Timber Roofs – Appropriate span range for a couple roof A traditional couple roof (without a collar tie) is generally suitable for which span range?

Introduction / Context:
Timber roof framing adopts different forms as span increases. A couple roof consists of two rafters leaning against each other at the ridge, with their feet bearing on the wall plates. Without additional ties, horizontal thrust on walls rises rapidly with span, limiting its practical use to short spans.

Given Data / Assumptions:

• Simple couple roof without collar tie or tie beam.
• Masonry loadbearing walls with typical wall plates.
• Residential-type roof pitches and loads.

Concept / Approach:
Because there is no member to restrain rafter spread, couple roofs are limited to short spans to keep horizontal thrust within acceptable limits. For longer spans, a couple-close roof (with a tie at the feet), collar roofs, or trussed roofs are adopted.

Step-by-Step Solution:
Identify structural form: two rafters meeting at ridge, no tie.Evaluate thrust: increases with span and roof pitch.Adopt conventional limit: ≈ 3.0 to 3.5 m; choose “3.5 m or less”.

Verification / Alternative check:
Textbook tables recommend couple roof up to about 3–3.5 m; beyond that, use couple-close roof (≈ 3.5–5 m) or trusses for greater spans.

Why Other Options Are Wrong:

• Spans beyond 3.5 m require ties or trusses to control thrust and deflection.

Common Pitfalls:
Using a pure couple roof over wider rooms causes wall spreading and cracks; always check horizontal reactions and provide adequate wall plate anchorage.

Final Answer:
3.5 m or less