Shed and equipment pads are usually reinforced slabs over compacted crushed rock, while workshops, garages and ADUs need thicker slabs, footings and often a vapor retarder. Foundations for additions and ADUs go through the building permit and inspection process. Oregon requires a CCB license for concrete work and an 811 locate at least two business days before digging.
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Sheds, workshops, detached garages, hot tubs, accessory dwelling units (ADUs) and additions all need something solid underneath. Around Portland, backyard ADUs and garage conversions have made slab and footing work a common homeowner project. Small pads are fairly simple. Anything that will be lived in or attached to the house becomes building code work with permits and inspections.
Common slab and foundation projects
- Pads for storage sheds, studios and garden buildings
- Slabs for workshops, detached garages and carports
- Pads for hot tubs, heat pumps and generators
- Foundations for ADUs, additions and covered patios
Small structures and pads
A shed or equipment pad is usually a reinforced slab over a compacted crushed-rock base, often with a thickened edge. Size it to the building plus a little extra, level it for the structure while letting water shed at the edges, and keep it out of setbacks, easements and drainage paths. Cities around the metro, including Portland, set size and height thresholds for when accessory structures need a permit, and zoning rules apply either way. HOAs often add their own rules, so check before you pour.
Workshops, garages and ADUs
A slab that holds vehicles, heavy tools or living space needs more thickness, reinforcement and footings to carry the walls. Because Portland-area ground stays damp much of the year, ask about a vapor retarder under any slab you plan to heat, finish or coat, plus insulation and where plumbing and conduit go before the pour. ADU foundations go through the full building permit process, with inspections before concrete is placed.
Additions and structural work
Foundations for additions and other habitable space must follow the building code your city has adopted. That typically means a permit, footings sized for the soil and load, inspections, and sometimes engineered drawings. Oregon's Construction Contractors Board licenses contractors, and concrete is among the kinds of work it says require a license. On structural jobs, a concrete contractor often works under a general contractor who holds the permit, so make sure everyone's role is clear.
Portland-area site factors
- Wet soil: footings should bear on firm, undisturbed soil, and digging in the drier months makes that easier to see and protect. Saturated, muddy footing trenches are a reason to pause.
- Hillsides: on steep lots in the West Hills, Lake Oswego, West Linn and the Gresham buttes, check DOGAMI's landslide maps and expect the city to ask for geotechnical input on larger cuts or fills.
- Footing depth: your local building department sets the required depth below grade. Ask rather than guess.
- Drainage: keep the slab above surrounding grade and slope the ground away. Plan where new roof runoff will go.
- Utilities: Oregon requires an 811 locate request at least two business days before digging.
Questions to ask
- Does this project need a permit, and who will pull it?
- How thick is the slab, and what reinforcement, base and vapor retarder are included?
- Do you need engineered drawings, and when are inspections scheduled?
- What do you need from the shed, ADU or addition builder before you start?
Planning a building or addition? Request a slab or foundation contractor and include the structure's size and use.
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Frequently asked questions
Do I need a permit for a shed slab around Portland?
It depends on the structure's size and height and your city. Portland and the suburbs set thresholds for when accessory structures need a building permit, and zoning setbacks apply either way. Check with your city's permit office and your HOA.
Should a slab have a vapor barrier in Portland?
For any slab you plan to heat, finish, carpet or coat, a vapor retarder under the slab is commonly recommended because Portland-area ground stays damp much of the year. It isn't usually needed for an open shed pad.
Can I pour footings on a steep lot?
Often, but steep lots in the West Hills and other slopes may need geotechnical input, especially with large cuts or fills. DOGAMI publishes landslide susceptibility maps, and your city may require an engineer's report.
Sources and further reading
- Oregon Construction Contractors Board: license requirements
- Oregon CCB: Guide to home improvement contracts
- Portland Bureau of Transportation: driveways
- Portland Bureau of Transportation: sidewalk repair notices
- Portland Permitting & Development: outside work permits
- Portland Bureau of Environmental Services: Clean River Rewards
- Portland Water Bureau
- City of Gresham Permit Services
- OSU Extension: clay soil challenges for Oregon gardeners
- DOGAMI: landslide hazard and risk study, Multnomah County (IMS-57)
- National Weather Service Portland
- Oregon 811: call before you dig
- American Concrete Institute (ACI)
- Portland Cement Association (PCA)