Timber trellis along a garden walkway leading to a roofed gazebo

Pergola Kit Install, Step 3 of 12: Square Up the Posts

Step 3 of 12 in our DIY timber pergola kit install series: squaring up the posts. Before a single post goes up, you mark exactly where each one lands and confirm the layout is square, so the beams, knee braces, and rafters all fit the way they were cut. Western Timber Frame™ has shipped more than 7,000 structures since 2008, and a square layout is the step that makes every one of them go together cleanly.

What this step covers

  1. Measure the pad. In this example the concrete patio is just over 10 feet across, and the posts are set 9′ 10″ apart, outside to outside, so each post sits about 1¾ inches in from the edge.
  2. Mark the post locations on the slab.
  3. Square the layout with a laser square, the 3-4-5 method, or by matching diagonals (all three are explained below).
  4. If you’re ever unsure of the spacing, measure the beam itself, as shown below.
Gloved hands holding a measuring tape against the edge of a concrete patio to mark where a pergola post will sit Yellow laser square tool set on a concrete patio corner to project a 90 degree line for squaring up pergola posts

Three ways to check for square

A laser square. A small laser tool projects a true 90-degree line from your first mark. It’s the fastest and most accurate method, and the best choice when something like a hot tub sits in the way and you can’t measure corner to corner.

The 3-4-5 method. From one corner mark, measure 3 feet along one side and make a mark, then 4 feet along the other side and make a mark. If the distance between those two marks is exactly 5 feet, the corner is square. For a larger footprint, use 6-8-10 or 9-12-15 for more accuracy.

Matching diagonals. Measure from corner to corner both ways. If the two diagonals are the same length, the layout is square. If they differ, adjust the marks until they match.

Dark stained timber pergola built over an oversize hot tub beside a stone home, a case where a laser square helps with layout
A pergola set over a hot tub, where a laser square makes layout easier.

Check the spacing against the beam

If you’re ever in doubt about exactly where a post should go, measure the beam. Measure from the outside edge of the beam, not from the dovetail, down to the matching edge at the other end. That distance is your post spacing.

Arrow pointing to the outside edge of a timber pergola beam, the spot to measure from instead of the dovetail joint
Measure from this outside edge, not the dovetail.
Arrow pointing to the matching outside edge at the other end of a stained timber beam, marking the post spacing
Down to the matching edge at the other end.

Why it matters

A square layout is what lets a pre-cut kit go together the way it was designed, from a backyard pergola to a pavilion over a deck.

Dark stained timber pergola beside a gabled timber pavilion over a backyard patio and deck, both built from pre-cut kits
A pergola and pavilion built from pre-cut kits.
Stained timber gazebo with arched knee braces and a shingled roof built over a hot tub on a backyard deck
A gazebo set square over a hot tub.

Continue the install series

← Step 2 — Getting Started Step 4 — Knife-Plate Layout →

New here? Start at Step 1 — Time-Lapse Install, or see every video at Pergola Kit Installation University. For the tools you’ll need, see tools to install a timber frame kit.

Have questions about your kit?

Every Western Timber Frame kit ships pre-cut, pre-stained, and pre-drilled with a step-by-step manual, so most pergolas go up in an afternoon to a day. Request a free design consultation, or call (877) 870-8755 (9 am–5 pm MST).

Western Timber Frame™ — Payson, Utah | Since 2008 | 7,000+ structures in all 50 states
2026 SBA Manufacturing Award | HGTV Design Excellence Award | 28 Best of State Awards

Content Writer, Architectural Photographer & Outdoor Living Author, Western Timber Frame

Marilynn Thompson is a content writer, architectural photographer, and outdoor living author at Western Timber Frame™, where she specializes in custom timber pergolas, pavilions, gazebos, and heavy timber construction. Since joining the company, she has helped document thousands of custom projects through technical research, homeowner interviews, jobsite photography, and educational articles that explain complex building concepts in clear, practical language.

Her work combines engineering research with real-world project experience, covering topics such as structural design, building permits, installation, timber species, joinery, maintenance, and outdoor living design. She regularly collaborates with Western Timber Frame's designers, engineers, and project specialists to ensure every article reflects current building practices and accurate technical information.

As the company's architectural photographer, Marilynn has photographed custom timber structures across the United States, documenting how they perform in different climates, landscapes, and architectural styles. Her writing is informed not only by research, but by conversations with homeowners and business owners who have lived with these structures for years.

Her goal is simple: to help homeowners understand the decisions behind a custom timber structure so they can build with confidence, not guesswork.

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