Wooden structures: a closer look at NF DTU 31.1

Although not a mandatory regulation, NF DTU 31.1 is crucial when it comes to designing and building wooden structures. Outlining technical guidelines and specifications, it is designed to help carpenters and joiners to build structures that conform to professional standards.

WHAT IS NF DTU 31.1 ?

Replacing the previous NF P 21-203 from May 1993, NF DTU 31.1 (NFU DTU stands for Norme Française Document Technique Unifié – the standards for France’s construction industry) outlines the regulations relating to designing and building wooden structures. There are certain parts of it that construction professionals really should be familiar with.

• What sectors it applies to: this standard is applicable to post-and-beam structures, joisting, timber frames and timber-framed houses and wooden or mixed (wood and metal) belfries, and applies to both new builds and renovations made to existing structures.
• What types of wood it applies to: for use on structures, this standard is applicable to glulam (glued laminated timber), finger jointed lumber, laminated veneer lumber (LVL) and cut framework components. For panels this standard is applicable to LVL, solid wood panels, strand board (oriented and waferboard) and cement bonded particle board.
• Geographical coverage: NF DTU 31.1 is applicable across France, irrespective of the climatic or natural zone.
• Users: as well as carpenters and joiners, all contracting authorities and general contractors are advised to follow this standard for design and construction. This standard is not mandatory unless stated in the contract. However, it is used as a reference document for court cases and insurance proceedings.

HOW TO USE NF DTU 31.1 ?

The standard NF DTU 31.1 outlines the regulations governing the design and construction of wooden structures. There are three main parts to it:

• The technical specifications: this contains specifications regarding calculations and the construction of the structures in question (framework, post and beam, joisting, timber frames and belfries), which must be adhered to.
• General criteria for selecting materials: this contains recommendations regarding the types of wood that can be used for each type of structure, taking factors such as gradient into account.
• Special conditions of contract: this explains the special conditions which apply to private contracts, specifying obligations between companies and the contracting authority.

WHAT IS NEW IN NF DTU 31.1 ?

The main difference with NF DTU 31.1 is that it is now based on Eurocode 5, a standard that is mandatory for public tenders and which includes more precise figures on sizing tolerance. However, for private contracts, joiners and carpenters may continue to follow the CB71 calculation regulations, other than in areas with special restrictions due to seismic activity. These regulations are outlined in the special conditions of contract section.

Previously, the standard NF P 21-203 also dealt with the design and building of wooden staircases, but this now has its own separate standard: NF DTU 36.3. It should be noted that other types of structures are also covered by DTU 31.1, such as wooden flooring, timber frames with shear walls and wooden structures with timber connectors.