Every tree follows its own path toward the sun. Because the beams of Underhill House are round timbers, purposefully chosen because of their graceful arcing lines, our roof swells and rolls down from east to west and from south to north. That makes for a swooping, organic roof line, but it adds some significant challenges in construction.
Our first choice was to put a standing seam metal roof on Underhill House. That’s the kind of roof we have on the barn. It looks great and has a long life expectancy. But with our undulating house roof line we chose to go with a sod roof instead. In fact, the choice of the name Underhill comes in part because we will be living under a roof topped with soil. That, plus the partial earth sheltered nature of our site, made Underhill an appropriate name for our new home.
Our builder, WholeTrees Architecture and Structures, has designed a custom-built truss system that attaches to the top of each un-milled rafter, in order to modulate some of the irregularity in our roof surface.
These trusses are being attached with screws to the length of each rafter. They are made of two conventionally-milled 2x4s connected with metal runner tracks, typically used to attach the tops and bottoms of metal 2×4 wall. studs. The runners have been snipped and bent to join our 2x4s, making a solid set of trusses that can be customized to adapt to our irregular roof surface and smooth it out just a bit.
The majority of them have a 12 inch depth, and the space they create between the rafters and the roof decking boards above will be filled with blown in insulation, which will provide much of the R-value above our heads in our living quarters.
But at the east and west ends of the house the trusses become progressively more narrow. On the east, the roof is an extended soffit, and to the west the roof covers a screened in porch, the trusses become progressively more narrow because no insulation is needed above these areas.
The gradually decreasing truss heights will also create an added downward curve to the roof along the east and west ends.
With our prevailing westerly winds, tucking the roof downward to the west a little should help us weather the kinds of storms climatologists are predicting we can expect with increasing frequency in our warming world.
These types of trusses have been used on earlier WholeTrees houses but in our case the thickness of the metal runner tracks has been increased from 25 gauge to 20 gauge, which will make the trusses even stronger. Bryan Dalstrom, our construction manager, suggested this change towards beefier construction, and he’s happy with the improved strength.
The next step after all the trusses are attached is to cover them with 5/8” pine decking boards made by milling up a clump of white pine trees that were planted 15-20 years ago immediately to the south of our building site.
These trees were growing on a south-facing slope, and in southern Wisconsin, that’s not a good ecological niche for pine trees. We’ve known for years that we were going to repurpose this sunny slope into a passive solar house site, and we spent a lot of time trying to think of a good use for these pines. Milling them for the roof boards on a house that will stand only a few feet from where these trees were grown seemed perfect.
Doug was up on the roof today, but I am waiting till all the trusses and decking are in place before I venture up there. He says he thinks we won’t have any worries about “trussing” the strength of our new roof, and he says the view is great.