Tag Archives: dropped top chord

Dropped Chord End Truss Framing

How to Frame a Roof Corner With a Dropped Top Chord End Truss

Josh and I are becoming better acquainted with each other. Josh happened to not have invested in a Hansen Pole Buildings’ engineered post frame kit package and as such – didn’t have a source of Technical Support which could guide him through the process (nor the Hansen Pole Buildings’ Construction Manual). Because we truly care about all buildings post frame, we help everyone – we believe in paying it forward.

JOSH writes: “I have another question for you. I have drop top gable trusses (not really sure what the name is)? What is the proper way to frame the corner overhangs since the last purlin and eave girt will not extend over the truss? I don’t want the snow to sag the corner. These trusses were built by Trusstek in Bonners Ferry. Thanks so much.”

Mike the Poe Barn Guru Responds:

I can tell by your framing you have been reading my articles and perhaps eyeballing the sample plans and details on the Hansen Pole Buildings website. I really like your having doubled the interior trusses and nailed them face-to-face, rather than placing blocking between them. By joist hanging the purlins into the double trusses, you will be able to pre-drill the roof steel. While this may sound time consuming, it truly makes for a quick roof install, all of the roof screws will be in beautiful, straight rows and you won’t have any shiners which would result in roof leaks down the road.

One thing I probably would not have recommended is using a drop top chord truss on the endwalls. Not for the framing challenge you have encountered, but because I would want the raised heels on the interior trusses to allow for full insulation depth in the attic to the outside edge of the walls. By using a raised heel, the bottom chords of all trusses can be placed at the same height, making it easy to attach ceiling joists without added framing members.

To address your question – when you install the varge rafter across the ends of the overhanging purlins, it will extend out far enough to be able to attach the fascia board to the back side of it. You will want to nail through the fascia into the overhanging “tail” of the end truss. Confirm with the engineer of record who designed your building, but I would typically recommend the use of four 10d common hot dipped galvanized nails through the fascia into the end truss tail, as well as through the varge rafter into the fascia and overhanging purlins. Using a loner nail will usually result in splitting the wood you are nailing in to and a smaller diameter reduces greatly the lateral shear value of the nails.

Let me know if you have any other questions. Hansen Buildings is here to help!


Dropped Chord Truss

The Following is an Actual Instant Message Conversation between Rachel, one of our in-house building designers and myself.

Rachel: Builder on the phone.

Rachel: He is wondering why we wouldn’t make the trusses smaller so all the pole lengths would the same?

Polebarnguru: I have no idea yet what he is asking.

Rachel: He says the trusses with overhangs on the endwalls are not at the same height.

Polebarnguru: Correct, they are lowered, so what is the point?

Polebarnguru: He can’t read a tape measure?

Rachel: Couldn’t they just have a smaller truss?

Rachel: So I figured out his question.

Polebarnguru: The dropped top chord truss, will rarely work.

Rachel: Perfect.

All too often end wall overhangs are supported by builders cutting into the top chord of an end truss, and laying 2×4 “outriggers” flat into notches. As a teenager, I remember doing this myself. Once I entered the prefabricated wood roof truss industry I found out – never cut into a truss in the field!!

The correct method for supporting a roof overhang past the endwall of a building is to either have what is known as a “dropped top chord”, or otherwise have the end truss height adjusted to allow structurally adequate framing to pass over the top of the end truss.

With a dropped chord truss, the bottom of all trusses is at the very same level. This works well in stick frame (stud wall) construction, where wall plate lines are all at equal heights.

pole barn trussesIn a post frame building – the dropped chord truss does not work so well, as the ends of the trusses are a point, at the corner columns, where the narrow tapered portion of the truss is unable to adequately transfer roof loads to the limited bearing surface of the corner columns.

With roof purlins on edge, to adequately support the imposed roof loads of trusses eight foot or more on center, the end trusses can be adjusted in height on the corner and end columns to compensate for the thickness of the roof purlins.

This difference of height does not change the length of any of the supporting columns, only the location of the trusses vertically.

Lowering the endwall trusses by the height of the top chord – really does not seem all that complicated.  It does of course involve being able to accurately read plans and use a tape measure.