Tag Archives: ganged wood trusses

Ganged Wood Trusses & Closed Cell Spray Foam Post Frame Condensation Control

Ganged Wood Trusses and Closed Cell Spray Foam Post Frame Condensation Control

Ganged wood trusses are most usually two individually fabricated metal connector plated roof trusses, fastened together with either nails or even better Simpson Drive Screws (https://www.hansenpolebuildings.com/2017/03/simpson-drive-screws/), so they work together as a conjoined pair.

True doubled trusses (not two single trusses spaced apart by blocking) afford many structural advantages (https://www.hansenpolebuildings.com/2018/09/true-double-trusses/). However if closed cell spray foam is being used to control condensation underneath steel roofing, a little extra prevention is worth a pound (or two) of cure.

Most often conditioned post frame buildings are designed around having a flat (or slightly sloped using scissor trusses) ceiling. Warm moist air from this conditioned space rises into building’s attic and hopefully has a place to go. Most generally best design solution involves venting this dead attic space. Appropriate amounts of air intake provided by eave soffit vents and air exhaust utilizing a vented ridge will eliminate most moisture.

As those of us who did not nap during science classes are aware – warm air rises. Some of this warm air will get trapped below roof purlins or other attic framing members and not exhaust as imagined.

There are many methods of controlling or eliminating this warm and moist air from coming into contact with cooler roof steel. Least expensive (although potentially labor intensive if windy) would be a reflective radiant barrier (https://www.hansenpolebuildings.com/2017/05/effective-reflective-insulation/). One step up in investment, but very easily installed, would be an Integral Condensation Control (https://www.hansenpolebuildings.com/2017/03/integral-condensation-control/).

Some folks opt to sheath over trusses and roof purlins with OSB (Oriented Strand Board) or plywood, with 30# asphalt impregnated paper (roofing felt) placed between sheathing and roof steel. This can tend to run up one’s investment, as not only will more material and labor be directly involved, but trusses also must be appropriately designed for added weight carrying ability.

Enter closed cell spray foam. Long time readers have grown tired of me solving condensation challenges by people who did participate in one of these solutions and are now faced with a drip-drip-drip. Two inches of closed cell spray foam applied beneath a steel roof between purlins and trusses will create an almost entirely effective thermal break and take care of nearly all condensation issues.

Except…..
Metal connector plates trusses have pressed steel plates on each side. These plates project slightly from lumber faces and when two trusses are joined together, some gaps will occur between them. Gaps wide enough to allow for a significant flow of warm moist air to reach your roof steel, condense and start wreaking havoc.

There is, however, a simple fix, easily done during building framing. Before conjoining two or more trusses, place enough urethane or acoustic caulking between top cords to provide a complete air seal when in service!

True Double Trusses

True Double Trusses

ASABE (American Society of Agricultural and Biological Engineers) published ANSI/ASABE S618 “Post Frame Building System Nomenclature” in December 2010. For those who are unfamiliar ANSI stands for American National Standards Institute (www.ansi.org). ANSI is a private non-profit organization overseeing development of voluntary consensus standards for United States products, services, systems and personnel.

In ANSI/ASABE S618, a Metal plated connected wood truss would be described as, “A truss composed of wood members joined with metal connector plates (also known as truss plates). Metal connector plates (MCP) are light-gauge, toothed steel plates. The most common type of light wood truss.” Ganged wood trusses are defined as, “A truss designed to be installed as an assembly of two or more individual light wood trusses fastened together to act as one.”

Reader RON in FORT BENTON writes: I built a 24 x 36 x 10 pole building from a kit 30 yrs ago. And I have had a 20 x 28 x14 built by local professionals about 8 yrs ago. They each have 12 or 14 foot distances between posts. The trusses are the regular double 2 x 6 construction with 1 on opposite sides of the posts with blocking between them at the bottom. The side girts have been 2  2x6s configured in the L shape. I am not sure what you call that. Now looking at examples of kits, they seem to use (double?) 2×4 construction for the truss. Has there been a big change or am I just missing something. I like the 2×6 approach and am not sure how much difference it makes in final costs. I am looking into a 48x36x? monitor type building in, at times, very windy location. Thanks.”

Even though some very high grades of 2×4 lumber are available to metal plated connected wood truss manufacturers (such as 2850msr), only in very small spans and light loads would they work for top chords of double trusses spaced upon 12 or 14 foot centers. For bottom chords, it might be possible to get to 30 or 36 foot width spans, provided loads were light.

Learn about Machine Rated (MSR) lumber here: https://www.hansenpolebuildings.com/2012/12/machine-graded-lumber/.

For Hansen Pole Buildings, any time we are using a “real” double (more specifically ganged) truss system, we specify top chords to be a minimum of 2×6, regardless of loads. I say “real” because placing a single truss along each side of a column (as you have described) is not a double truss. They are two single trusses, acting independently from each other. A true double truss system, such as used by Hansen Pole Buildings, features trusses physically attached face-to-face by means of mechanical connectors (e.g. nails, bolts, etc.). This allows for two members to actually load share, reducing probabilities of one weak single truss failing and pulling a roof system down with it.