Georgia-Pacific Corporation
Engineered Lumber Division
4300 Wildwood Parkway
Atlanta, GA 30339
Web site: http://www.gp.com
The ultimate goal in the design of a floor or roof system is the end
user’s safety and satisfaction. Although joists used at spans indicated
in this guide meet or exceed minimum code criteria and will safely support
the loads imposed on them, judgment must be used to adequately meet user
expectation levels. These expectations may vary from one user to another.
• The specifier should consider the meaning of a given deflection limit
in terms of allowable deflection and the effects this could have on the
system. For example, L/360 (span/360) for a 30’ span is 1” of deflection.
L/240 would be 1-1/2” and L/180 would be 2” of deflection. Consideration
might also be given to cases in which a joist with a long span parallels
a short span or a foundation end wall. For example, a 30’ span with up
to 1” of allowable live load deflection could be adjacent to an end wall
with no deflection, causing a noticeable difference in floor levels under
full design load.
• A stiffer floor will result from using a live load deflection limit
of L/480 versus the code minimum L/360. A roof system with less total
load deflection than the code required L/180 may be achieved by using a
criterion of L/240.
• In addition to more stringent deflection limits, several other factors
may improve overall floor performance. Reducing joist spacing and/or
increasing the subfloor thickness will lessen deflection between adjacent
joists and increase load sharing. Floor stiffness can be improved by gluing
the subfloor to the joists before nailing or screwing rather than nailing
alone. For additional stiffness, glue tongue and groove joints. Surfaces
must be clean and dry before gluing.
• As with any construction, it is essential to follow proper installation
procedures. Joists must be plumb and anchored securely to supports before
system sheathing is attached. Supports for multiple span joists must be
level. To minimize settlement when using hangers, joists should be firmly
seated in the hanger bottoms. Leave a 1/16” gap between joist end and header.
• Vibrations may occur in floor systems with very little dead load,
as in large empty rooms. A ceiling attached to the bottom of the joists
will generally dampen vibration as will interior partition walls running
perpendicular to the joists. If a ceiling will not be attached to the bottom
of the joists, vibration can be minimized by nailing a continuous 2x4 perpendicular
to the bottom of the joists at midspan running from end wall to end wall.
Where future finishing of the ceiling is likely, x-bridging or Wood I Beam
blocking panels may be used in place of the 2x4.
Literature
(800) BUILD G-P
(800) 284-5347
Eastern Sales Center
(800) 839-2588
Western Sales Center
(800) 830-7370