Pergola span calculator

Enter how far the beam and the rafters have to span. We show how far each board can go, how much of that your pergola uses and which check sets the limit.

Your doubled 2x12 beam and 2x6 rafters work. The beam can span up to 13'-6-1/4" and yours is 12'-0". The rafters can span up to 12'-10-3/4" and yours is 10'-0". The part working hardest is the beam, at 79% used, limited by bending.

Allowable span and how much of it your design uses
Member	Size	Allowable span	Your span	Used	Limited by
Beam	doubled 2x12	13'-6-1/4"	12'-0"	79%	bending, snow case
Rafters	2x6	12'-10-3/4"	10'-0"	60%	bending, snow case

The rafter span counts a 12 in overhang past the beam, and the beam counts a 12 in overhang past the end posts. The snow load used is 25.2 psf on the roof, after the flat-roof factors.

Design this pergola in 3D, free

Carries over: sizes, post count, lumber, rafter spacing and snow load. The width and depth are the post and beam centers plus the post width, so they read a few inches over your spans. Not carried: wind, soil and frost, which start at the designer defaults. A span check covers the beam and rafters only, not the posts or footings.

Planning figures from the same engine as our pergola designer, not an engineer's design or a permit. Your building department has the final word, and local rules may be stricter.

How the pergola span calculator works

A pergola roof is a ladder of boards. The rafters rest on the beams, and the beams rest on the posts. The pergola span calculator checks two spans: how far the beam goes between posts, and how far the rafters go between beams or from a beam to the house. For each one it shows the longest span the board can take, the share of that limit your span uses, and which check sets the limit.

Beam span and rafter span

The beam span is measured from the center of one post to the center of the next. The rafter span is measured from the center of one beam to the center of the other, or from the beam to the face of the ledger on an attached pergola. Spans are taken center to center, which is slightly longer than the clear gap and so errs on the safe side (our own conservative choice, not a code rule).

What the percent used means

At 100% a board is exactly at its limit. Under 100% it passes, over 100% it fails. In the default example, a double 2x10 beam over a 12'-0" span uses 110% and 2x8 rafters over a 10'-0" span use 38%.

The checks behind the answer

Bending

The board bends under load, and bending sets the span for most pergola rafters. Wet outdoor lumber is weaker than the dry values in the usual tables, so the calculator reduces the bending strength to 85% for wet service. A tall, narrow board can also twist sideways, which the calculator checks with the NDS beam stability factor. The top edge is held by the boards above it.

Deflection

A pergola can pass on strength and still sag. The calculator limits the sag to span over 180 under snow or live load and span over 120 under all loads together. Long beams in softer lumber are often limited by this, not by strength.

Shear and overhang

Shear is checked near the supports, where short, heavily loaded boards can split. Overhangs past the beam add bending and sag of their own, so the calculator checks them too. Keep an overhang under a quarter of the back span (25% rule, carried over from deck framing).

Choosing the boards

Rafters

Closer spacing lets a rafter go further. At 12 in on center a 2x8 spans 18'-5-3/4", at 16 in 16'-2-1/2" and at 24 in 13'-4-3/4", in Southern Pine under the default snow load. Rafters carry the slats too, but in a pergola the slats are light.

Beams

A double beam, one board on each side of the posts, is the strongest and the simplest to build. A beam sitting on top of the posts needs a post cap and two or three plies. If a beam fails by a small margin, one more post is usually cheaper than a deeper board: it cuts the span in half for the middle bay.

Posts

The posts are checked as columns in the designer and the pergola plan, not in this calculator. A 6x6 is the safer choice for any pergola taller than 8 ft or with a long beam.

What this calculator does not check

It does not check wind uplift, the footings, the post connections or the ledger on the house. The pergola designer checks all of them for the whole pergola and shows a pass or fail for each. Snow sliding off the house roof onto an attached pergola is not in the loads. Local rules and homeowner associations can set their own limits.

Building a patio cover instead? A pergola plan lists every board and fastener for a size you pick.

Where the numbers come from

Pergolas are not in the span tables of the International Residential Code, so these spans are calculated, not looked up. The wood strengths are the National Design Specification values for No. 2 Southern Pine and Hem-Fir, reduced for wet outdoor service. The same engine sizes the members in the 3D pergola designer.

The loads are 20 psf of roof live load, snow worked out from the ground snow load you enter, and the weight of the boards plus 2 psf for hardware. Deflection is limited to span over 180 under live or snow load and span over 120 under all loads. Wind on the posts and the footings is checked in the designer, not here.

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Questions

Anything else? Write to us or see every calculator.

Questions people ask

It depends on the boards, the load and the number of plies. With a double beam in Southern Pine, 6x6 posts and 30 psf of ground snow, a pair of 2x8 boards spans 9'-8-1/2", a pair of 2x10 boards 11'-5-1/2" and a pair of 2x12 boards 13'-5", carrying 10 ft rafters. Enter your own spans above and the calculator checks them.

As a pergola rafter at 16 in on center, a 2x10 spans 19'-0-1/4" between beams in Southern Pine with 30 psf of ground snow. As a double beam carrying 10 ft rafters it spans 11'-5-1/2" between posts. The beam is the lower figure because it carries the rafters as well as the roof load.

Pergolas have no span table in the building code the way decks do, so we work each span out from the wood properties and the loads instead of reading a chart. The result is the same kind of answer: a rafter spans 12'-10-3/4" in 2x6, 16'-2-1/2" in 2x8, 19'-0-1/4" in 2x10 and 22'-2" in 2x12 at 16 in on center under the default snow load. The calculator shows yours.

A lot where it snows. A 2x8 rafter at 16 in on center spans 18'-6-1/2" with no snow load and 16'-2-1/2" with 30 psf of ground snow. Ask your building department for the ground snow load of your address and enter it.