A concrete patio in the Seattle area should sit on a compacted gravel base, slope slightly away from the house, and have control joints plus an isolation joint at the foundation. Plan where runoff will go, since dense glacial till soaks up little water, and choose a textured finish for traction. Check city stormwater rules and HOA approvals first.
Last updated
Around Seattle, a patio has to earn its keep in two seasons: the dry summer months, when everyone wants to be outside, and the long wet stretch from fall through spring, when the slab mostly needs to shed water and not turn green. A well-built concrete patio does both. The fundamentals are a stable base, a clear plan for drainage and well-placed joints.
Planning the patio
- Size for real use. Lay out the table, chairs, grill or fire feature and leave room to walk around them.
- Sun and shade. North-facing and tree-shaded patios stay damp longer and grow moss and algae faster. A sunnier spot dries out sooner after rain.
- Cover. Many homeowners here add a roof or pergola to use the patio in drizzle. Posts for a covered structure usually need footings, and covered structures generally need a building permit, so plan them from the start.
- Access. Backyards on hillside or fenced lots often need concrete pumped or wheeled in, which affects cost and scheduling.
Drainage comes first
A patio should slope slightly away from the house so rain runs off instead of toward the foundation. On many Puget Sound lots, the soil under the topsoil is dense glacial till that water barely soaks into, so runoff from a new slab can collect in a low corner or flow toward a neighbor. Think about where the water goes next: a lawn edge, a rain garden or an approved drain. Adding hard surface can also trigger stormwater requirements in some cities, including Seattle, depending on the size of the project. Seattle Public Utilities and your city's permit office can tell you what applies.
Base, thickness and joints
Patios are commonly about four inches thick over a compacted gravel base. Soil next to a house foundation is often backfill that was never compacted well, and it can settle under a new slab if it isn't compacted or replaced. Control joints divide the slab into roughly square panels so shrinkage cracks form along them. An isolation joint where the patio meets the house lets the two move independently.
Finish options for a wet climate
- Broom finish: economical and slip-resistant when wet.
- Exposed aggregate: a long-time Northwest favorite. The surface paste is washed off to reveal stone, which gives texture and good traction.
- Stamped or colored concrete: patterned to resemble stone or slate. See our stamped and decorative concrete page, and ask about slip resistance.
- Smooth trowel: attractive but slippery in the rain, so it's rarely a good outdoor choice here.
Permits and HOAs
Whether a ground-level patio needs a permit depends on your city, the amount of new hard surface, and whether the lot includes a mapped critical area such as a steep slope. Many Eastside subdivisions in places like Sammamish and Redmond also have HOA approval rules for patios and colors. Check both before the pour is scheduled.
Moss and upkeep
Shaded slabs in the Seattle area grow moss and algae, which can make them slick. Keep gutters and downspouts from dumping onto the patio, sweep debris off, and clean the surface periodically. A penetrating sealer can slow water absorption.
Questions to ask
- How will the slab drain, and where will the water go?
- What base material and thickness are included?
- Where will joints be placed, including at the house?
- How will you protect the pour if it rains, and how will you get concrete to the backyard?
Ready to plan it? Request a patio contractor and share a photo of the space.
See how it works
Frequently asked questions
How thick should a concrete patio be?
Patios are commonly about four inches thick over a compacted gravel base. Areas that will carry a hot tub, outdoor kitchen or roof posts may need extra thickness or footings, so mention future plans to your contractor.
Do I need a permit for a concrete patio in Seattle?
It depends on the size of the new hard surface, whether the lot has a mapped critical area such as a steep slope, and whether the patio will be covered. Covered structures generally need a building permit. Check with your city's permit office and your HOA.
How do I keep a shaded patio from getting slippery?
Choose a broom or exposed aggregate finish, keep downspouts from dumping onto the slab, sweep off needles and leaves, and clean moss and algae periodically. A penetrating sealer can slow water absorption.
Sources and further reading
- Washington State Department of Labor & Industries: Register as a contractor
- Washington Utilities and Transportation Commission: Call before you dig
- Seattle Department of Transportation: Property owners' responsibilities
- Streets Illustrated (Seattle right-of-way manual): Construction permits
- Seattle Department of Construction and Inspections: Environmentally Critical Areas code
- Seattle Public Utilities
- City of Bellevue: Right-of-way permits
- City of Redmond: Right-of-way use permit
- City of Tacoma: Sidewalks
- City of Renton Municipal Code: Critical areas regulations
- Washington State University Extension
- USDA NRCS Web Soil Survey
- American Concrete Institute (ACI)
- Portland Cement Association (PCA)