.
These are water-shedding relationships, not a substitute for specified lap distances, attachment patterns, or material requirements. Those details depend on the product and assembly. The James Hardie installation guidance also describes step flashing at shingle courses, kick-out flashing, and integration with the WRB; check the current document for the specific cladding product.
Does the same detail work for metal or membrane roofing?
Not necessarily. The DOE Building America Solution Center says metal and rubber membrane roof coverings use continuous flashing rather than individual shingle-style step pieces. The specific roof-system manufacturer’s detail governs the transition, including compatible materials and how the flashing connects to the roof and wall layers.
Do not adapt the shingle sequence by simply substituting a continuous strip of metal or membrane. A roof-system transition can depend on its membrane, seams, substrate, and manufacturer-defined installation method. If the project combines unfamiliar materials or the roof-system detail is unclear, ask a qualified roofer or exterior-envelope contractor to resolve the connection before it is covered.
How does new construction differ from a retrofit?
New construction gives the trades a chance to establish the drainage layers in the correct order before they are hidden. Coordinate the roof and wall sheathing, the transition membrane, roof underlayment, WRB, step flashing, shingles, siding, kick-out, and gutter location on the same plan. Decide which trade owns each transition and schedule an inspection while the flashing and WRB are still visible.
A retrofit requires more investigation. Remove enough siding to see the WRB and open enough of the roof-side edge to assess the sheathing and framing. Look for rot or other damage rather than installing new metal over an unknown substrate. Any compromised sheathing or framing should be repaired before the new water-control layers are installed. The existing-home guide describes this exposure and inspection step because a concealed defect can remain a leak path even when new flashing is visible.
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After exposure, preserve the intended drainage direction while tying new materials into old ones. The transition membrane must be compatible with both substrates and extend into the wall and roof areas as specified for the assembly. Do not assume that a dimension shown in a retrofit illustration applies to every wall, siding type, or membrane.
What mistakes commonly cause leaks at this joint?
- Stopping step flashing short: The pieces must run along the full roof-wall intersection, not only around a suspected leak.
- Reversing a lap: Higher pieces must shed onto lower pieces. A reversed overlap can send water behind the flashing or shingles.
- Leaving the WRB behind the flashing: The upper WRB layer needs to lap over the vertical leg so wall drainage exits outward.
- Omitting the kick-out: Water descending the wall flashing needs a low-end diverter that sends concentrated flow toward the gutter.
- Blocking the outlet: Siding, trim, or gutter placement should not obstruct the kick-out’s drainage path.
- Using incompatible materials: Confirm compatibility among the roof covering, flashing metal, membranes, WRB, cladding, and fasteners. Corrosion resistance alone does not establish compatibility for every assembly.
- Relying on exposed caulk: Sealant cannot replace interlocked roof and wall drainage layers.
- Copying another product’s fastening detail: The DOE illustration’s instruction not to fasten its one-piece diverter to the sidewall is specific to that detail. Follow the selected product’s instructions instead.
What should you check before the joint is covered?
Before the siding and remaining roofing conceal the work, check that the complete drainage path is present and integrated—not merely that metal is visible at the edge of the roof.
- Step flashing follows the full wall-roof intersection and is coordinated with the shingle courses.
- The roof covering laps over the roof leg, and the pieces overlap in a water-shedding direction.
- The vertical flashing leg is integrated with the wall drainage plane, with the WRB lapped over its upper edge.
- A kick-out is installed at the low end and directs runoff away from the wall toward the gutter.
- The diverter’s installation and attachment match its own instructions; a detail shown for one product is not assumed to apply to another.
- Siding clearance and the gutter-end drainage path comply with the relevant cladding and roofing instructions.
- Any damaged sheathing or framing found during preparation has been repaired before the layers are closed up.
Photographing the open joint can help document the layer order for the owner and for the next trade. Verify that the transition membrane, underlayment, WRB, flashing, and kick-out are all present before siding covers them. If a component cannot be identified or its lap is hidden prematurely, stop and resolve the uncertainty rather than depending on a later bead of sealant.
Roofing and siding may be done by different trades, which makes sequencing important. Agree in advance who installs each transition layer and when the WRB, flashing, shingles, siding, and kick-out are inspected. If one trade covers another’s work without coordinating the laps, the finished joint may look complete while the drainage layers are incorrectly ordered.
Which code and product requirements should you follow?
Use the locally adopted building code and current installation instructions for the actual project; do not rely on an old code example as a current, universal specification. The DOE guide references dimensions and material requirements from the 2015 International Residential Code and discusses older IRC editions. Those references provide context, but they do not establish which edition, amendments, or provisions apply to a particular garage today.
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Before choosing or installing components, confirm the requirements that affect the assembly: flashing material and dimensions, required laps, compatibility with the roof and wall materials, attachment, kick-out geometry, and siding clearance. These can vary with jurisdiction, roof covering, wall construction, cladding, and manufacturer instructions. If the applicable code detail is unclear, confirm it with the local building department or a qualified building professional rather than guessing a dimension.
When comparing component options, assess compatibility with the roof covering and wall cladding, corrosion resistance and material compatibility, required leg dimensions and laps, kick-out capacity and geometry for the roof runoff, and each manufacturer’s installation requirements. The available sources do not identify a universally best brand or model.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How should you compare flashing components?
Start with the roof system and wall cladding, not with a preferred brand. For step flashing, compare whether the piece is approved or specified for the shingle and wall materials, whether its material resists corrosion in the project environment, and whether its legs and laps meet the locally applicable requirements. The flashing must also work with the transition membrane and WRB without creating a compatibility problem.
For a kick-out diverter, compare its geometry and runoff capacity with the roof slope, roof area draining to the intersection, gutter location, and cladding clearance. Confirm how the manufacturer directs the diverter to be integrated with the first step-flashing piece and shingles, and whether it specifies sidewall attachment. Do not select a diverter solely because it resembles one shown in an illustration for another product.
For transition membranes, verify adhesion and compatibility with the roof sheathing, wall sheathing, WRB, and flashing materials. A membrane that works on one substrate may not be suitable for every adjacent material. Follow the current instructions for surface preparation, laps, and attachment, and keep the membrane’s upper edge integrated with the wall drainage plane.
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When should a professional handle the work?
A qualified roofer or exterior-envelope contractor is especially appropriate when the roof is metal or rubber membrane, when the wall has concealed damage, when several incompatible materials meet, or when the manufacturer’s transition detail is unclear. Professional help is also sensible when siding must be removed extensively or when the gutter and roof geometry make the kick-out difficult to integrate.
Ask the contractor to identify the roof-system detail, the WRB and transition-membrane sequence, the flashing material and dimensions, and the kick-out installation method before work begins. Request a review while the layers remain exposed. This focuses the inspection on the water-control path rather than on the appearance of the finished siding.
Frequently asked questions
Frequently Asked Questions
Can I flash a garage roof-wall joint without removing the siding?
A retrofit detail needs access to the wall’s water-resistive barrier so the flashing can be integrated with it. The DOE’s existing-home guide calls for removing enough siding to expose the WRB and inspecting the concealed sheathing and framing; covering an inaccessible junction does not establish the required lap.
Is caulk enough to seal where a garage roof meets a wall?
No. Exposed sealant alone does not create the water-shedding sequence of roofing, step or continuous flashing, and the wall drainage layer. Sealants may have a role where a product’s instructions call for them, but they are not a replacement for correctly integrated flashing.
What should I ask a contractor before the roof and siding work are scheduled?
Ask who is responsible for the transition membrane, step flashing or continuous flashing, WRB overlap, and kick-out, and when those layers will be checked before they are covered. Also ask which product instructions and locally adopted code provisions determine the material, dimensions, and clearances for your assembly.
How far along the wall should step flashing extend?
It should run along the full roof-wall intersection, with a piece integrated into each shingle course. The exact leg dimensions and overlap distances depend on the roof, wall, cladding, product instructions, and locally adopted code, so do not assume one universal measurement.
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The low end needs a kick-out diverter to send concentrated runoff away from the wall and toward the gutter. Keep its outlet clear and integrate it with the first step-flashing piece and shingles according to the selected diverter’s instructions.
The Bottom Line
For a shingle-covered garage roof, use corrosion-resistant step flashing at every shingle course, lap the wall WRB over the flashing leg, and install a kick-out at the low end to direct water toward the gutter. Expose and repair concealed damage, coordinate the trades, and follow the current local code and product instructions. Metal and rubber membrane roofs generally need a manufacturer-specific continuous-flashing detail instead.
