How to Flash a Shed Roof Against a Wall

Short answer: First identify where the roof meets the wall. If the wall runs alongside the sloping edge, it is a sidewall condition; asphalt-shingle roofs commonly use step flashing woven into the shingle courses. If the roof ends against the wall at its high edge, it is a headwall (or front-wall) condition, and the detail is different. In either case, the flashing must work with both the roof covering and the wall's water-drainage layer. Caulk alone is not a flashing system.

Sidewall or headwall? Identify the joint first

The phrase “roof-to-wall flashing” is a useful search term, but it does not identify one standard part. Look at the direction water travels down the roof and how that roof edge meets the wall:

• Sidewall: The wall follows the roof slope along one side. On many asphalt-shingle roofs, step flashing pieces are installed course by course with the shingles. The wall's drainage plane must be connected to the flashing so water can exit onto the roof.
• Headwall or front wall: The upper edge of the roof runs into a wall across the slope. That is not the same as sidewall step flashing. The 2024 IRC's asphalt-shingle provision for flashing against a vertical front wall points to the shingle manufacturer's instructions.
• Metal panels or other roof coverings: Use the exact roof-system manufacturer's sidewall or endwall detail. Do not transfer a shingle step-flashing sequence or dimensions to a metal-panel roof.

The 2024 International Residential Code (IRC) places flashing at roof-wall intersections and calls for water to be diverted away where a sloped roof's eave meets a vertical sidewall. It is a model code, not automatically the code adopted where your shed is built. Local editions, amendments, permits, and the roof system all matter.

Understand the water path before measuring

A roof-to-wall junction has at least two drainage paths to connect: water that runs down the roof covering and water that gets behind the wall cladding onto the wall's water-resistive barrier (WRB), housewrap, or other drainage plane. In a typical shingled sidewall, step flashing is interwoven with the shingles, while its upturned leg connects to the wall drainage layer. That is why simply fastening an L-shaped strip on top of the finished joint—or covering a gap with sealant—may leave water trapped behind the assembly.

Building Science Corporation describes step flashing as a transition between the wall's vertical drainage plane and the roof's drainage plane. It also identifies a kick-out flashing at the lower end of the sidewall run as a way to move concentrated roof runoff away from the wall. The correct bottom termination depends on where the roof drains, how the shed is guttered, and the wall assembly.

Dimensions and details to confirm for your shed

Before buying or bending metal, collect the product-specific values below. The measured roof-to-wall run and the flashing cross-section are different things; label them separately. Take photos from a safe location and record the roof and wall product names so a roofer or supplier can check the same assembly.

How to plan the repair without guessing

1. Identify the roof covering and the wall assembly.

   Look for shingle or panel labels, leftover materials, manuals, invoices, and siding information. Note the roof pitch if known, the wall cladding, visible trim, gutter location, and whether the joint is a sidewall or headwall. Do not remove sound roofing or siding just to read a hidden layer.

2. Check for signs that the repair is larger than flashing.

   From a safe position, look for lifted or missing shingles, loose panel trim, staining, soft or swollen siding, rotted wood, or dampness inside the shed. If water has reached wood framing, insulation, or sheathing, ask a qualified roofer to find and repair the source before new metal covers it.

3. Make a labeled sketch and record the dimensions in the table.

   Separate the run along the roof from the profile's legs and bends. Add photos of both ends, the low-end discharge, the wall cladding, and any visible seams. Never use the old bent or corroded flashing as the only template; it may not show the original shape or correct overlap.

4. Compare the detail with the exact product instructions and local code.

   For asphalt shingles, the code section and shingle-maker details are not interchangeable with metal-panel directions. For anchored masonry veneer, adhered veneer, stucco, or concealed cladding layers, ask a qualified professional to verify how the roof flashing integrates with the wall's water-control layer.

5. Decide who should open the roof or wall.

   If the correct detail requires lifting shingles, removing panels, cutting cladding, or exposing the wall WRB, this is no longer just a surface measurement. Use a roofer who can restore the roof and wall layers in the correct sequence. A custom-bent part cannot fix damaged sheathing or a failed water-control layer.

6. Get a written part and installation detail before ordering custom metal.

   Ask the roof-system supplier, roofer, or fabricator to confirm the flashing type, cross-section, material, piece lengths, joints, attachment method, and how the wall drainage layer will lap. Keep that written detail with your sketch and photos.

Why one set of dimensions does not fit every roof

The 2024 IRC is a model code whose local adoption must be checked. In its asphalt-shingle sidewall section, R905.2.8.3 allows continuous or step base flashing and specifies minimum height and width of 4 inches; it also says the vertical leg is continuous under siding. That scoped code text is not a universal custom-flashing specification for every roof covering, wall cladding, climate, or jurisdiction.

Manufacturer instructions can add conditions. For example, GAF's technical bulletin for its asphalt-shingle systems describes 5-inch-by-5-inch step flashing as its standard and permits a 4-inch-by-4-inch alternative only with a specified 36-inch-wide leak barrier at the roof-to-wall transition. This example applies to the GAF detail described in that bulletin—not to every asphalt shingle, metal panel, or shed. Use the current instructions for the actual products and ask the local building department which code edition and amendments apply.

The IRC separately addresses flashing at a vertical front wall against asphalt shingles by directing the installer to the shingle manufacturer's instructions. That is another reason to identify a headwall rather than reuse sidewall measurements.

When to stop and call a roofer

• You cannot reach the area without climbing onto a steep, wet, icy, windy, fragile, or otherwise uncertain roof.
• The repair would require removing shingles, metal panels, siding, stucco, or masonry to expose hidden flashing layers.
• The shed leaks, has soft wood, has visible rot, or has damage that could extend into sheathing or framing.
• The roof uses a proprietary metal-panel system, or the wall has masonry veneer, stucco, or a hidden drainage plane you cannot identify.
• You cannot verify the roof manufacturer's flashing detail, the wall-layer sequence, or the locally adopted code requirements.

OSHA's ladder guidance is written for construction work and is not a homeowner-specific fall-risk assessment. If you are not certain you can access and measure the joint safely, do not go onto the roof to collect dimensions. Have a roofer inspect it and provide the profile measurements.

Where Trimgy can help—and where it cannot

After a roofer, roof-system supplier, or fabricator confirms the required cross-section, Trimgy can help a signed-in user draw and price a custom sheet-metal trim profile. The drawing grid is in inches and snaps in quarter-inch increments. Use the confirmed dimensions and profile—not a guess based on the visible gap.

Trimgy does not identify whether a shed needs sidewall step flashing or a headwall detail, choose a roof-compatible flashing, design the wall-layer sequence, or certify code compliance or leak performance. In particular, do not use its profile drawing as a substitute for a step-by-step shingle assembly if the shingle maker specifies individual step-flashing pieces.

The online trim-price estimate is based on the drawn profile and selected order inputs; it excludes shipping and tax. Review the separate order flow for applicable shipping, tax, and the full total before payment. Save and order actions require signing in, and the order interface has its own material, edge, and footage requirements.

Related guide

This article covers the wall-to-roof joint. If your project also needs eave or rake trim, the separate shed drip-edge measurement guide explains how to record roof-edge runs and a trim cross-section without confusing those measurements with the roof-to-wall detail.