Frost Line Depth in Mason County, Washington
- Mason County’s adopted code
- Checked Oct 2026
- Sources
Verified
12in
30 cm · 1 ft
Set in the building code Mason County has adopted.
Source and how we checked
Document: masoncountywa.gov
Climate: 46 nights a year at or below 32 °F (NOAA 1991–2020, Cushman Powerhouse #2, 1.5 mi).
Checked Oct 2026. Report a correction
How deep is the frost line in Mason County?
Mason County's building authority sets the frost line depth at 12 in (30 cm). Footings, piers and other permanent supports must reach at least this depth (IRC R403.1.4.1).
Who sets the frost line depth in Mason County?
Washington leaves the frost line depth to each city or county, and Mason County fills it in when it adopts the residential code. The value above comes from Mason County’s own document on masoncountywa.gov. See how Washington sets frost line depth.
How deep do footings, decks and water lines need to be in Mason County?
House or garage footing in Mason County: dig at least 12 in (30 cm). Below the frost line and at least 12 in deep (IRC R403.1.4).
| Project | Depth | Rule |
|---|---|---|
| House or garage footing | 12 in | Below the frost line and at least 12 in deep (IRC R403.1.4) |
| Deck footing (attached) | 12 in | Below the frost line and at least 12 in deep (IRC R507.3) |
| Water service line | 18 in | 6 in below the frost line and at least 12 in deep (IRC P2603.5) |
| Fence post bottom | 12 in | No code rule; common practice is below the frost line |
Work out posts, concrete and footing sizes with the fence post depth calculator or the deck footing calculator.
How cold does it get in Mason County?
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Mason County never averages a low below freezing. 46 nights a year reach 32 °F or colder; Feb is coldest, with an average low of 35.2 °F.
NOAA 1991–2020 climate normals, Cushman Powerhouse #2 station, 1.5 mi away, elevation 21 ft.
Monthly temperatures as a table
| °F | Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Low | 35.7 | 35.2 | 37.2 | 40.1 | 45.8 | 50.1 | 54 | 54 | 49.9 | 44 | 38.6 | 35.2 |
| High | 45.3 | 48 | 52.5 | 57.7 | 65.3 | 69.6 | 75.6 | 75.5 | 69.3 | 58.4 | 49.7 | 44.2 |
Mason County has fewer freezing nights than 30 of the 47 other Washington places we track. The Washington median is 54. Coldest: Chelan County; mildest: Lakewood.
What frost line depth do towns near Mason County use?
4 official codes near Mason County are verified; each links to its document.
| Place | Verified frost line | Distance |
|---|---|---|
| Bremerton | 12 in | 25 mi away |
| Lynnwood | 18 in | 52 mi away |
| Pierce County | 18 in | 52 mi away |
| Redmond | 12 in | 54 mi away |
How likely is the soil in Mason County to heave?
USDA rates the sampled soil around Mason County as 86% none, 14% low for frost action (main soils: Hoodsport, Dungeness variant, Skokomish).
USDA NRCS soil survey, 9 sample points within 2 mi. Higher frost action means more risk of heave; see frost heave.
How do you confirm the frost line before you dig in Mason County?
Mason County at a glance
- Frost line depth
- 12 in
- Population (2023 est.)
- 68,389
- Mean annual temperature
- 51.3 °F
- ZIP codes (15)
- 98312 · 98380 · 98502 · 98524 · 98528 · 98541 · 98546 · 98548 · 98555 · 98557 · 98560 · 98563 · 98584 · 98588 · 98592