Modified Larsen Truss Retrofit: I-Joists on a Roof

February 20, 2017

 

Framing a roof with I-joists over the existing roof makes a ton of room for insulation and offers a short lesson in vapor movement through assemblies

 

A little while ago, we looked at a deep energy retrofit striving for Passive House certification by thickening the walls with I-joists, screwed to the outside.

Turns out, they also thickened the roof.

A new roof was framed on top of the existing roof deck, which was air sealed with high performance tape.

When I arrived, much of the roof had been framed, but I was able to climb up for a little valley framing.

Because the framing is overlaid, there is no valley rafter. Instead, the compound bevel at the bottom of the jack rafters are reversed, to flush into the existing roof. This means laying down a strip of roof decking before framing is complete.

Another benefit of framing on top of an existing roof deck is that you can walk on decking between the rafters. It makes carrying lumber a lot easier. To look at the valley flashing, we are going to follow along with Ryan Olivieri—the Carpenter of the Year—and Ben Bogie his biggest fan.

The valley line is snapped on the decking strip and rafter locations marked along the line. This can be done with a stringline and a tape measure, a framing square, or math.

Even still, Sometimes rafters need to be trimmed to fit. They they put it on the line and nail it.  

This over roof adds an extra 16 inches for cellulose insulation.  Because the existing deck is sealed, this roof assembly needs to be able to dry upward if it gets moist or wet. Above the i-joists, Ben and Ryan install a smart vapor membrane, which allows drying and prevents bulk water intrusion.

A ventilation space is created above this membrane with various furring strips in a cross hatched pattern to allow continuous airflow under a top layer of roof decking and a metal roof.

This roof ventilation scheme is called a Sarked Roof. And the strips are called sarking.

They begin with a ledger strip installed perpendicular to the rafters so they can butt rafter tails to establish a straight fascia. These ripped-down 2x4s are screwed into the i-joists. And spaced evenly with a jig. 

With one side of the roof ready, Ben cuts a couple of gage blocks so they can stringline the roof framing to remove any last humps.

Now, they can fill in the rest of the sarking, cutting the hips in place and flushing the tops with the first course to ensure a great-looking cap on an energy efficient overachiever.

 

—Ben Bogie owns Built to Last Design & Build, LLC, in New Milford, CT

 


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