When clients begin designing their timber frame homes and structures, one of the first questions that comes up is how to optimize their layout for both cost and performance. We’ve previously discussed how far a timber truss can span, which largely dictates building width. The length of a timber frame, however, is controlled by the number of bents and the distance between them.
At Vermont Timber Works, we can design around nearly any architectural intent, but there are dependable engineering rules of thumb that help establish efficient, cost-effective starting points for bent spacing.
- Structural Efficiency Drops Quickly as Beam Length Increases
We do the engineering for all of our projects, but understanding a little about why length so greatly impacts member size is a great starting point when thinking about bent spacing.
The governing design property for most horizontal members—joists, girts, rafter plates, purlins, tie beams—is bending moment capacity. For a simple span, the bending moment is calculated with the formula:
M=(1/8)*w*L^2
Where:
- M = bending moment (ft-lb) – used to design member size
- w = applied load (lb/ft)
- L = beam length (span) (ft)
Because length is squared, moment increases exponentially with added length. As bent spacing increases, every intermediate timber works harder and must grow in size and/or grade to compensate.
For typical residential loading, 12’–16′ bent spacing usually maximizes both structural performance and material cost.
When loads are high, small spacing changes matter
Regions and use cases where this is especially true:
- Heavy snow load areas (Northeast, Rockies, Pacific Northwest)
- High-wind coastal or hurricane zones
- High live load spaces (event venues, public assembly, commercial occupancies, hay lofts)
In these situations, jumping from 14′ to 16′ or 18’ can drive significant increases in member size, often requiring:
- Larger or deeper rafter plates
- Heavier floor girts or tie beams
- More closely spaced or larger roof purlins
- Higher grade timbers, hybrid steel, or reinforced connections
Structurally, it is still possible to achieve large bent spacings, but costs rise disproportionately because the intermediate timbers grow faster than the bay length.
For high-load environments, 10’–14′ is generally the most efficient starting point.
- Timber Length Availability
Material procurement quietly influences cost-related design choices. Solid-sawn heavy timbers are stocked in 2-foot increments, and cost per board foot stays relatively stable up to certain lengths for different species:
- Douglas Fir or Southern Yellow Pine: stable pricing up to ~12’–16′
- Eastern species (White Pine, Hemlock): stable pricing up to ~10’–14′
- White Oak: stable pricing up to ~12’–16′
This means bent spacing that aligns with those increments—10′, 12′, 14′, 16’—tends to be the most economical, as there is less waste with members that frame between bents, and there is not a premium board-foot cost on the member lengths.
- Balancing Structure, Cost, and Aesthetics
Timber frames are a blend of engineering, fabrication efficiency, and architectural vision.
Longer bays provide:
- Fewer bents
- Larger open volumes
- More dramatic frames
Shorter bays provide:
- More structural redundancy
- Smaller intermediate timber sizes
- More cost-effective load paths
Good design is ultimately a balance of structure, aesthetics, and budget—and bent spacing is one of the first places those considerations intersect.
Every Vermont Timber Works Frame Is Fully Custom
The guidelines above are reliable engineering starting points, but every VTW frame is designed from scratch. If your project requires long spans, wide-open bays, or a specific truss system, we can engineer it.
If you’re early in schematic design or evaluating cost-saving approaches, these principles are an excellent place to begin. If you’re planning a project and want to explore what’s feasible, our engineers and craftsmen at Vermont Timber Works are here to help.
📧 Email: [email protected]
📞 Call: 802-886-1917
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