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Pergolas

How to Build a Pergola: Footings to Slats, With Member Sizes Calculated

Most pergola guides give you a list of boards and a hope. This one gives you the order of work and tells you which sizes have to be calculated: footings for uplift, beams and rafters for your span and snow, and bracing for wind.

By The Patio PlannerUpdated 13 min readChecked against 5 sources

The short answer

To build a pergola, set footings below the frost line, fix the posts on post bases, carry the beam on notched or capped posts, lay rafters and slats on top, then brace the frame against wind. No building code publishes a span table for pergolas, so beam and rafter sizes are calculated for your spans, snow and wind. A footing must hold the posts down in wind, not only up under weight.

If you want to know how to build a pergola, you will find a lot of videos that show the carpentry and almost none that show the sizes. That is the hard part. A pergola is a roof frame in the wind: it needs footings that hold the posts down, a beam and rafters sized for the span, and braces. This guide gives you the order of work for a DIY pergola, and for each step it says what is a code rule, what we calculate and what we are still unsure about.

The parts, from the ground up.

We are not your building department or your engineer. Where a number is our own design choice, we say that too.

Before You Build a Pergola: Size, Height, Site and Permit

Four decisions come before the first hole: how big, how high, where, and whether you need approval.

Size and height

No building code we read sets a pergola size or height, so those are your decisions. What the size does is set the boards. A longer beam span needs a deeper beam, and a longer rafter needs a deeper rafter or a closer spacing. The pergola span calculator shows how far a beam and a rafter can go for your lumber and snow load, so you can pick a size that works with standard boards. For height, think about doors, headroom under the beam and how the shade falls. A taller pergola also catches more wind on the posts.

The site

Look for utilities before you dig, and have them marked by your local locating service. Check where water drains, how level the ground is, and whether trees or overhead lines are in the way. If the pergola attaches to the house, read our guide to a pergola attached to the house first, because that changes the layout.

The permit

Local approval is not something we can check for you. Pergola permit rules, setbacks, HOA and zoning vary, so confirm with your local building department before you buy materials. Our pergola permit guide explains what changes the answer. For decks, see the deck permit requirements.

The order of work at a glance

Here is the whole build in one pass, so you can see where each section below fits.

  1. Draw the pergola, check the member sizes and get the cut list.
  2. Ask your building department about a permit, setbacks and HOA approval.
  3. Have utilities marked, lay out the post centers and square the corners.
  4. Dig and pour the footings with the post bases on a template.
  5. Set the posts plumb and fix them to the bases once the concrete has cured.
  6. Lift the beams onto the posts and fix them with the notch, cap or bolts you designed.
  7. Fix the rafters to the beams with ties, then lay the slats across them.
  8. Add the braces, check every fastener and walk around the frame for movement.

Layout and squaring

Strings and stakes set the post centers. Check the corners square with a 3, 4, 5 triangle, or a multiple of it such as 6, 8, 10 ft, and measure both diagonals. They should match. A pergola that is out of square by an inch shows up in the last slat, and it is far easier to fix in the string line than in the concrete.

Tools you will need

A post-hole digger or auger, a level, a string line, a framing square, a circular saw, a drill and a socket set cover most of the work. A ladder or two helps with the beam, and a second pair of hands is not optional for lifting it.

Lumber and Hardware for a DIY Pergola

Buy lumber and hardware for the loads you calculated, not for the look of a kit.

Treated lumber for the posts

DCA61 calls for pressure-treated lumber rated for the exposure. Above ground it uses a UC3B rating, and for wood in contact with the ground it uses UC4A. Cuts made on site are treated with a preservative. Burying a wood post is not typically recommended by Simpson2 Strong-Tie, because of rot, and a post that goes into the soil needs a UC4B rating at minimum.

Species for beams and rafters

Our designer checks members in Southern Pine No. 2, and in Hem-Fir or Douglas fir-larch, which it treats with the Hem-Fir values. It leaves out SPF because we could not confirm the shear value. The beam and rafter sizes depend on the species, so check that the lumber you buy matches what you designed.

Board lengths

Board lengths vary by store, and we have not verified them. Our plan flags 24 ft boards as likely special order, based on lengths we have not confirmed at a store. Check what your yard stocks before you fix a span, because a joint in a beam must land over a post.

Post bases, caps and ties

You will need a base at the foot of each post and a cap or notch at the top. One example of a base is the Simpson2 CBSTQ cast-in standoff, and an example of a cap is the BCS2-2/4Z for a two-ply beam on a 4x4. Our plan tells you the load each rafter tie must resist. It does not tell you what any particular tie can hold, so check the rating in the maker’s table.

Pergola Footings and Post Bases

The footing of a pergola is sized for uplift. That is the biggest difference from a deck footing, and the reason our pergola footings guide exists. Here is the short version.

Depth

A footing goes at least 12 in below the undisturbed ground, or below the frost line, whichever is deeper. Use your local frost depth, which our deck footings guide explains how to find.

Size

Our designer starts from a round footing sized for the load and the soil, at least 6 in thick, then enlarges it until 60% of the concrete’s weight beats the uplift. A footing wider than 36 in or deeper than 60 in goes to an engineer. Those limits are our own.

The post base

A cast-in base is set in the wet concrete. The Simpson2 CBSTQ is rated at 2,680 lb of uplift on a 6x6 and 4,070 lb on a 4x4, with 485 lb lateral load, from the maker’s table that we read on October 26, 2026. Set the bases on a template, so they are square and in line before the concrete sets.

Pergola Posts and Beams: Notch, Cap or Sandwich

How the beam sits on the post decides whether the joint holds in the wind. There are three common ways.

Each rafter needs a tie that resists uplift.

Notched post

The beam sits in a notch cut into the top of the post. With a two-ply beam on a 6x6, the DCA61 detail uses two 1/2 in bolts through the notch. The notch is the strongest and the cleanest, and it takes the most careful cutting.

Post cap

A metal cap holds the beam on top of the post. DCA61 calls for a cap on a three-ply beam, and the maker’s examples pair a two-ply beam with a 4x4 and a three-ply with a 6x6. Use the cap that matches your post and beam.

The sandwich beam, and an open question

In a sandwich beam, one board sits on each side of the post, held by bolts. DCA61 prohibits a beam fastened to the side of a post with no notch on a deck. On a pergola our designer accepts the sandwich only with the bolts calculated: 1/2 in bolts in double shear, counted from the NDS3 2018 yield equations with the end, edge and spacing distances checked. When the bolts cannot carry the weight, each board sits in a 1-1/2 in deep notch cut into the post top, and the bolts carry only the uplift and the sideways force. Whether to allow this at all is a question we are putting to an engineer, so treat it as our judgment and not a code rule.

Pergola Rafters and Slats: Calculated, Not Looked Up

This is the step where most online guides guess. The IRC and DCA61 have no span table for a pergola, so a rule of thumb for rafter size is a guess.

How the members are checked

Every member is calculated by the National Design Specification for wood, 2018 edition. Our designer runs six checks on each beam and rafter: bending in the span, bending in the overhang, shear, deflection under live or snow load, deflection under total load, and the tip of the overhang. It shows which check governs and the percent used, so you can see how much margin a board has. The method is ours, and it is declared in the plan.

The loads we use

We apply 20 psf of roof live load over the whole footprint, the figure the IRC gives for a flat roof, even though open slats shed part of it. That is conservative. For snow, you enter the ground snow load for your town, and the designer starts from 30 psf if you do not. We could not read the ASCE 7 text, which is paid, so treat the snow results as a check and not a final answer. The deflection limits are L/180 under live or snow load and L/120 under total load. They come from a building code table that we have not read in the text, and applying the roof row to a pergola is our choice.

A check against the IRC

To test the method, we ran a Southern Pine 2x6 and 2x4 rafter at 16 in on center with 20 psf live and 10 psf dead load. The calculation gives 160.5 in and 107.3 in. The IRC rafter table, section R802.4.1, gives 162 in and 108 in. They agree within about 1%.

Fixing the rafters

Each rafter needs a connection to the beam that resists uplift as well as sliding. Our plan gives the load that each rafter tie must resist, so you can match it to the maker’s table. Space the rafters as drawn, since the checks above assume that spacing, and put one over each post so the post top is tied into the roof grid. Cut all the rafters to one pattern and test the first against the beam before you cut the rest.

Overhangs and slats

We keep an overhang to a quarter of the span behind it as a warning level only, and calculate the overhang itself. Slats are lighter members laid across the rafters, and they are checked the same way.

Try it

Draw it first, then cut

Open the pergola designer and set your size, snow and wind. It sizes the beam and rafters, shows the percent each uses, and builds a cut list. The pergola span calculator answers a single question: how far can this board go. Prepared to NDS3 2018 methods, not engineer-stamped. Check with your building department.

Bracing a Pergola Against Wind

A pergola frame is open, but its posts, beams and slats still catch wind, and a gust can rack the frame sideways.

Knee braces

Our designer uses 2x6 braces at 45 degrees, bolted along each beam at every post. A freestanding pergola gets braces across the beam as well, and the rafter above each post ties the post top to the roof grid. The brace leg runs 24 to 48 in. The designer picks the shortest leg that works, often 30 in, and flags a brace whose foot lands below 6 ft 8 in for head clearance. Those choices are ours, because the code gives no lateral table for a freestanding frame.

Posts set in the ground without braces

You can skip the braces if the posts are buried deep enough. The IBC formula for a post in the ground gives a deep hole: an 8 ft 6x6 held only by the soil needs about 62 in in our design. We use a conservative soil rating, and we have an open question with an engineer about the factor for isolated posts. Braces are usually less work than that hole.

How wind is read

The pressures come from the IRC components and cladding table, with exposure B or C. The default wind speed is 115 mph, and above 140 mph the designer stops and tells you to see an engineer.

How to Build a Pergola on a Slab, a Deck or the Ground

The ground you build on changes how the posts are fixed.

On the ground

This is the default, with a footing and base under each post, as above.

On a concrete slab

The post still needs a way to resist uplift. A cast-in base only works in new concrete. For an existing slab, an adjustable retrofit base bolts on, and its rating must be checked in the maker’s table because we have not verified one. Slab thickness and edge distance matter too, so ask the maker or an engineer.

On a deck

A deck was built for floor loads, not for a post pulling up in the wind. Our pergola over deck guide explains the three options and why we rank them as we do. Our pergola designer does not check posts that rest on the deck frame.

Attached to the house

A pergola can hang one side on a ledger. The wall decides whether that is allowed, and our guide to a pergola attached to the house gives the test.

Try it

Get the plan, with the lists

The $29 pergola plan has the plan view, elevations, details, member schedule with checks, a cut list, a shopping list with hardware counts and a build sequence. Choose a size in the pergola designer or start from a pergola plan preset. Two separate plans, a deck and a pergola on its own footings, are $59. Prepared to NDS3 2018 methods, not engineer-stamped. Check with your building department.

Sources

Every number in this guide that does not come from our own deck designer was checked against these 5 documents, numbered in the order the text first cites them and read between Oct 25, 2026 and Nov 4, 2026. 4 are code guides and official pages; 1 is a secondary source (secondary pages), marked, used only where no code guide or official page gives a number.

  1. 1American Wood CouncilPrescriptive Residential Wood Deck Construction Guide (DCA 6, 2015 IRC version) (2015)
  2. 2Simpson Strong-TieCBSTQ cast-in standoff column base, load table (2026)
  3. 3AWC, National Design SpecificationNational Design Specification for Wood Construction, 2018 edition (free view) (2018)
  4. 4City of Seattle2015 Seattle Residential Code (IRC 2015 adoption), chapter 3 (2015)
  5. 5Missouri University of Science and TechnologySecondary sourceCourse problem 4.30, Hem-Fir No.2 posts and timbers design values (NDS Table 4D)

Questions

Anything else? Write to us or browse every guide.

Questions people ask

Start with a small, freestanding rectangle with four posts, on footings you can dig by hand. Draw it first, so the beam and rafter sizes are checked before you buy lumber. Build in order: footings, posts, beam, rafters, slats, braces. Ask your building department about a permit before you dig.

You need a design with sizes that are calculated, treated lumber for the posts, concrete and post bases, beam and rafter lumber, hardware for each joint, and braces. Add tools for digging, cutting and leveling. A cut list and a shopping list for your own size save most of the guesswork.

Often yes, but check two things first. Ask your building department whether the pergola needs a permit and whether an owner can pull it. Then size the structure properly, because a pergola has to resist wind and snow like any other frame. A design that shows the percent used by each member helps.

Fix each post to the slab with a base rated for uplift, and check what the slab can hold. A cast-in base cannot be added to a slab already poured, so you need a retrofit anchor with a published rating for your slab. When in doubt, cut through the slab for a footing.

Plan more than one weekend. We have not timed builds, so we will not give you hours. The long pole is the concrete, because footings need time to set before the posts take load. Lay out and cut the lumber while it cures, and put the frame up once the footings are ready.

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