Essential structural considerations in roof design

The structural commentaries, since 1985, explain these load allowances are for use and occupancy loads, not a minimum snow load intended to provide for maintenance loadings, workers, and so forth and the load allowances are not reduced as a function of area or roof slope.

The U.S. has had minimum load provisions since 1943. The provisions, in their present form, first appeared in the Uniform Building Code (UBC) in 1949. They allow the minimum load to be reduced based on the tributary area the roof member is supporting and on the roof’s slope. The requirements are:

  • 0.9 kPa (20 psf) reducing to 0.6 kPa (12 psf), depending on a combination of tributary area and roof slopes.
  • Greenhouses, lath houses, and farm accessory buildings 0.48 kPa (10 psf).

In 1988, a lower value for the minimum roof load was allowed if repair work is done from the ground and approved by the local authority.

Dead loads

With roofs, the contractor needs to know what weights are always on the roof structure and what allowances are provided for items that may or may not be there. There are many terms for describing these dead loads that might be there. This author prefers the term “collateral dead load.”

The load allowances for mechanical ductwork and architectural bulkheads are examples of collateral dead loads. The reason for identifying these is when:

  • Checking for uplift due to wind load on the roof structure, the structural engineer should only use the dead load.


  • Checking for loads which are pushing down on the roof structure, the structural engineer should use the dead load and collateral dead load.

The locations of mechanical units, ductwork, and architectural bulkheads should be reviewed with the structural engineer. Often, these items are handled by delegated design and the structural engineer is never made aware of them. This is especially true for architectural bulkheads and ceilings.

Connection details for hanging loads have been identified as an area structural engineers should be aware of and not left to a sub-trade to do. As a minimum, the engineer for the sub-trade should know the type of detail that is acceptable to the structural engineer of record.

Project manual

Design of the roof and its systems and components involve all the design disciplines, for example, roofing Division 7, structure Divisions 3, 5, or 6, reflected ceiling plans and bulkheads gypsum board assemblies Division 9, lighting Division 26, and HVAC ductwork Division 23.

Roof design can be handled in many ways: by the owner’s design team, delegated designers, or a combination of both. Previous articles published in Construction Canada have recommended processes for delegated design. The same considerations also apply if the owner’s design team (professionals of record) do all or part of the design work. The transfer of design information is the same for all these approaches.

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