The parts of a pergola
A freestanding pergola has three layers of lumber:
- Posts set in the ground or on concrete footings.
- Beams that span between posts along the length, often doubled (two boards bolted on either side of the posts).
- Rafters that cross the beams, spaced evenly, creating the shade.
Many designs add a fourth layer of thin purlins or slats on top of the rafters for more shade.
The calculation
- Posts per side = length ÷ maximum post spacing, rounded up, plus one. Posts = 2 × posts per side.
- Beam length = pergola length + overhang at both ends.
- Rafters = length ÷ rafter spacing, rounded up, plus one.
- Rafter length = pergola depth + overhang at both ends.
- Post length = height above ground + embedment depth.
Example
A 12 × 10 ft pergola with posts at most 12 ft apart, rafters every 16 in and 1 ft overhangs needs 4 posts 10 ft long (8 ft above ground, 2 ft in the ground), 4 beam boards 14 ft long (two doubled beams) and 10 rafters 12 ft long: 216 linear feet of lumber, about $648 at $3 per foot. Setting the posts in 10 in holes takes about 5 bags of 80 lb concrete.
| Part | Common size |
|---|---|
| Posts | 6×6 (or 4×4 for small, light pergolas) |
| Beams | doubled 2×8 to 2×10 |
| Rafters | 2×6 to 2×8 |
| Purlins | 2×2 or 2×3 |
Spans and spacing
Beam spans of 8 to 12 ft are typical for doubled 2×8s or 2×10s; longer spans need bigger beams or an extra post. Rafters can span 10 to 14 ft depending on their size and wood species. Check a span table or ask your lumberyard for your species and grade.
Building tips
- Use pressure-treated lumber rated for ground contact for posts, or set posts on raised metal bases over concrete piers to keep the wood dry.
- Cut decorative tails on beams and rafters before assembly.
- Brace posts diagonally until the beams and rafters are fixed.
- An attached pergola or one in a snowy or windy region may need a permit and engineering.