Considerations for Flat Roof Construction

When designing and installing a flat roof, it is essential to provide for drainage to avoid leakage problems and pooling water on roofs. The International Residential Code (IRC) states that roofs must have a minimum of 1/4:12 slope that allows water to drain properly (Chapter 9)

Flat Roofs with a slight slope can be achieved using Trus Joist® TJI® joists and solid section products. It is the responsibility of the designer of record to address items like ponding, snow/drift loads, lateral loads, wind loads, bracing, ventilation, and insulation when adding a slope to roofs. 

Weyerhaeuser suggests the following variations for creating slope with Trus Joist products. 

Raised Bearing: At one end of the joist raise the plate height and add a vertical 2x to the side of the TJI bottom flange to provide a flat ceiling surface.To achieve a minimum 1/4" per foot of slope, bearing wall elevations need to be raised the thickness of a 2x plate for every 6 foot of joist span. 

Taper Cut Trus Joist Structural Composite Lumber: A designer of record may specify 1- 3/4" (or wider) TimberStrand® LSL, Microllam® LVL, or Parallam® PSL roof joists to be field modified with a taper cut.

Tapered Rigid Insulation: Often seen on large scale projects, tapered rigid insulation can be used to create minimum slope on top of a TJI® joist roof system. Contact insulation manufacturer for thermal and installation information.

Ripper Strips on TJI® Joist Top Flange: Rip dimensional lumber diagonally to the desired slope and fasten to the top flange of the TJI® joist with nails or Simpson Strong-Tie® Ripper Clips. Extra blocking will be required.


For more information such as proper connections and bracing contact tech support.

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