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Couple closed roof and mansard roof truss

Short answer

A couple-close roof uses rafters and a tie beam at feet, eliminating thrust (span ~5 m). A mansard truss has steep lower slopes (60–70°) and flat upper slopes, creating an attic room; it combines queen/king post elements for large spans.

Elaborate couple closed roof and mansard roof truss with neat sketches.

Couple closed roof

A couple-close roof is a single roof in which each pair of common rafters meeting at the ridge has its feet connected by a horizontal tie beam. The tie takes the horizontal thrust, so only vertical loads reach the walls. Span about 4.5-5 m.

          ridge
           /\
          /  \ rafters
         /    \
        /      \
     __/________\__
    wall plate  tie beam  (wall plate)

Features:

  • Rafters about 50 x 100 mm to 50 x 125 mm at 400-600 mm c/c, ridge piece 40 mm thick; tie beam 50 x 100 mm or more.
  • Rafters notched/spiked to wall plate and tie.
  • Advantage: no outward thrust, simple. Limitation: rafters sag if span exceeds 5 m, so a collar roof or double roof is used.
  • Used for small houses, sheds and verandahs.

Mansard roof truss

A mansard roof truss has a double slope on each side: the lower slope is steep (about 60-70 degrees) and the upper slope is flat (about 15-30 degrees). The truss is a combination of a queen post and king post type, with a tie beam. It encloses a room in the roof space (attic) and is used for buildings that need extra living space or large spans.

          ________
         /        \     upper slope 15-30 deg
        /          \
       /            \
      |              |   lower slope 60-70 deg
      |______________|
        tie beam

Elements: tie beam, principal rafters (lower steep and upper flatter), king/queen posts, struts, collar, purlins at the break line, common rafters and battens. The wide span and the attic room under the truss make it useful for large halls and buildings that need extra accommodation.

✓ Answer checked by Claude Opus 5.5 (high effort)

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