I received the following email last week from a consumer who has questions about using splines in SIP walls and roofs.
I’ve been researching SIP manufacturers and have a question no one has satisfactorily answered for me. Panels that I have seen in demonstration use a block spline for joining panels although one had framing material (like a 2×6)(on a 6-1/2” wall) doubled to make a join with the next panel. I actually liked that.
The two companies that use a block spline said they might also use what I have come to know as a “single spline” like a little piece of OSB that is put in a notch of both panels. I’m being told that is as strong as framing lumber. My mind does not equate to that. Looks like to me that the actual lumber would be stronger and easier to assemble than OSB strips, especially for a roof panel. What do you think? I watched some of your videos but could not determine how you assembled the roof.
Let me try to make sense of this for you.
A structural panel can span specified distances and carry maximum specified loads without the additional strength of a connecting spline. The type of spline that adds NO strength (yet still joins the panels at the joint) is the surface OSB spline or the “block spline” sometimes called a super-spline. These components allow the panels to do all the work and the load design charts reflect the panels capabilities when this detail is used.
However, if the span is too great for the panel to carry the load – For example: a roof panel spans a large distance between ridge and eave wall in a northern climate with significant snow loads or a tall wall with large windows in a wind zone that exceeds the loading capacity of the panel – then a spline joint can be used to supplement the capacity (bending strength) of the panel. The designer can typically choose what type of spline is best suited for the work its being asked to perform based on the structural loads. For instance, a designer (or engineer) might choose a double dimensional lumber joint or a single LVL spline, or even a wood I-Joist spline. Depending on the manufacturers typical details and test data, the designer can easily choose from a variety of options to meet the needs of the structural loads. You are correct that a single OSB spline is not as strong as a dimensional lumber spline!
If you look at the critical sealing requirements, you will quickly see that an OSB spline is easier to seal against air leakage, eliminates the thermal bridging associated with other types of spline joints, it eliminates the cost of lumber, it allows jumbo panels to be used, and it will speed the installation process. It’s a veritable win, win, win, win scenario. However, if your overall design demands large spans, large windows, tall walls and is built were the snow piles high and the wind blows hard, you may be forced to incorporate a structural lumber spline.
This explanation is why I always instruct designers and homeowners to consider a thicker panel as a means to a more efficient envelope, and one that assembles more quickly. Everybody’s engineering requirements are unique and everyone should be aware that they are best served when they work with reputable SIP supplier that uses a tested panel (with load design charts) and involves the experience of an engineer or at the very least a qualified SIP designer. It would be appropriate to point out the SIPA community requires its members to maintain both a 3rd party QC program and current test data that results in load design charts. This fact alone is worth considering when looking for your next SIP quote.