Collar Tie
/ ˈkɒlə taɪ /
Also known as: collar beam, collar, tie beam, ceiling tie
Definition
A collar tie is a horizontal timber member connecting a pair of opposing rafters in a pitched roof, positioned in the upper third of the rafter height. It acts in tension, pulling the rafter feet together and resisting the outward thrust that sloping rafters exert on the supporting walls. Essential in traditional cut roofs where no other means of restraining rafter spread is provided. Must not be removed during loft conversions without a structural engineer's assessment and alternative measures. Fixed to rafter faces with bolts or structural connectors - skew nailing alone is inadequate for resisting tension.
In practice
In older houses with traditional cut roofs, collar ties are often found at mid-height or even lower - sometimes these were originally ceiling ties (fixed at wall plate level) that have been raised by a previous owner seeking headroom, which significantly reduces their structural effectiveness. A collar tie at mid-height exerts higher forces on its connections to resist the same spread as one in the upper third. In a loft conversion survey, the position and condition of all collar ties must be assessed before any are relocated or removed.
Modern trussed rafter roofs (pre-engineered factory-made trusses) incorporate the collar tie function within the truss geometry - each truss is a self-contained structural unit that resists rafter spread internally. The wall plates in trussed rafter roofs still receive a horizontal thrust, but the magnitude is much lower than in a comparable traditional cut roof with high collars. Where a traditional cut roof is being replaced with trussed rafters, the new trusses typically exert less lateral load on the walls than the old roof, which is beneficial for older masonry walls.
See also