When designing a yurt roof, the same considerations that are factored into stressors on conventional housing roof rafter chords come into play. That is, you need to consider the tensions (both lateral and gravitational) on the angled chords. Truss chords endure two primary stress forces: the tendency of the bottom of the chord to push walls outward and the pull of gravity that causes slump in the riser chord. Use of collar ties works to ameliorate the gravitational warping, but, simultaneously, actually increases the stress on the top plate-to-chord heel point of contact. Fortunately, yurt roofs are so light that collar ties and webs usually are not needed.
In my prototype solid-wall yurt, I employ several redundant reinforcements for the chords. A series of mending plates, hurricane ties, aircraft cable and unique angled top plates create a structure that resists very significant outward stress. These concepts will be presented in future articles.
The rafter ring design in this article consists of two layers of ½ or 5/8 inch oriented strand board (or plywood, if OSB is unavailable) and a collage of two-by-four blocks. Other materials needed include a pound and a half of 3 ½ construction or deck screws, a pound of 1 ¾ inch construction screws, enough 3 ¼ inch framing nails to allow for four nails per block, and a quart of carpenters glue or three tubes of construction adhesive.
Begin by cutting a four by eight sheet of OSB into four-by-four pieces. Scribe a circle four feet in diameter in the first piece, and a circle three inches smaller in the second. These will form the upper and lower layers of the “sandwich” ring.
Cut as many four to six inch lengths of 2 by 4 as you will have truss chords. Lay out the pieces around the perimeter of the larger OSB circle, equidistant apart, with the pieces pointing toward the exact centre of the ring. Mark the location of each piece. Apply a layer of carpenter’s glue to each piece, and re-secure them in the spots as marked. Once they have dried sufficiently, turn the assembly over and secure the pieces using two 1 ¾ inch screws per block.
Measure the distance between each block at the inner edge. Cut pieces of 2 by four that will fit accurately between each 4-6 inch piece. Do not worry too much about angling the cut edges precisely, as these pieces simply act as stabilizers for the main blocks. Apply glue to the long edges of these blocks, slide them into place between each 4-6 inch block and secure them using 3 ¼ inch screws, toenail angled into place. Use one nail per longer block to nail the spacers into place.
Turn the assembly over again, apply glue to the exposed edge of all of the blocks, and attach the second ring, with its centre aligning exactly with the centre of the larger ring. Use two screws per block, as in the prior side of the sandwich. Turn the assembly over once again, and screw in two screws per spacer block.
This rafter ring is much lighter than the laminated version described in prior articles, and is much easier to raise into place. With the smaller ring on the lower side of this sandwich design, the truss chords, once cut on the proper angle, will slide into the notches quite easily and will hold themselves in place as each truss in installed. However, the drawback to this design is the tendency for the OSB to expand and weaken if it gets wet, or for the screws to pull through if they are set too deep in the OSB.