Calculators / Sheds, pergolas and gazebos
Pergola plans for your size
Put in how wide, how deep and how tall. You’ll get the beams, rafters, posts and footings that carry it, a drawing from above, and the lumber to buy.
Updated · How we check our numbers
Pergola plan
12 × 12 ft, 4 posts
3-2x12 beams on 6x6 posts 10 ft apart, with 2x10 rafters 16 in on center, in southern pine (pressure-treated).
- Beams
- 3-2x12span up to 10 ft 6 in
- Rafters
- 2x1016 in o.c., span up to 14 ft
- Posts
- 4 × 6x66x6 allowed to 14 ft here
- Footings
- 27 in roundor 24 in square, 11 in thick
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Estimate for planning only, not professional advice. Check the manufacturer’s instructions, your local code and a licensed professional before you buy, build or install. Use at your own risk. Terms.
rafter span = 12 ft → 2x10 at 16 in (DCA 6 Table 2 allows 14 ft)
post spacing = (12 ft − 2 × 1 ft) ÷ 1 = 10 ft → 3-2x12 (Table 3A)
posts: 8 ft vs 14 ft max for 6x6 (Table 4)
Posts (dark) under the two beams, rafters across them.
Cut list
Southern pine (pressure-treated)- 4 posts, 6x68 ft to the underside of the beam, on post bases4 × 8 ft
- 2 beams, 3-2x1212 ft long, 3 plies of 2x12; posts 10 ft apart, beam runs 1 ft past the end posts6 × 12 ft
- 10 rafters, 2x1014 ft each, 16 in apart; span 12 ft, tails 1 ft10 × 14 ft
- 4 footings27 in round or 24 × 24 in, 11 in thick, bottom at least 12 in below grade or below your frost line, whichever is deeper27 in dia
Lengths to buy round up to the next even foot. Treated or naturally durable lumber; hot-dip galvanized or stainless fasteners (DCA 6).
Simpson Strong-Tie suggests knee braces on pergolas over 8 ft tall, with bays over 10 ft, in high wind, or with a solid roof. Yours doesn’t need them for size, but they stiffen any pergola.
DCA 6: “All 3-ply beams shall be connected to the post by a post cap.” Set every beam on the posts in a post cap, or a notch; don’t just bolt it to the side.
These sizes are from a deck guide designed for 40 lb per sq ft, so they’re generous for an open pergola. Add a solid roof, a snow load, or attach it to the house, and it needs to be designed: Apex, NC, for one, says a pergola with a roof is a gazebo and must carry snow and wind loads.
How we work it out
There's no code table for pergolas, so we size every part with the American Wood Council's DCA 6, the prescriptive deck guide that building departments use. Its tables are built for a deck's load, 40 lb per sq ft plus 10 for the wood itself, which is far more than an open pergola carries, so these sizes are on the safe side.
rafters: DCA 6 Table 2 · beams: Table 3A · post height and footings: Table 4
The rafters span from beam to beam, so they're sized as joists for that span and spacing. Their tails can hang past the beam up to the table's overhang, and never more than a quarter of the span. The beam has to span from post to post with that rafter span on it; we use the fewest posts a two- or three-ply beam can manage. DCA 6 requires a post cap on three-ply beams, and you shouldn't just bolt a beam to the side of a post. The beam can run a quarter of the post spacing past the end posts.
6x6 posts can go as tall as Table 4 allows for your beam span, rafter span and species; past that, 8x8 posts work up to 14 ft. Footings come from the same table, at 1,500 psf soil, and go at least 12 in below undisturbed ground, or below your frost line if that's deeper.
Knee braces: Simpson Strong-Tie says to consider them on pergolas over 8 ft tall, with bays over 10 ft, in high winds, or with a solid roof. DCA 6 braces the corner posts parallel to the beam with ½ in lag screws, and never the center posts.
A pergola with a solid roof is a different structure. The Town of Apex, NC, puts it plainly: once a roof is added it's a gazebo, and it must carry snow and wind loads. Have that, or a pergola attached to the house, designed. Our deck calculator uses the 2021 IRC tables, which differ slightly from DCA 6's; we use DCA 6 here because only it gives post heights and footings to match.
Where the numbers come from
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