History of Timber Roof Framing

Timber has framed pitched roofs for thousands of years. The central problem is always the same: sloping rafters push outwards at their feet, and that thrust must be contained or the walls spread and the roof sags. The history of roof framing is largely the history of solving that problem with longer spans and less timber.

Coupled rafters

The simplest roof uses pairs of rafters leaning together at the ridge, tied at the bottom by a horizontal beam. These coupled-rafter roofs were structurally weak and prone to racking, meaning sideways collapse, especially over wider buildings.

Medieval trusses

Medieval carpenters developed the truss, a rigid triangulated frame placed at intervals to carry purlins and common rafters. Variants included:

• King post trusses, with a central vertical post from tie beam to ridge;
• Queen post trusses, with two posts; in Italy the type is called palladiana because Andrea Palladio used it so often;
• Arch-braced trusses, with curved braces forming an arch;
• Hammerbeam trusses, with short projecting beams that let a roof span further than any single timber.

The masterpiece of the period is Hugh Herland's hammerbeam roof at Westminster Hall, installed in the 1390s and spanning just over 20 m (66 ft).

Joints

Early trusses were held by mortise-and-tenon joints cut on site and secured with oak pegs. Carpenters drilled peg holes slightly off-centre so that driving the peg drew the joint tight, a technique called draw-boring. Shrinkage of unseasoned green timber could crack joints over time.

Exchange of ideas

Historians have traced how roof-framing knowledge moved from Italy to England in the 17th century through architects such as Inigo Jones and Christopher Wren. German settlers in the United States built liegender Stuhl (lying chair) trusses in the 18th and 19th centuries.

Modern trusses

In modern construction, factory-made trusses of dressed lumber joined by metal connector plates have replaced much site-cut framing, particularly in the United States. They use less timber and are made with machine accuracy, and a shortage of skilled carpenters has accelerated the move to factory production.