What Snow Load Does a Boise Deck Need to Carry?

Which snow numbers apply to a Boise deck?

Snow-load questions about decks usually go wrong because three different design values get treated as one. Here is what each means and where Boise's figures come from.

Boise's numbers are modest by mountain standards. NOAA's 1991–2020 normals put annual snowfall at the Boise Air Terminal at 17.6 inches, with about 6 days a year bringing an inch or more. Code values aren't averages, though; they cover the rare heavy, wet storm.

Why does 40 psf govern a Boise deck?

The 2018 IRC's live load table assigns 40 psf to exterior balconies and decks. The deck span tables, from joists in Table R507.6 to footings in Table R507.3.1, are built on the higher of that live load or the site's ground snow load. The joist table footnote reads "Ground snow load, live load = 40 psf, dead load = 10 psf," and the footing table says it is based on the highest load case, dead plus live or dead plus snow.

With a 20 psf ground snow load, Boise never reaches the point where snow controls an open deck. Build to the IRC tables and the deck is already designed for twice Boise's ground snow. The ledger fastener table in R507.9.1.3 is labeled for deck live load 40 psf, dead load 10 psf and snow load at or under 40 psf, and the AWC's DCA 6 guide says it does not apply to decks with snow, drift or sliding snow loads above 40 psf.

What 40 psf looks like on a real deck

Forty pounds per square foot sounds abstract until you multiply it out. A 12 × 16 foot deck has 192 square feet, so the code designs it for 7,680 pounds of live load spread across the surface, on top of 1,920 pounds of assumed framing and decking. Twenty adults at an average of 180 pounds come to 3,600 pounds. A foot of fresh, dry snow typically weighs far less than 40 psf; dense, wet spring snow or a crusted drift weighs more, which is why drifting is the case to watch rather than ordinary snowfall.

The span tables also control bounce. The joist table footnotes set a deflection limit of L/360 at the main span, meaning a 12-foot joist may sag no more than about 0.4 inch under full load, and L/180 at a cantilever with a 220-pound point load at the end. That is why a deck built to the tables feels solid with a crowd on it.

Those species are what Treasure Valley lumberyards most often stock for deck framing. Southern pine spans run slightly longer in the same table. Decking also limits spacing: composite boards follow the manufacturer's rating, and many residential composite lines call for 16 inches on center, which is why most composite decks here are framed at 16 inches or closer even when the joist table would allow more.

When does snow change the design?

Sliding and drifting snow

A deck tucked under a house roof can receive snow that slides off a pitched roof or drifts into the corner where walls meet. That concentrated load can exceed 40 psf over part of the deck even when the ground has a few inches. Boise's design criteria page notes local amendment language clarifying drifting. If your deck sits under a valley, a long eave or a metal roof that sheds, have the designer check it, because the prescriptive tables don't cover it.

Covered decks and patio roofs

A roof over a deck is a roof. Boise's design criteria require a uniform roof snow load of at least 25 psf, and Boise requires a permit for any attached patio cover regardless of size. Covers that meet the city's Residential Patio Cover Construction Policy may not need an engineer; others do. A roof also changes what the deck below carries: its posts bring roof load down to footings that must be sized for it, which is a different calculation from the open-deck footing table. Our covered deck ideas page covers options.

Higher-elevation subdivisions

Snow loads climb with elevation. Eagle's residential checklist requires structural stamped plans for homes in its Avimor and Valnova hillside developments, citing special ground snow load areas among the reasons. If you're building in the upper Foothills or a hillside subdivision, ask the building department for the site's ground snow load before you assume Boise's 20 psf.

Hot tubs and planters

Concentrated loads are a separate problem. DCA 6 lists hot tubs as beyond its scope, and Trex's installation guide tells owners to consult a local engineer or inspector for span recommendations under hot tubs and heavy planters. Snow piled on a tub cover adds to that. See our hot tub deck ideas for the framing approach.

Snow load scenarios at a glance

How should you clear snow from a deck?

You don't need to shovel a properly built open deck for structural reasons after a typical Boise storm. Clear it for safety, to keep doors working, and to stop ice forming where melt refreezes at night. When you do, follow the board maker's instructions:

• TimberTech's care guide calls for an all-plastic shovel and says metal shovels or plastic shovels with a metal leading edge will damage the surface. It allows most calcium chloride ice melts and says to avoid rock salt and other abrasive products.
• Trex's care page says never to use a metal shovel on Trex decking, notes that shovel scratches aren't covered under warranty, and allows calcium chloride or rock salt if rinsed off when practical.
• On wood, a plastic shovel pushed along the boards is gentler than chopping at ice. Rinse ice melt off in spring before staining.
• Don't pile snow from the driveway or roof onto the deck. A plowed pile is denser and heavier than snowfall.

Our deck maintenance guide covers the rest of the winter routine.

How we handle snow load on Boise decks

For an open deck, we frame to the IRC tables at 40 psf live and 10 psf dead and check where the house roof sheds. For anything with a roof, we size the cover for Boise's 25 psf minimum plus any drift at the house wall, bring its posts down to their own footings, and use the city's patio cover policy or an engineer as the design requires. The covered decks and pergolas page explains the build.

Shortcuts to avoid

1. Hanging a solid cover off the deck's rim joist. The deck framing was not sized for roof loads.
2. Using the open-deck footing table for posts that carry a roof.
3. Assuming Boise's 20 psf applies in a hillside subdivision with its own snow requirements.
4. Ignoring the roof valley that dumps onto one corner of the deck every February.
5. Using a metal shovel or ice chipper on composite or PVC boards.
6. Putting a hot tub on a deck framed only for 40 psf.

Related resources

Snow load flows down to the ground through the posts and footings; our footing depth guide shows how footings are sized for it. For covered decks and roofs, see the covered deck cost guide, and if you're unsure whether an older deck was built to carry these loads, our repair-or-replace checklist walks through it. Ready to plan? Request an estimate.