Bottom Chord Attic Truss

The bottom chord is also called a scissors truss.

Bottom chord attic truss.

I have this above my own garage. I remember on one house he told me that you never ever ever ever cut a truss. If the roof slope is steeper than 9 inches in 12 inches of run it doesn t take much width to end up with a nice room 12 feet wide with an 8 foot ceiling. If the bottom chord of a roof truss functions as a floor joist such as in an attic room it carries a live load that also varies by use but a typical live floor load for a residential space is about 40 pounds per square foot.

In doing so the attic width exceeds 20 which necessitates a deeper bottom chord. Due to the overall height the truss is manufactured as a piggyback truss to allow for transportation. Look at this photo of the space created by attic trusses at my home. Those trusses were engineered with a heavy critical dependency on the bottom chords which are in tension.

In floor truss applications 2x4 or 2x3 material is typically used with the wider portion of the board resting on the bearing or what is known as a 4x2 configuration. Cutting the bottom chord of an attic truss i worked for an old school new england carpenter for a couple years when i was 15 16 years old. The roof system is basically a hinge now. This truss is a variation of the one above and stays truer to the classic gambrel shape.

Truss capacity is listed by pounds per square foot based on the following load rating criteria. The bottom chord of the cap truss is typically moved up 1 1 2 for 2x4 continuous lateral restraints over the flat top chord of the base truss or the thickness of whatever material may be used 3 4 for plywood osb etc. The bottom chord establishes the lower edge of a truss. The bottom chord therefore carries combined stress of both tension and bending of the truss.

Removal has left them extremely vulnerable to collapse due to spreading especially under snow loads but also under just the load of the roof itself. In some cases a live load can be applied to a bottom chord either for storage purposes or if there will be movement within the truss such is the case in an attic truss.