Enter Pergola Details
Example Data Table
| Example | Pergola length | Spacing (max) | Projection | Overhangs | End doubled | Rafters (total) | Rafter length |
|---|---|---|---|---|---|---|---|
| Imperial | 12.00 ft | 16 in | 10.00 ft | 0.50 ft + 0.50 ft | Yes | 13 | 11 ft 0 in |
| Metric | 4.00 m | 400 mm | 3.00 m | 0.20 m + 0.20 m | No | 12 | 3.400 m |
Formula Used
- Rafter length = projection + front overhang + back overhang.
- Base rafter count = ceil(pergola length ÷ max spacing) + 1 (includes both ends).
- Actual spacing = pergola length ÷ (base count − 1).
- Total rafters = base count + (2 if end rafters are doubled).
- Order rafters = ceil(total rafters × (1 + waste%)).
Material estimates use board feet (Imperial) or timber volume (Metric). These are planning values and may differ from supplier lengths and grading.
How to Use This Calculator
- Select your unit system and enter the pergola length.
- Enter your maximum rafter spacing for the layout.
- Enter projection and both overhangs to get rafter length.
- Optionally double end rafters and set a waste factor.
- Press Calculate, then export CSV or PDF if needed.
Planning Pergola Rafters with Confidence
Rafter layout controls shade density, material cost, and overall stiffness. This calculator turns a few site measurements into a practical rafter count and length. For typical garden pergolas, spacing commonly ranges from 12–24 inches (about 300–600 mm). Tighter spacing increases shade and reduces rafter deflection, but also raises timber volume and hardware needs.
Spacing and shade coverage
The tool uses your maximum spacing to ensure the final layout does not exceed it. For example, a 12 ft run at 16 in maximum spacing produces rafters at slightly less than 16 in actual spacing, keeping alignment clean at both ends while protecting the target shade pattern.
Length allowances and overhangs
Rafter length is calculated as projection plus front and back overhangs. A 10 ft projection with 6 in overhangs on each side becomes an 11 ft rafter. Overhangs improve rain drip lines and appearance, but they add leverage, so confirm beam and ledger details.
Material takeoff and waste factor
Material estimates are shown as board feet (Imperial) or timber volume (Metric). Many builders carry 8–12% waste to account for knots, checking, and selecting straighter pieces for the outer edges. Decorative end cuts may justify a higher waste setting.
Field checks that save rework
Before ordering, verify post spacing, beam straightness, and ledger level. Check diagonal measurements to confirm the frame is square. Small layout errors compound across repeated rafters, so measure twice and keep reference lines snapped along the full length.
Finally, translate the takeoff into an order that matches supplier stock lengths. If rafters are sold in fixed lengths, you may optimize by stepping up to the next length and trimming. Plan fasteners at a consistent pattern—many crews use two structural screws or bolts per bearing point—and choose corrosion‑resistant hardware for outdoor exposure. Sealing end grain and allowing small expansion gaps can reduce splitting and seasonal movement.
FAQs
1) Does the calculator include both end rafters?
Yes. The base count includes a rafter at each end, then it adds additional rafters so the spacing stays at or below your maximum setting.
2) What does “double end rafters” mean?
It adds one extra rafter at each end (two total). This can stiffen the perimeter and create a cleaner edge line for trim or decorative details.
3) Why is my “actual spacing” slightly smaller than my input?
The calculator uses your input as a maximum. It increases the rafter count when needed, so the final spacing is equal to or less than your limit.
4) How do I choose a waste percentage?
For standard, straight cuts, 8–12% is common. Increase waste if you expect knots, want premium visual faces, or plan angled/decorative end cuts.
5) Are board feet the same as linear feet?
No. Linear feet measures length only. Board feet estimates volume based on thickness, width, and length, which helps compare material needs across sizes.
6) Can I use metric inputs with standard timber sizes?
Yes. Enter the section dimensions you plan to buy (for example 45×145 mm). The calculator then estimates total linear length and timber volume.
7) Does this confirm structural safety?
No. It is a planning tool for layout and materials. Always verify spans, connections, wind uplift, and local code requirements for your site and roofed conditions.