Table of Contents
Journal of Structures
Volume 2015, Article ID 563025, 10 pages
Research Article

Effect of Composite Action on the Strength of Wood Roofs

1L.A. Fuess Partners, Dallas, TX 75219, USA
2Department of Civil Engineering, University of New Mexico, Albuquerque, NM 87131, USA
3Sandia National Laboratories, Albuquerque, NM 87185, USA

Received 30 September 2014; Revised 12 February 2015; Accepted 17 February 2015

Academic Editor: Lucio Nobile

Copyright © 2015 Ivan A. Campos Varela et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.


Engineering certification for the installation of solar photovoltaic modules on wood roofs is often denied because existing wood roofs do not meet current building codes. Rather than requiring expensive structural retrofits, we desire to show that many roofs are actually sufficiently strong if the effect of composite action produced by joist-sheathing interaction is considered. In a series of laboratory experiments using a limited number of two-by-four wood joists with and without sheathing panels, conventionally sheathed stud-grade joists, surprisingly, exhibited between 18% and 63% higher nominal strength than similar bare joists. To explain this strength increase, a simple model was developed to predict the strengths of the nailed partially composite sections, but the model only justifies a 1.4% to 3.8% increase in bending strength of joists with an allowable bending strength of 1000 psi. More testing is indicated to resolve this discrepancy between laboratory results and analytical modeling results. In addition to elucidating nonlinear partial composite behavior of existing roof systems, this paper shows that, with minor changes in roof framing practices, strength increases of 70% or more are achievable, compared to the strengths of conventionally sheathed joists.