Strut

What is a strut in a roof frame?

A strut (Slovak vzpera) is an inclined supporting member of a roof frame that carries compressive force along its own length. The Pamiatkový úrad SR roof glossary defines vzpera as an inclined supporting or stiffening member of a roof frame that transfers compressive forces. The role follows from the force. A member that the load pushes along its length acts as a strut. A member that the load pulls does not.

Steel construction guidance states that the principal force in each member of a truss is axial tension or compression. Every member therefore carries a push or a pull along its own length. A strut carries the push. A roof truss or a traditional timber frame uses struts to give the load a path to a support.

Where do struts sit in a traditional Slovak roof frame?

The glossary lists three basic support frames (stolice). Each uses struts or braces for stiffening in the longitudinal direction of the roof. It also names members that work as struts in other positions.

Frame or member	Where the strut or brace appears	What it does
Stojatá stolica	Inclined struts and corner braces (vzpery, pásiky) in the longitudinal direction, next to vertical posts, purlins and sometimes sill beams	Gives longitudinal bracing (zavetrenie)
Ležatá stolica	Horizontal rozpery and inclined vzpery and pásiky next to lying posts and purlins	Gives longitudinal bracing and carries compression between posts
Šikmá stolica	Inclined vzpery and pásiky next to inclined posts set against the rafters	Gives longitudinal bracing
Vešadlo (hanger frame)	Arms of the hanger (inclined vzpery) that support the hanger post from the side	Holds the post in line
Zápora (heel strut)	An inclined strut at the foot of a vertical member	Holds the member against shifting or tilting, which matters when the frame is erected

The king-post truss entry in this glossary calls the diagonal between the post and the purlin a brace. The Slovak methodology treats it as a vzpera, defined by the compression it carries. A pásik is a short corner member that stiffens a frame between a post and a purlin, a ližina or a rozpera.

How does a strut carry load and why does it buckle?

Steel construction guidance says that the upper chord of a truss is in compression under gravity loading. It adds that purlins and the transverse roof wind girder usually restrain that chord sideways. The same guidance says that the bottom chord is in compression under uplift. Compression can therefore occur in more than one member and in more than one load case.

The same guidance states that members in compression must be designed against out-of-plane buckling. It says that a reduction factor controls buckling and that the factor comes from the slenderness of the member. A slender member can bend sideways, so its length and its restraint matter. The guidance addresses steel members. For timber, the JRC Eurocodes site names Eurocode 5 (EN 1995). It applies to the design of buildings and other works in timber.

How is a strut different from a tie, a rozpera or a rafter?

The Slovak glossary defines each member by its job and position. The table gives the definitions in short form.

Member	Glossary definition
Strut (vzpera)	An inclined supporting or stiffening member that transfers compressive forces
Rozpera	A horizontal member that transfers compressive forces between posts or other members
Pásik	A short corner member that stiffens a frame between a post and a purlin, ližina or rozpera
Hambálok (collar beam)	A horizontal beam that spreads the rafters and pulls them together, joined with carpentry joints
Väzný trám (tie beam)	The basic horizontal transverse beam of the roof, usually resting on wall plates
Krokva (rafter)	An inclined member, usually paired, in each transverse bay that carries the roof covering
Väznica (purlin)	A horizontal longitudinal beam that carries the rafters

The Czech heritage guide that the Pamiatkový úrad SR republishes adds a point about tie beams. They usually carry posts or inclined struts. They also give the roof its transverse stiffness by taking the horizontal force from inclined members, which are the rafters and the struts. A tie beam and a strut therefore work together.

What are the common misconceptions about struts?

The first misconception is that every diagonal in a truss is a strut. Steel construction guidance shows otherwise. In a conventional Pratt truss the diagonals are in tension under gravity loads. In a Warren truss they are alternately in tension and in compression. The force in a diagonal depends on the truss type and on the direction of the load.

The second misconception is that a strut is only a timber part. Steel trusses have compression members as well, and the same two force states apply. The third is that a strut is a minor part that a carpenter can cut out to make room in a loft. The roof glossary lists struts and braces among the members of the frames that stabilise the roof. A designer must check what removing one does to the frame.

What timber does a strut need?

STN EN 338 sets strength classes for structural timber. The Slovak edition has the title Konštrukčné drevo. Pevnostné triedy. The ÚNMS catalogue lists it as valid. It came out in August 2016 as an identical adoption of EN 338:2016. The French edition of the standard describes its purpose as a system of strength classes of general use for structural design codes. This page gives no strength values. The designer takes them from the standard for the class that the project specifies.

Why should a strut be checked before any change?

A Czech heritage guide republished by the Pamiatkový úrad SR says that setting up a loft always means some intervention into the roof frame. It says that a tie beam which carries posts or inclined struts needs a reliable replacement for all its functions if it goes. The same guide says that for a protected house the owner must discuss the intent with the heritage authority first.

Cutting a strut, moving it or adding load at its joint can therefore change the load path of the whole frame. Owners who plan a loft conversion or a larger opening should ask a structural engineer to check the existing frame before any work starts.