Pergola· 3 min read
Freestanding Pergola 5 × 4 m: How Many Posts and How Far Apart
How many posts a 5 × 4 m freestanding pergola needs at the front and back, where to set them and when a third row appears — worked out for several spacings.
A freestanding pergola has nothing to lean on, so it has two rows of posts — front and back — each carrying a beam. How many posts go in a row comes down to one number: the maximum post spacing under the beam. Here is how to turn it into a post count and positions.
The brief
- Width along the beams 5,000 mm, depth along the rafters 4,000 mm (about 16 × 13 ft).
- Posts 120 × 120 mm, 2,400 mm above ground, on post bases.
- Beam overhang 300 mm beyond the outer posts, rafter overhang 300 mm at front and back.
- Maximum rafter span 3,500 mm; 60 mm rafters with a clear gap of at most 600 mm.
- Timber in 3, 4, 5 and 6 m lengths.
Posts for different maximum spacings
The outer posts stand where the overhang ends, 300 mm in from the beam ends — 4,400 mm apart. The calculator adds posts evenly until no spacing is larger than your maximum.
| Max. spacing | Posts per row | Total | Actual spacing | Timber to buy |
|---|---|---|---|---|
| 4,500 mm | 2 | 4 | 4,400 mm | 2 × 5,000 mm |
| 2,500 to 4,000 mm | 3 | 6 | 2,200 mm | 3 × 5,000 mm |
| 2,000 mm | 4 | 8 | about 1,467 mm | 4 × 5,000 mm |
Note that a maximum of 2,500, 3,000 or 4,000 mm gives the same result: three posts per row, 2,200 mm apart. A 2,400 mm post always comes twice out of a 5 m length. The rest stays the same in every case: 2 beams of 5,000 mm without a joint and 9 rafters of 4,000 mm with a clear gap of about 558 mm, so 18 rafter-to-beam connections.
With three posts per row, the post centres are at 300, 2,500 and 4,700 mm from the left end of the beam. Use these figures when you set out the footings.
A deep pergola: a third row of posts
The beams sit 300 mm in from the front and back edges, so the rafters span 3,400 mm. If your supplier allowed at most 3,000 mm, the calculator would add a third row in the middle of the depth (at 2,000 mm). Result: 9 posts, 3 beams and 27 rafter connections instead of 18.
Different post counts front and back
In “I choose” mode you set the front and back posts separately. For example 3 at the back and only 2 at the front to keep the entrance clear: 5 posts in total, but the front beam then spans the full 4,400 mm. Such a beam must be designed for the longer span — check with the supplier or an engineer.
Step by step
- 1Get the maximum post spacing and rafter span for the sizes you want to use from your supplier or an engineer.
- 2Enter the sizes, overhangs and “Freestanding”; leave the post count on “From max. spacing”.
- 3For a different layout switch to “I choose” and set the front and back posts — and have the new spacing checked again.
- 4Transfer the post positions from the plan to the site before you set the bases.
Before you build
- Post spacing and beam size go together: a longer span needs a stronger beam. Take the maximum from the kit instructions, span tables or an engineer.
- A freestanding pergola has no wall to brace it, so anchor the posts firmly — for example into pad footings. The post base manufacturer specifies how to fix it.
- A post base with a stand-off keeps the bottom of the post away from moisture.
- Set out the post centres from the calculator’s plan before pouring concrete, and check the diagonals.
The calculator handles geometry and materials, not structural design. Sections, post spacing, spans, connections, wind and snow loads and foundations must follow the supplier’s system or an engineer’s design and local building rules.