Southwest Washington Reference
The numbers I build to in Cowlitz and Clark County. Rafter spans, ground snow by city, and the details that actually get flagged at inspection here.
This is a reference, not an engineered or stamped drawing. The values below are transcribed from published code tables and regional design criteria. Your building department has the final say, site conditions vary, and anything unusual belongs with a licensed engineer. Use this to understand what you are looking at and to check that a bid makes sense.
Most span tables you find online are written for somewhere else. That matters more than people expect, because the single number that drives a roof structure is ground snow load, and it changes inside a twenty-minute drive around here. A patio cover detailed for Battle Ground is carrying half of what the same structure carries in Toutle.
I size to 30 psf ground snow across this service area. That is one notch above Kalama and it covers every city in the table below except Toutle. It means some jobs get a slightly heavier member than the minimum, which I would rather explain than defend a sagging roof.
Everything here is what I hand a customer when they ask why a bid says what it says. Thirty years in the field, licensed in Washington, permits pulled in house.
These print in the corner block of a permit sheet in this region. If a drawing you have been handed does not carry them, it was not drawn for here.
Published ground snow load, in pounds per square foot, for cities across Cowlitz and Clark County. This is the number that decides your rafter size before anything else does.
Elevation moves this faster than distance does. Toutle sits under 500 feet and carries nearly double what Battle Ground does. Above roughly 400 feet, Cowlitz County is treated as a case study for snow, which means a licensed engineer sets the site value rather than a table. If your property is up in the hills, that is a conversation to have before anything is drawn.
Maximum clear span at 30 psf ground snow and 10 psf dead load, which covers metal roofing or up to two layers of asphalt. Heavier roofing such as tile or slate runs shorter and is not listed here.
One thing worth knowing about these particular numbers. A patio cover has no ceiling attached underneath it, so the code minimum for that case is a looser deflection limit than what is printed above. I size to the stricter ceiling-attached family instead. The practical result is a stiffer roof that does not bounce or telegraph a wave along the fascia, for the cost of stepping up a size on some jobs. If you compare this table against another one and mine looks conservative, that is why.
I also hold back about six inches from any published maximum. A member used right at the edge of its table is legal and it is not what I want holding up a roof someone sits under.
Beams are the part people most want a chart for and the part a chart serves worst. A beam size comes out of post spacing and rafter span together, and the published patio cover tables that circulate online were mostly written for climates with no snow at all. Run one of those grids at our load and its reach collapses. The table will happily tell you a beam will span twenty feet. Here it will not.
My working line is twelve feet. Up to twelve feet between posts I will build it in dimensional lumber. Past twelve feet it goes to engineered glulam and an engineer sizes it. That is not a code number, it is a field number, and it comes from what I have watched hold up and what I have watched sag.
This is also the real trade in front of most customers. Fewer posts means a cleaner view and engineered lumber. More posts means dimensional lumber and a lower number. Both are legitimate. Anyone who tells you the wide span is free is not sizing the beam.
Spans get most of the attention and are rarely what an inspector catches. These are.
Twelve inches is a habit a lot of people carry from unpermitted work, and it is under depth for a permitted footing in this region. Frost depth drives it. If your footings are already poured at twelve, that is a correction on an inspection report, and it is the most common one I see on a deck.
A flat looking patio cover is not flat. A quarter inch of fall per foot is the floor, and water that has nowhere to go will find the ledger and then the wall behind it. Slope also has to be planned against the door header height before anything is framed, because you cannot add it later without lowering the whole roof.
Below a 2:12 slope you are in metal or a built up roof, not asphalt shingles. This decides the look of the finished structure and it gets settled at design, not at the roofing stage. A lot of covers that want to stay low end up in metal for exactly this reason.
A patio roof with open sides is a wing. Wind lifts it. Ties go post to beam and rafter to beam, both, and they cost almost nothing while the framing is open. They cannot be added once the roof is on. At 110 mph ultimate wind speed this is not a formality.
Half inch lags at sixteen inches on center, staggered, with roof-to-wall flashing detailed so water leaves the house rather than soaking into it. A ledger fastened into framing that will not hold it is the most common way a structure fails, and it does not fail slowly. On an older house nobody can tell you what is behind the siding from the driveway. It has to be opened up.
Cowlitz County charges by project valuation plus a plan review percentage rather than a flat fee, which is why nobody can quote you a permit number over the phone. I built a calculator off the adopted fee schedule if you want to run yours.
Cowlitz County permit cost calculatorUnincorporated Cowlitz County only. City limits and Clark County run their own fee schedules.
Nobody can bid a job that has not been drawn, and the county will not take a permit application without a drawing. Send dimensions and a couple of photos and I will come back with a scaled project sketch.