Skip to main content

Couple closed roof versus collar roof

Short answer

A couple-close roof has a tie beam at rafter feet, removing outward thrust and spanning up to 5 m, but limits headroom. A collar roof places a collar beam higher (1/3–1/2 rise), allowing more headroom and slightly longer spans (~5 m), though walls must resist some thrust.

Compare couple closed roof and collar roof over couple roof with suitable sketch.

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.

Collar roof (collar over couple roof)

A collar roof is a couple (or couple-close) roof in which a collar beam (horizontal tie) joins each pair of rafters at about one-third to one-half of the rise above the wall plate. The collar stops the rafters sagging and spreading, and allows slightly larger span (about 5 m) than couple-close roofs.

            ridge
             /\
            /  \
     collar/----\ collar beam
          /      \
         /        \
      __/__________\__
     wall plate   wall plate

Members: wall plate (75 x 100 mm), common rafters (50 x 100 mm at 450 mm c/c), collar beam (40 x 100 mm, bolted), ridge board (40 mm thick), battens and covering.

Comparison

PointCouple closed roofCollar roof
Tie memberTie beam joins feet of rafters at wall plate levelCollar beam joins rafters at 1/3 to 1/2 of rise
Outward thrust on wallsEliminated by the tieReduced but not fully removed
SpanUp to about 4.5-5 mUp to about 5 m (slightly larger)
HeadroomTie reduces headroomMore headroom under collar
Rafter stiffnessRafters may sag at middleCollar reduces sagging
CostSlightly lessSlightly more

✓ Answer checked by Claude Opus 5.5 (high effort)

Related questions

All Building Technology chapter 4 questions →