Meeting New Code Requirements with TimberStrand® LSL Floor Joists

June 30, 2016 in Wood Products - #tech talk

By Jerad Bankston, P.E.

TimberStrand® LSL has traditionally been specified as an efficient and cost effective solution for beam or wall framing member (click here for more information on TimberStrand® LSL).  Its high shear strength and dimensional stability make it the first choice for many residential and multifamily projects.  One application that is gaining popularity is using TimberStrand® LSL as floor joists.

A key driver for the increasing popularity of TimberStrand® LSL floor joists is a provision in the International Residential Code (IRC) which requires floor framing to be installed with a ½” gypsum ceiling membrane for fire protection (section R501.3 in the 2012 IRC, section R302.13 in the 2015 IRC). However, 2×10 or larger dimension lumber or structural composite lumber (SCL) are exempted from this membrane requirement.

The designation of TimberStrand® LSL being a SCL, as well as the design properties needed for floor member design, can be found in our ICC-ESR 1387 code report.  The superior design properties and longer lengths of TimberStrand® LSL are proving to be an effective solution to overcome the allowable span and length limitations of traditional dimensional lumber.  By allowing builders the same design flexibility they are accustomed to with engineered wood products, but not requiring membrane protection on the underside of the floor framing, TimberStrand® LSL is answering the demand for a high quality SCL floor framing alternative.

TimberStrand® LSL floor framing is available in traditional 2×10 and 2×12 nominal sizes as well as 9-1/2”, 11-7/8” and 14” deep members consistent with typical engineered wood product depths.  TB-712 provides span and hole charts for 2×10 and 2×12 1.5E TimberStrand® LSL floor joists and TB-711 provides span and hole charts for the 9-1/2”, 11-7/8” and 14” deep product.

Even in finished basements where the membrane protection has been installed as part of the finishing process, there is a place for TimberStrand® LSL floor joists.    The IRC permits up to 80ft^2 of area (per floor) to be unprotected when using light weight framing.   This exemption is typically used in the mechanical room to allow for mechanicals to be routed from the basement to the floors above.  However, many mechanical rooms are greater than the 80 ft^2 exemption, so fire blocking and membrane protection ends up being required somewhere within this room.   By strategically placing TimberStrand® LSL floor joists over the mechanical room area, a builder can greatly simplify the finishing process by eliminating membrane installation in this area.One common question we get with using TimberStrand® LSL as a floor joist is in regards to drilling holes.   Due to its high shear strength, TimberStrand® LSL can accommodate much larger holes (and closer to bearing) than other traditional engineered lumber products.  The chart below shows the allowable hole size and zone where holes can be drilled.  Forte, our single member sizing program, can be used to analyze specific situations outside the scope of this chart.

(Click image below to enlarge)

Just like our other Trus Joist products, TimberStrand® LSL floor joists can be analyzed with the TJ-Pro Rating® system.  This valuable tool can help guide your decision on floor performance and customer satisfaction.

Whether you have questions about TimberStrand® LSL floor framing or any of our Trus Joist products, feel free to contact us at 1-888-453-8358, by email, or online.


Jerad Bankston, P.E. is a Weyerhaeuser engineer supporting Trus Joist Engineered Wood Products who is based out of St. Paul, Minnesota. During his 14+ years with Weyerhaeuser, Jerad has provided technical support and training for residential, multifamily and industrial applications.