truss modifications

As you may have already noticed, one of the primary design features of ‘el sombrero’ is the inverted truss roof form. Along with many other useful building materials and an already built CMU foundation, we found ourselves with 20 ‘traditional’ roof trusses and 2 gable end trusses. Utilizing the existing trusses in a ‘not-so-traditional’ upside down manner required modifications as all of the top chords now became bottom chords and new beams hidden within the truss cavity needed additional support. With much assistance from Rick (UCD Professor extraordinare) and Chris (Structural Engineer), we were finally able to complete the modification of 20 trusses using lumber already on-site as part of the home kit previously delivered. Two gable end trusses are awaiting modification (maybe) and five new trusses will need to be built in order to round out the new roof framing construction. In order to expedite modifications and get a consistent result (we hope), our newly sheathed floor, a.k.a. the dance floor, became a large level surface to set up a jig. Details of the jig and the steps involved are outlined below.

east side truss modification - step 1

A – existing truss to be modified
B – existing truss alignment block (ensures that each truss is positioned the same)
C – top & bottom chord separator (ensures that existing chords (A) don’t move when cut is made)
D – truss raising block (raises truss 1/2″ to prevent cutting into floor sheathing)
E – skilsaw leveling block (creates level surface for skilsaw between top & bottom chord cuts)
F – truss modification alignment block (creates proper angle and location for cut)

east side truss modification - step 2

G – removable cutting fence (this 2×6 and the alignment blocks (F) are set to the proper angle and distance so that measurements are not needed for each truss, just place the cutting fence against the alignment blocks and cut.

east side truss modification - step 3

H1 – new truss modification member (after the existing truss is cut, a new vertical member is added).

west side truss modification - step 1

H2 and H3 – new truss modification members (on the west side of the trusses no cutting was required, only the addition of a new vertical member as well as new top chord bracing. the top chord bracing shown (H3) was eventually re-done to butt up against the new vertical member (H2) in order to satisfy the specified overlap. Again, alignment blocks (F) are set to the proper angle and location so measurement of each truss is not needed).

truss modifications - final step

Partially modified truss seen in the jig on the ‘dance floor’. The final step involved flipping the truss over and adding additional top chord bracing (as seen in the stack of finished trusses)

the end

Completed stacks of modified trusses (and 2 gable ends to be modified).

Alright. I must confess. This process didn’t exactly proceed as smoothly as this post may seem. Although the lumber already on site was too short for added top chord bracing per Chris’s specifications, we decided to move forward with an alternate solution in the interest of time. Well, this sorta backfired. After thinking we were finished with half of the trusses, we received word that our alternate solution wasn’t sufficient due to an excessive cantilever on the west side of the roof. Needless to say, we became really good at stacking and re-stacking and re-re-stacking the trusses until the final photo seen above. Not too much extra time lost in the end and lots of good lessons learned.