Carport slab in Austin: slab first, carport second
Order the carport, get its drawing, then pour. The posts land at fixed spacing, and each one needs enough concrete around its anchor. A slab poured to a guess means posts that land on an edge or a joint. A carport slab is otherwise a driveway slab under a roof: 4 to 6 inches of reinforced concrete per HomeBlue's Austin guide, thick enough for the vehicles under it.
Carport slab sizes and Austin costs
| Carport | Area | projectcosted Austin | HomeBlue Austin |
|---|---|---|---|
| One car, 12x20 | 240 sq ft | $1,351 to $2,702 (computed) | $1,440 to $2,400 (computed) |
| Two car, 20x20 | 400 sq ft | $2,252 to $4,503 | $3,400 to $5,900 |
| Two car roomy, 24x24 | 576 sq ft | $3,243 to $6,486 (computed) | $3,456 to $5,760 (computed) |
| RV or three car, 20x30 | 600 sq ft | $3,377 to $6,755 | $5,100 to $8,800 |
| Large RV, 24x40 | 960 sq ft | $5,405 to $10,810 (computed) | $5,760 to $9,600 (computed) |
Swipe the table sideways to see every column →
projectcosted (August 2026) and HomeBlue (September 2024) Austin figures; computed rows multiply area by $5.63 to $11.26 and $6 to $10 per sq ft.
Anchors, edges and drainage
- Post spacing and anchor type from the carport maker's drawing
- Any minimum slab depth or edge distance the anchors need
- A few extra inches beyond the posts so anchors aren't at the very edge
- Fall toward the open side so roof runoff and rain drain off the slab
- A clean meeting point with the driveway, with no lip to trip on
Rain off a carport roof lands in a line along the low edge, so the slab should carry it away from the house. Within 10 feet of a house, the residential code wants paved surfaces sloped at least 2 percent away (IRC R401.3). Austin averages 36.25 inches of rain a year, with May the wettest month at 5.04 inches (NOAA 1991 to 2020 normals, Camp Mabry).
Permits: the slab and the carport are separate questions
The City of Austin exempts residential concrete flatwork outside the public right-of-way from a building permit. The carport is a different matter. The exemption for detached accessory structures stops at 200 square feet and 15 feet high, and the City's residential application counts detached covered parking as building coverage. A 20x20 carport is 400 square feet. Setbacks and HOA rules apply either way.
