I attached more sheathing yesterday, and had a relaxing sit-down up in the loft area. The spring weather has finally made a confident showing, while my level of sureness has waned. This house is tall. As I sat, I looked down through my structure and tried to imagine all of the weight and forces on each of the components. Gravity is the easiest to picture. Forces of gravity and wind can exert themselves on weak components and cause the parts to unravel. A heavy load of snow can crush a roof. Heavy winds can rip one completely off with an unexpected upward force. A heavy roof exerts a downward force on the lower framing members – but the rafters also have an outward force on the walls.
An outward force on walls. I looked at the screws holding the upper wall framing to the lower walls. Would these screws be enough to counteract the force of the rafters pushing out on the walls? I thought that yes, they probably would. But my level of confidence was far from high enough to put me at ease. This was the first time it had occurred to me to consider these outward forces in my planning, and I’ll tell you: it unsettled me.
A typical roof construction using rafters (as opposed to pre-built trusses) requires that the rafters be attached and triangulated in a way that counteracts the outward forces. The rafters have a tendency to push outward and spread apart under the weight of the roof. There are a couple ways to counteract this force: attached ceiling joists or rafter ties.
IRC (international residential code) requires that roofs incorporating rafters (not trusses) also have ceiling joists attached to the ends of the rafters where the rafters meet the top plates of the walls. In this setup, the rafters are attached to the ridge board and also tied together at the bottom by the ceiling joists. This forms a triangle, which will keep the walls from spreading apart. If there are no ceiling joists, then IRC requires the use of “rafter ties.” Rafter ties are horizontal members which span the rafters and attach in the lower third of the triangle formed by the rafters. Rafter ties counteract the outward forces in much the same way as ceiling joists would. Code requires one or the other.
I didn’t know any of this. I uploaded some screenshots of my Sketchup model to the design forums at small-cabin.com and let the wolves pick them apart. I got plenty of helpful suggestions and advice. I was torn about what to do, but I made a difficult informed decision.
I’m not building to code. I haven’t gotten any permits, and I’m not going to see any inspectors. I get to make my own decisions here. The rules outlined in the IRC exist to give residential homes a minimum level of strength and safety. The dogmatic requirements are probably a good idea for typical large houses, but when a structure is only 8 feet wide, the weight and forces are much smaller. Also, a house that’s only 12 feet long has the top plates at either end preventing the rafters from drifting apart. This is probably more than enough in my case.
So – I think my design was fine. But I’m changing it anyway. Let’s just call it a “change of heart” and move on. Tomorrow, I will remove the upper wall framing and begin the process of moving toward a stronger design and a smaller-still house. I will use ceiling joists across the double top plates, and the ceiling joists will be the floor joists for the loft. I will lose some headroom in the loft. The lumber comprising the upper walls will be reassigned to future tiny house projects. I will be left with a simple and strong tiny house. Everything will seem just a hair easier and more manageable. I won’t have a tall-skinny house supported on orange buckets of buried concrete. I trust my foundation, but I don’t want to push it. The taller they are, the harder they fall. I don’t know what an incredibly strong wind would do to the tall house – but the forces exerted on it would be much greater than the design that I am ultimately switching to.
I wanted small and simple. This decision puts me at ease.