Underpurlins
Underpurlins are so called because they are fixed under the rafters. They are fixed horizontally and perpendicular to the direction of the roof slope. In a conventional roof frame the underpurlins support the underside of the rafters. This allows for the most economic rafter size and prevents the roof from sagging and collapsing.
Underpurlins must be in single lengths where possible. They must be in straight runs at right angles to the direction of rafters. Where underpurlins are joined in their length, the joint needs to be made over a point of support, with the joint halved, lapped and nailed.
