Snow Load for Decks and Outdoor Structures

Decks, balconies and open porches are outdoor structures exposed to snow accumulation. They do not have a roof to protect them; they accumulate the full ground snow load (or close to it), and their framing must be designed accordingly.

Are decks designed for snow?

Most modern building codes require residential decks to carry a minimum live load of 40 psf for decks intended for occupancy. In high-snow regions, the design snow load can exceed 40 psf, in which case the snow load governs. A deck in Rochester, NY with a design roof snow load of 38 psf would typically be framed to a higher standard than the minimum 40 psf live load, but they are close.

Ground load vs roof load for an open deck

A deck is not a roof. It has no exposure factor reduction (no sheltering) and no thermal factor (no heat below it). In practice the design snow load for an open deck is often taken directly as the ground snow load Pg, or a simplified fraction of it per the adopted code. Check with your local building department for the design value they require for decks.

Critical spots: columns and connection points

The most common deck failure under snow is at the connection between the deck ledger board and the house band joist, or at post-to-beam connections. These are designed for the tributary area of deck they support; a large deck with a long snow-load season multiplies the stress on every connection.

What about covered porches?

A porch with a solid roof is treated as a roof structure and follows the full ASCE 7 snow load calculation with Ce, Ct and Is. An open pergola or lattice cover allows snow to pass through and is generally designed like an open deck. A partially covered structure (e.g. spaced-slat pergola with 50% coverage) requires engineering judgement; check with your local AHJ.