Pergola· 3 min read
A 4 × 3 m Pergola Against the House: How Many Posts, Rafters and Beams
A wooden 4 × 3 m pergola fixed to a house wall: how many posts, rafters and timbers to buy, how to cut them and what changes with a longer beam overhang.
A pergola against the house has one advantage over a freestanding one: the back ends of the rafters sit on a timber fixed to the wall — the ledger. That removes the back row of posts and the back beam. Let’s work out the materials for a 4 × 3 m (about 13 × 10 ft) pergola.
The brief
- Width along the wall 4,000 mm, depth from the wall 3,000 mm including the rafter overhang.
- Posts 120 × 120 mm, 2,400 mm above ground, on post bases — so no extra post length.
- Maximum post spacing under the beam 3,000 mm and maximum rafter span 3,500 mm. These are values for this example — for your pergola they must come from the system supplier or a structural engineer.
- Single-timber beam overhanging the outer posts by 300 mm; 60 mm rafters overhanging the beam by 300 mm.
- Rafters with a clear gap of at most 600 mm, outer rafters flush with the edge.
- Timber sold in 3, 4, 5 and 6 m lengths, 3 mm saw kerf.
The result
| Member | Pieces | Buy |
|---|---|---|
| Posts | 3 × 2,400 mm | 1 × 5,000 + 1 × 3,000 mm |
| Beam | 1 × 4,000 mm | 1 × 4,000 mm |
| Ledger | 1 × 4,000 mm | 1 × 4,000 mm |
| Rafters | 7 × 3,000 mm | 7 × 3,000 mm |
| Rafter connections (ties, hangers) | 14 | — |
The beam sits 2,700 mm from the wall — the 3,000 mm depth minus the 300 mm rafter overhang. The rafters span 2,700 mm, less than the 3,500 mm maximum, so one row of posts at the front is enough.
The outer posts stand 300 mm in from the beam ends, so they are 3,400 mm apart. That is more than the 3,000 mm maximum, so a third post goes in the middle and the spacing drops to 1,700 mm.
There are 7 rafters with a clear gap of about 597 mm (about 657 mm centre to centre). Each rafter sits on the ledger and on the beam, so there are 14 connections — one tie or hanger each.
Posts: two from one length
Two 2,400 mm posts come out of a 5 m length, leaving 194 mm. The third comes from a 3 m length, leaving 597 mm. The calculator cuts each member type separately, because posts, beam, ledger and rafters usually have different sections.
What if the beam overhangs more
With a 500 mm overhang instead of 300 mm, the outer posts are exactly 3,000 mm apart and 2 posts are enough — both cut from one 5 m length. A longer overhang and a longer span load the beam more, though. Whether it can take that is for the supplier or an engineer to say, not the calculator.
Using the calculator
- 1Choose “Attached to a wall” and enter the width along the wall and the depth from the wall.
- 2Add the sizes and limits from your supplier: post size, ledger thickness, maximum post spacing and maximum rafter span.
- 3Set the overhangs and rafter spacing, and enter the timber lengths you can actually buy.
- 4Check the plan: beam, post and rafter positions. Then open the cutting plan for each member.
Watch out at the wall
- Fix the ledger into load-bearing masonry. The type and number of anchors depend on the wall — follow the anchor manufacturer or an engineer.
- On an externally insulated façade you risk damaging the insulation and creating a thermal bridge. There are fixing systems that mount at a distance in front of the façade; otherwise consider a freestanding pergola.
- A post base with a stand-off keeps the end grain of the post out of standing water. Sealing the post end before fitting is worth it too.
- Check the ledger height against windows, doors and the gutter before you drill.
The calculator handles geometry and materials, not structural design. Sections, post spacing, spans, connections, wall fixing and foundations must follow the supplier’s system or an engineer’s design and local building rules. Check with your local authority whether you need a permit.