December 10-21, 2018: Watching the crane swing the roof trusses in place was a thrill, and once all were set into place the house began to take on a shape you could feel.
Manufactured roof trusses are an economical choice and at 24” deep can clear-span the width of my house. The deep cavity provides a built-in space to fill with insulation (more on this in a later blog post).
The style of my house calls for deep overhangs and generous rakes. Like the brim of a hat, the eaves will deflect rain away from the walls and provide the shade I need to help keep the house cool in summer. I used Sketchup, a 3D modeling software to visualize my design. I played around with thickness and depth of eave to get the look I wanted, and dialed in day of month and time of day to find out where shade lands on the Summer Solstice, when the sun is highest in the sky. I found out I needed a 30” overhang to get full shade on my south-facing windows. On the Winter Solstice, when the sun is lowest in the sky, the windows are in full sun, capturing free heat energy.
The problem is that mid-June isn’t the hottest month, and December isn’t the coldest month. To optimize shade, I should look at July which weather data says has the most “cooling degree days”. To optimize passive solar heat gain, I should make sure my windows are in full sun in February, the month with the most “heating degree days”. I found that cutting the overhang back by 8” gave me full sun on February 15th , but exposed the windows to way too much sun on July 15th. (Note to fellow building science geeks: also play around with truss heel height, wall height, and window placement).
To build a strong, wind-resistant overhang, I upgraded the truss “tail” (top chord) from the usual 2×4 to a 2×6. For each rake, I specified two dropped top chords supporting 2×4 flat “lookouts”. These cantilevers were then reinforced with 2×4’s on edge.
Instead of the usual 2×6 “sub-fascia” covered with a 1×6 “finish fascia”, I kept things simple and straightforward with a 2×8 cedar fascia screwed directly into the truss ends. Cedar is naturally decay-resistant and needs no paint or stain. In this way, I haven’t “sandwiched” materials that might get wet and stay wet, hiding rot from view.My overhang is what’s called a “boxed-in eave”. You see it on farmhouses around here that retain their vintage forms, although their roof edge would have a “square cut” not a “plumb cut” like mine. I’ll need the plumb cut to attach the large gutters I have planned.
Newer homes or remodeled older homes usually have “soffited eaves” that while practical, can look awkward. High-end newer homes or Arts & Crafts homes sport “exposed eaves” which I love but are considerably more complicated and expensive to build.
Each roof truss was secured to the wall framing with a Simpson truss screw instead of the usual hurricane anchor. It’s simpler and faster and can be installed from the inside. I upgraded the nailing protocol for the roof sheathing to match the wind-resistant specifications I used for the wall sheathing. For underlayment, the crew rolled out a double layer of Titanium UDL 25—a synthetic air, water, and vapor barrier—and attached it with cap nails. It’s rated to perform up to 240 degrees, a requirement where PV (photo-voltaic) panels are installed.
It’s become common practice to roll “ice & water” peel-and-stick membrane at the lower edge of a roof as a hedge against ice dams. I’ll save the expense because my roof will have:
1. uniform heat loss across the entire roof area (insulation doesn’t thin @wall)
2. minimal heat loss with 20 inches of dense-pack cellulose
3. minimal heat loss because of air-sealed ceiling cavity
4. vent chute above insulation keeps underside of plywood sheathing cooler
5. metal roof promotes snow shedding
December 21st was a pretty sweet day because the crew got the underlayment down even as night was falling and we enjoyed a beautiful sunset. It was our last day before the holidays and as it turned out, our last day. Winter arrived and hasn’t let up. We’ll be back in March……for sure by April!