Roof Joist Spacing Chart & Guide (Australia, UK, NZ, Canada)
Roof joist spacing determines the strength, stability, and load-bearing capacity of your roof. In most homes, roof joists are spaced 16 to 24 inches on center, depending on the joist size, span, roof type (flat or pitched), and load requirements.
This comprehensive guide explains roof joist spacing standards, rafter distances in mm, and regional codes (Australia, UK, NZ, and Canada) — including a detailed roof joist spacing chart for both flat and pitched roofs.
Key Takeaways
- Most roofs use 16 inches (400mm) on-center joist spacing.
- Larger joists or lighter loads can use 24 inches (600mm) spacing.
- Joist size, roof load, and span affect spacing requirements.
- Local building codes differ across Australia, the UK, New Zealand, and Canada.
- Flat roofs need closer spacing than pitched or trussed roofs.
- A printable roof joist spacing chart is provided below.
Why Roof Joist Spacing Matters
Roof joist spacing decides how strong, stable, and safe your roof is. Whether you’re working on a flat, pitched, or shed roof, the spacing between each timber joist affects how well the structure carries weight — from tiles to snow to ceiling boards.
When I built my first shed roof, I thought, “How far apart can I set these joists before the thing sags?” Turns out, spacing isn’t just guesswork — it’s math, code, and a bit of common sense. So, let’s break it down clearly.
Roof Joist Spacing Basics
Most roof joists are spaced 16 to 24 inches (400 to 600mm) apart, measured “on center.” That means the center of one joist to the center of the next. The tighter the spacing, the stronger the structure — but also the more material and cost.
| Spacing (Inches) | Spacing (mm) | Typical Use |
|---|---|---|
| 12" | 300mm | Heavy loads, smaller joists |
| 16" | 400mm | Standard residential roofs |
| 19.2" | 480mm | Specific code applications |
| 24" | 600mm | Large joists, light loads |
Quick Fix: If in doubt, stick with 16 inches (400mm). It’s the safest standard for most timber sizes like 2x6 or 2x8.
Factors That Affect Roof Joist Spacing
Roof joist spacing depends on several factors — the size of the joist, roof span, the weight it must carry, and even your country’s building code.
| Factor | Effect on Spacing |
|---|---|
| Joist Size | Larger joists = wider spacing |
| Span Length | Longer spans = closer spacing |
| Roof Load | Heavier loads = closer spacing |
| Timber Type | Stronger wood allows wider spacing |
| Building Codes | Must meet regional standards |
When I asked a carpenter in Melbourne about his rule of thumb, he said, “Bigger joist, bigger gap — but only if the inspector says yes.” That sums it up: check local rules before you lift that hammer.
Flat Roof Joist Spacing
Flat roofs need tighter joist spacing, usually 12" to 16" (300–400mm), because they collect rain, debris, and sometimes snow. Unlike pitched roofs, water doesn’t slide off easily.
| Joist Size | Maximum Span (ft) | Recommended Spacing | Metric Span (m) |
|---|---|---|---|
| 2x6 | 10 ft | 12" (300mm) | 3.0m |
| 2x8 | 12 ft | 16" (400mm) | 3.6m |
| 2x10 | 15 ft | 16" (400mm) | 4.5m |
| 2x12 | 18 ft | 24" (600mm) | 5.5m |
Common Pain Point: Flat roofs sag fast if spacing is too wide — even a 1-inch sag can pool water.
Practical Tip: If you live in rainy or snowy areas like Canada or NZ, don’t exceed 16" spacing unless your joists are large and well-supported.
Pitched Roof Joist Spacing
Pitched roofs are more forgiving since gravity helps with drainage. Standard spacing ranges from 16" to 24" (400–600mm) depending on span and rafter size.
| Joist Size (Timber) | Span (ft) | Recommended Spacing | Span (m) |
|---|---|---|---|
| 2x6 | 9 ft | 16" (400mm) | 2.7m |
| 2x8 | 11 ft | 19.2" (480mm) | 3.3m |
| 2x10 | 13 ft | 24" (600mm) | 4.0m |
Common Pain Point: People often confuse rafters with joists. Rafters slope; joists stay horizontal.
Quick Fix: If your roof is steep or tiled, 24" spacing is fine with large rafters like 2x10 or 2x12. For lighter builds, stick with 16".
Ceiling Joist Spacing
Ceiling joists hold up drywall or plaster, not roof loads, but still need strong support. The typical spacing is 16" (400mm).
| Ceiling Joist Size | Max Span (ft) | Spacing |
|---|---|---|
| 2x4 | 8 ft | 12" (300mm) |
| 2x6 | 10 ft | 16" (400mm) |
| 2x8 | 12 ft | 24" (600mm) |
Personal Note: When I installed my own ceiling joists, I learned that drywall sags if you go over 16 inches. I had to redo half of it — lesson learned the dusty way!
Shed Roof Joist Spacing
Shed roofs are usually smaller and lighter. Builders often use 2x4 or 2x6 joists, spaced 16" to 24" (400–600mm), depending on roof size.
| Shed Width (ft) | Recommended Joist Size | Spacing |
|---|---|---|
| Up to 8 ft | 2x4 | 16" (400mm) |
| 8–12 ft | 2x6 | 16" (400mm) |
| 12–16 ft | 2x8 | 24" (600mm) |
DIY Tip: If you’re building a shed, keep the joists close — 16" is best unless you’re using metal trusses or engineered beams.
Regional Roof Joist Standards (Australia, UK, NZ, Canada)
Each country has its quirks when it comes to joist spacing. Codes vary slightly, but the general rules are consistent.
| Region | Common Spacing | Code Reference |
|---|---|---|
| Australia | 450mm or 600mm | AS 1684.2 |
| UK | 400mm or 600mm | BS 5268 |
| New Zealand | 400mm | NZS 3604 |
| Canada | 16" or 24" | NBC Part 9 |
Note: Always confirm with your local council or engineer before building. The internet’s great, but codes change faster than you’d think.
Joist vs Rafter vs Truss
| Type | Position | Purpose | Spacing |
|---|---|---|---|
| Joist | Horizontal | Supports ceiling or floor | 16–24" |
| Rafter | Sloped | Supports roof sheathing | 16–24" |
| Truss | Triangular frame | Pre-built roof structure | 24–48" |
Pain Point: Mixing up terms can confuse inspections.
Solution: If it’s angled, it’s a rafter. If it’s horizontal, it’s a joist. If it’s pre-built, it’s a truss.
Common Questions About Roof Joists
Q: Are ceiling joists 16 or 24 on center?
A: Usually 16" (400mm), especially for drywall ceilings.
Q: Can I use 4x2 for rafters?
A: Yes, for short spans (under 8 ft), like sheds.
Q: How far can a 2x6 span for a roof?
A: About 10 ft (3m) at 16" spacing.
Q: What’s the spacing for steel roof joists?
A: Usually 600mm (24"), depending on load.
Q: Are trusses better than rafters?
A: For large spans, yes — they’re lighter, stronger, and pre-engineered.
Roof Joist Spacing Chart
Here’s a PDF-ready table you can copy or print for quick site reference:
| Joist Size | Span (ft) | Span (m) | Flat Roof Spacing | Pitched Roof Spacing | Ceiling Spacing | Shed Spacing |
|---|---|---|---|---|---|---|
| 2x4 | 8 | 2.4 | 12" (300mm) | 16" (400mm) | 12" (300mm) | 16" (400mm) |
| 2x6 | 10 | 3.0 | 12" (300mm) | 16" (400mm) | 16" (400mm) | 16" (400mm) |
| 2x8 | 12 | 3.6 | 16" (400mm) | 19.2" (480mm) | 24" (600mm) | 16" (400mm) |
| 2x10 | 15 | 4.5 | 16" (400mm) | 24" (600mm) | 24" (600mm) | 24" (600mm) |
| 2x12 | 18 | 5.5 | 24" (600mm) | 24" (600mm) | 24" (600mm) | 24" (600mm) |
My Experience: Learning the Hard Way
When I built my small workshop, I used 24-inch spacing for 2x6 rafters. Looked fine — until the first rainstorm hit. The middle bowed just enough for a puddle to form, and I spent two hours with a mop and regret.
Lesson? Follow the chart, not your gut. Especially if your gut’s never held a hammer before.
Final Thoughts
Getting roof joist spacing right is a mix of math, material, and a bit of humility. Whether it’s a flat roof, pitched ceiling, or a shed, the difference between 16" and 24" can mean years of safety — or hours of fixing sagging beams.
If you’re unsure, ask a local builder or engineer. It’s cheaper than replacing a roof. Trust me — I’ve learned that lesson firsthand.
