Bottom Chord Attic Truss

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.

Bottom chord attic truss.

I remember on one house he told me that you never ever ever ever cut a truss. Due to the overall height the truss is manufactured as a piggyback truss to allow for transportation. This truss is a variation of the one above and stays truer to the classic gambrel shape. Top chord live load snow and other temporary loads top chord dead load roofing materials weight of the truss itself other permanent items attached to the roof bottom chord live load storage or living space and bottom chord dead load.

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 bottom chord is the bottom horizontal or inclined member of a truss. In doing so the attic width exceeds 20 which necessitates a deeper bottom chord. 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.

Those trusses were engineered with a heavy critical dependency on the bottom chords which are in tension. The bottom chord is also called a scissors truss. Truss capacity is listed by pounds per square foot based on the following load rating criteria. The bottom chord establishes the lower edge of a truss.

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. 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. 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. Attic trusses have a rectangular shape inside each truss.

The bottom chord therefore carries combined stress of both tension and bending of the truss. The roof system is basically a hinge now.