How to Determine What Size Truss You Need

Before ordering your trusses, it’s important to measure your available space and understand how truss sizing works. The right size ensures your building fits properly and avoids costly mistakes during installation. This guide will help you figure out exactly what you need.

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1. Measure Your Available Building Space

Start by measuring the clear width of the area where your building will go. This is usually measured between the outside edges of your posts or foundation.

- Measure in multiple spots to ensure the area is square and consistent.

- For gable buildings, measure the full width across where your endwall will be.

- For lean-to structures, measure from the wall of the existing building to the outermost post or edge where the roof will end.

✅ Tip: It’s a good idea to stake out the corners of your future building and double-check all measurements before choosing your truss size.

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2. Understand Truss Sizing vs. Inside Measurement (ISM)

Trusses are typically sold and labeled by their outside span, but your usable inside measurement (ISM) will be slightly less. This is because the trusses sit between your posts, not on the outside of them.

Gable & Monoslope Trusses

- ISM = Nominal Span – 11 inches

  - Example: A 40’ gable truss has an inside measurement of 39’1”.

Lean-To Trusses

Lean-tos have different standard ISMs:

- A 20’ lean-to truss has an inside measurement of 19’ 6.5”.

- A 16’ lean-to truss has an inside measurement of 15’ 6.5”.

💡 This ISM is key to ensuring your trusses fit between posts correctly. Always base your post placement on the inside measurement, not the nominal span.

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3. Match the Truss Size to Your Building Plan

Once you know your available width and understand the ISM, you can choose the right truss size:

- Gable Buildings – Choose a truss whose inside measurement matches the clear width between your posts.

- Lean-To Attachments – Measure from the wall of the existing structure to the outside post line, then select a truss with the corresponding lean-to ISM.

- Monoslope Structures – Use the gable ISM rule (subtract 11 inches) when planning post placement.

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4. Consider Pitch and Overhang

The pitch (roof slope) you choose affects your overall building height and may impact site selection or local building code compliance.

- Standard gable pitch is 4/12, and standard lean-to/monoslope pitch is 1.5/12.

- Custom pitches are available but will require a quote.

Overhangs may also extend beyond the nominal span, so ensure there’s adequate clearance on all sides of the building site.

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5. Double-Check Local Codes

Before finalizing your truss size, verify your local building codes for minimum setbacks, required clearances, and any size restrictions. This helps avoid problems during permitting or construction.

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6. Get Expert Help

If you’re unsure about sizing, contact our team for assistance. We’ll help you match the right truss size, style, and configuration to your space and project goals.

📞 (850) 623-1414
🌐 blackwatertruss.com

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Key Takeaway

The correct truss size starts with accurate measurements and an understanding of inside measurements. Use this guide to plan your structure confidently—and when in doubt, **we’re