A collar-tie roof is a structure of rafter pairs connected by transverse timber members called collar ties, whose position and connections directly shape how the entire roof truss performs. Before deciding, it is worth establishing whether the design allows the loft to be adapted without removing the collar ties, since this element often limits layout freedom. Check that the collar-tie-to-rafter connections match the solution assumed in the calculations — random screws or extra holes can weaken the timber. The choice of structure is governed by span, loads and how forces are transferred to the walls, not by a sketch alone.

The cross-section of a rafter is not determined by the roof covering alone, but by a set of structural parameters. Check the span between supports, spacing, timber class, support arrangement and loads — only these allow the beam's behaviour to be assessed. The length from eaves to ridge alone does not describe its performance, especially with intermediate support. When ordering, give the length from the documentation and the number of pieces; a dimension from a handbook does not replace a design for a specific house. Compare offers using the same material specification and scope of processing.