Timber roof truss

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Two king post trusses linked to support a roof.
Key:1: ridge beam, 2: purlins, 3: common rafters. This is an example of a "double roof" with principal rafters and common rafters.


Rafters have a tendency to flatten under gravity, thrusting outwards on the walls. For larger spans and thinner walls, this can topple the walls. Pairs of opposing rafters were thus initially tied together by a horizontal tie beam, to form coupled rafters. But such roofs were structurally weak, and lacking any longitudinal support, they were prone to racking, a collapse resulting from horizontal movement. Timber roof trusses were a later, medieval development.[2] A roof truss is cross-braced into a stable, rigid unit. Ideally, it balances all of the lateral forces against one another, and thrusts only directly downwards on the supporting walls. In practice, lateral forces may develop; for instance, due to wind, excessive flexibility of the truss, or constructions that do not accommodate small lateral movements of the ends of the truss.


Timber roof truss example


Closed trusses[edit]

Closed/open distinction is used in two ways to describe truss roofs.

Closed truss:

  1. A truss with a tie beam; or
  2. Roof framing with a ceiling so the framing is not visible.

Open truss:

  1. A truss with an interrupted tie beam or scissor truss which allow a vaulted ceiling area; or
  2. Roof framing open to view, not hidden by a ceiling.[4]

King post truss[edit]

King post truss.



Queen post truss[edit]

Queen post truss.

A queen post truss has two principal rafters and two vertical queen posts.[5] The queen post truss extends the span, and combined with spliced joints in the longer members extends the useful span for trusses of these types. As with a king post, the queen posts may be replaced with iron rods and thus called a queen rod truss. This truss is often known as a palladiana (Palladian truss) in Italy, as it was frequently used by the Renaissance architect Andrea Palladio.[6][a] Sometimes a Palladian truss is defined as a compound truss with a queen post and king post truss in the same assembly.[7]


Liegender Stuhl[edit]


Liegender Stuhl is a truss of German origin, the German name is used in America. This truss is found in some 18th and 19th-century buildings where Germans settled in the U.S. The literal translation is "lying chair", lying meaning the top chords are angled or leaning (and chair in the sense of a support, in this case a post or truss).[9] Carpenters in the Netherlands also used this truss where it is spelled liggende stoel.[10]

The opposite being a "Stehender Stuhl", which is the common roof truss type where simple vertical posts replace the more elaborate support structure in the image (highlighted in blue).

Open trusses[edit]

Arch-braced truss[edit]

A single arch-braced truss.


Hammerbeam truss[edit]

Chambers 1908 single hammerbeam truss



Scissor truss[edit]

A scissor truss



It has been proven that permanent bracing designs can improve the lateral stability of roof trusses, specifically with j chords. Continuous lateral braces can prevent negative effects of lateral forces by being diagonally set in to brace the chord. However, there are variable effects of permanent braces on the truss web, given that the length of the chord would determine the number of brace locations.[21]

Pre-fabricated wood trusses[edit]

Pre-fabricated wood trusses offer advantages in building construction through machine-made accuracy and tend to use less timber.[22] However, this does not take into account site-specific design alterations that require customized truss design.

Historic wood trusses[edit]

The challenges of maintaining wood trusses are not limited to newer building projects. Visual grading can be used to conduct condition assessments for historic trusses. This along with load carrying tests, can be used to determine the best possible solutions to repairing older truss systems. Metal plates and correct, period material can be used to repair, although 100% recovery of the material may be hard if the wood truss has deteriorated. MPC wood trusses are typically made from lumber, rather than timber.[23]

The interior of the barn at Bartram's Garden with a view of the wooden trusses.

Connections in wooden trusses[edit]

The earliest wooden truss connections consisted of mortise-and-tenon joints and were most likely crafted at the construction site with the posts. Since most early trusses were made from unseasoned posts, the subsequent shrinkage would create cracking at the mortise-and-tenon joints. Additionally, the mortise-and-tenon joints in older trusses were located at the weakest point in the post, accelerating failure. Much of the early truss connection designs anticipated structural behaviour under loads. This is why holes were drilled slightly off-centre, allowing the peg to naturally pull the posts together with gravity.[24]

See also[edit]





  2. ^ Steane (1984), p. 196
  4. ^ Domestic Buildings Research Group (Surrey, England), http://www.dbrg.org.uk/GLOSSARY/Open%20truss.html
  5. ^ a b Glossary of Australian Building Terms – Third Edition.(NCRB)
  8. ^ Yeomans (2003), p. 42
  11. ^ Harris (2003), pp. 75–77
  14. ^ Steane (1984), pp. 196–97
  15. ^ Webber, Frederick G.. Carpentry & Joinery. Methuen and Co., London: 1898. 152.
  19. ^ Wood, Margaret. The English Mediaeval House. London: Ferndale Editions, 1980, 1965. 319.
  21. ^ Permanent bracing design for MPC wood roof truss webs and chords, Forest Products Journal, 51(7), 73-81
  22. ^ "The benefits of pre-fabricated timber roof trusses." Civil Engineering (10212000) 24, no. 7: 47-48. Business Source Complete, EBSCOhost (accessed November 8, 2017)
  23. ^ Non-destructive assessment, full-scale load-carrying tests and local interventions on two historic timber collar roof trusses. Engineering structures, 140.
  24. ^ "Early Wooden Truss Connections vs. Wood Shrinkage: From Mortise-and-Tenon Joints to Bolted Connections" APT Bulletin, Vol. 27, No. 1/2, A Tribute to Lee H. Nelson (1996), pp. 11-23.


  • Harris, Richard (2003), Discovering Timber-framed Buildings, Shire Publications, ISBN 978-0-7478-0215-0
  • Steane, John (1984), Archaeology of Mediaeval England and Wales, Croom Helm, ISBN 978-0-7099-2385-5
  • Yeomans, David T. (2003), The Repair of Historic Timber Structures, Thomas Telford, ISBN 978-0-7277-3213-2
  • Hoffsummer, Patrick (ed.) (2009), Roof Frames from the 11th to the 19th Century. Typology and Development in Northern France and in Belgiums, Brepols, ISBN 978-2-503-52987-5CS1 maint: extra text: authors list (link)

External links[edit]