Adding a Lean-To to a Metal Building in Spokane: Frame Checks, Snow Drift, Permits and Roof Slope (2026)

A lean-to is the cheapest way to add covered space to a metal building, but only if the building you already have can carry it. A lean-to hangs one side of its roof on your existing sidewall columns, so it adds weight, wind load and a new snow drift to a frame that was never designed for them. Here's what Washington's code, the Metal Building Manufacturers Association (MBMA) and Spokane County's snow loads mean for a lean-to on a shop, barn or pre-engineered metal building (PEMB).

What a lean-to actually is

A lean-to is a single-slope roof framed off the side of a taller building:

• The high side bears on the existing building's sidewall columns
• The low side sits on a new, shorter column line with its own footings
• There's no ridge and no second full wall, which is why it's the least amount of steel you can buy per square foot of cover
• Typical uses: equipment and hay storage, RV and boat parking, a covered work area or an enclosed wing for storage

On a new building, a lean-to is just part of the design. The engineer sizes the shared column for the extra load from day one. Adding one to a building that's already standing is a different project.

Why the existing frame has to be checked first

MBMA's guidance on retrofitting metal buildings says the existing sidewall column, which becomes an interior column once the lean-to goes on, has to be reviewed for:

• Extra axial load from the lean-to rafters bearing on it
• Lost bracing: wall girts brace a column along its height. If the girts and siding come off so the lean-to opens into the main building, the column can lose that bracing. If it's critical, MBMA says the column needs reinforcement, commonly by welding plates to both flanges
• The foundation under it, which was sized for the original building's loads only

Metal Construction News makes the same point: taking off the girts and siding unbraces the column at the interface while it's picking up more load, and the foundation under it takes that load too. Their alternative is a separate building next to the existing one, with the gap set by structural analysis, so the old girts and siding stay in place.

Metal building suppliers say the same thing. Valley Steel's engineering manual says the project engineer of record, not the metal building supplier, has to investigate the existing building to make sure it stays adequate with the added loads.

What Washington's existing building code requires

Washington adopted the 2021 International Existing Building Code (IEBC) with state amendments, effective statewide March 15, 2024. For additions, IEBC Section 1103 says:

• Gravity loads: any existing element whose design gravity load goes up by more than 5 percent because of the addition has to be replaced or altered to carry the loads required for new construction
• Lateral loads: if the addition isn't structurally independent, the existing building and the addition are treated as one structure and have to meet the building code's wind (IBC 1609) and seismic (IBC 1613) requirements
• The 10 percent exception: an existing lateral-load element can stay as is if its demand-to-capacity ratio goes up by no more than 10 percent with the addition included

In plain terms, a lean-to that ties into your building is usually going to need an engineer to run the numbers on the old frame, not just the new one.

The snow drift problem at the roof step

Spokane County's residential permit packet lists a 39 psf ground snow load, and the County's SCOUT map shows higher loads in parts of the county. Our Spokane County snow load guide explains how to find your parcel's number.

A lean-to creates a roof step, where a lower roof sits against a taller wall. Under ASCE 7-16 Section 7.7, which Washington's 2021 IBC uses for snow loads:

• Snow drifts against the step: wind blows snow off the upper roof and piles it on the lower roof against the wall
• The drift is extra load: its height is figured from the length of the upper roof and the ground snow load, and it's added on top of the normal snow load on the lean-to roof
• Heavier snow in snowier places: ASCE 7-16 sets snow density at 0.13 times the ground snow load plus 14, capped at 30 pounds per cubic foot. At Spokane County's 39 psf, that works out to about 19 pcf
• Sliding snow: a steeper upper roof can also dump sliding snow onto the lean-to, which ASCE 7 treats as a separate load

That's why a lean-to roof and its rafters are often heavier than the same roof would be on its own, and why the low eave height you pick changes the design.

Roof slope and the tie-in

• Minimum slope: IBC 1507.4.2 allows lapped, non-soldered metal panels down to 3:12 without lap sealant, 1/2:12 with lap sealant and 1/4:12 for standing seam. A lean-to's pitch is limited by how much height is left under the existing eave
• The wall junction: where the new roof meets the old wall is where leaks start, so it needs proper flashing, not just sealant
• Water off the old roof: Valley Steel notes that water running off an existing building can void the Galvalume warranty on a new roof below it. Our metal rust guide explains why some metals don't mix

Do you need a permit?

Plan on it. Spokane County's permit exemption handout (BP-25) exempts one-story detached accessory structures up to 200 sq ft. A lean-to is attached to your building, and it changes that building's structure. Setbacks, lot coverage and height still apply, and the new column line has to clear your property lines. See our shop and pole barn permit guide and rural setback guide.

New footings for the lean-to's outer columns have to go below Spokane County's 24-inch frost depth. Our frost depth guide and PEMB foundation guide explain what those footings carry.

Three ways to add space, and when each makes sense

• Plan the lean-to into a new building: the cleanest option. The shared column and footing are sized for it from the start
• Tie a lean-to into an existing building: works when the original frame has the capacity or can be reinforced. MBMA says to start with the manufacturer's name (often on the ridge cap) and the order number (often on a plaque on a column) so the original design can be reviewed
• Build a separate, freestanding addition: a structurally independent building beside the old one. Under IEBC 1103.2, the existing lateral system can then stay unaltered, and you avoid reinforcing old columns

Extending the length of a building is another option. MBMA notes it depends on how the existing endwall was built (an expandable rigid frame, post-and-beam, or a half-load frame), and the footings may need changes.

What it costs

A lean-to addition is quoted per project, because the price depends on whether the old frame and footings need work. For a separate new building, Raven's going rate for an installed pre-engineered metal building is about $60 to $75 per sq ft, with electrical, plumbing and utility hookups not included. Pole buildings run about the same installed in our market.

Raven isn't tied to one manufacturer. We source steel from whichever manufacturer can deliver the engineered package fastest, which helps when you're matching an add-on to a building that's already up. Our PEMB lead time guide walks through a realistic timeline.

Get a lean-to or addition quote

Tell us your building's size, eave height and manufacturer if you know it, and what you want to cover. Call 509-350-2016 or request a free quote. We'll pour the footings and erect the steel. See our steel building services.

Sources

• Installed pricing: Raven Buildings' current going rate
• MBMA, Retrofitting/Reinforcing Metal Building Systems (PDF)
• Metal Construction News, "Don't Lean on Me"
• Valley Steel Product and Engineering Manual, Existing Building Tie-In Systems (PDF)
• IEBC Section 1103, Structural (UpCodes)
• Washington SBCC, Building Code Amendments (2021 IEBC, effective March 15, 2024)
• IBC 1507.4, Metal Roof Panels (UpCodes)
• ASCE, Drifts on Lower Roofs
• SK&A, Snow Drift Loads: ASCE 7-16 vs 7-22
• Spokane County BP-06, Residential Building Permits (design criteria)
• Spokane County BP-25, Permit Exemptions
• City of Spokane / Spokane County Joint Policy BUILD-07, Official Frost Depth (PDF)