Why a Structural Boundary Member Between a Truss/Rafter is Not Optional

Blocking or boundary member?

In my experience traveling across the country observing wood-framed construction, it was apparent that east of the Rocky Mountains, structural wood members in-line with supporting walls between roof framing cease to be installed. Some may call these wood members blocking and deem them as optional. And often in a humid environment, installation of these members may be ardently resisted in order to provide ample attic ventilation and prevent mold growth. It is important, however, to understand that this blocking creates the structural boundary members for the roof diaphragm and it is not optional.

Tabulated allowable diaphragm lateral design loads contained in the building code, are based on testing of unblocked and blocked diaphragms (structural wood members used at intermediate sheathing edges to increase the shear load in the diaphragm). In all cases a boundary member around the perimeter of the diaphragm, where the shear is the highest, was utilized. The boundary member in these tested assemblies was a minimum 2x structural member that collected the load from the diaphragm through the fasteners in the edge of the diaphragm. When a boundary member is missing, what other key element is also missing? That’s right, the fasteners! If a diaphragm boundary member and consequently sheathing fasteners are omitted, a weakened and unknown allowable roof diaphragm lateral load will exist.

 Example

If the roof framing is spaced at 24 inches on center in the picture below, what is the fastener spacing in the direction of the shear load if there is no boundary member? Yep, pretty easy, 24 inches on center. So what is the diaphragm allowable load with fasteners spaced at 24 inches on center?  The answer is unknown.

Roof Framing example

In addition to being a required structural member, the boundary member can be a big benefit when installed. There is an obvious direct connection between the roof diaphragm and lateral resisting system below (typically shearwalls) to transfer shear loads. Hurricane ties may be an alternative to transfer shear between the roof and wall, but the use of these connectors does not negate the need for the boundary member. Installing a boundary member will require additional detailing to attain the needed ventilation without compromising the structural integrity of the roof diaphragm.

Ventilation

Adequate attic ventilation can be developed through other detailing like holes and screens in the boundary member, orienting every third member horizontal, or gable end vents; but cannot be a reason for eliminating the diaphragm boundary member.

One possible method to eliminate the boundary member in-line with the wall below may be to soffit the eaves and provide a structural fascia board that will receive the diaphragm fasteners. Proper detailing, like shown below, and a structural soffit will be required to adequately transfer the shear in this method.

What are your thoughts? Let me know in the comments.

– Paul

Paul McEntee

Author: Paul McEntee

A couple of years back we hosted a “Take your daughter or son to work day,” which was a great opportunity for our children to find out what their parents did. We had different activities for the kids to learn about careers and the importance of education in opening up career opportunities. People often ask me what I do for Simpson Strong-Tie and I sometimes laugh about how my son Ryan responded to a questionnaire he filled out that day:

Q.   What is your mom/dad's job?
A.   Goes and gets coffee and sits at his desk

Q.   What does your mom/dad actually do at work?
A.   Walks in the test lab and checks things

When I am not checking things in the lab or sitting at my desk drinking coffee, I manage Engineering Research and Development for Simpson Strong-Tie, focusing on new product development for connectors and lateral systems.

I graduated from the University of California at Berkeley and I am a licensed Civil and Structural Engineer in California. Prior to joining Simpson Strong-Tie, I worked for 10 years as a consulting structural engineer designing commercial, industrial, multi-family, mixed-use and retail projects. I was fortunate in those years to work at a great engineering firm that did a lot of everything. This allowed me to gain experience designing with wood, structural steel, concrete, concrete block and cold-formed steel as well as working on many seismic retrofits of historic unreinforced masonry buildings.