Unique Yurt Top Plate Design Provides Exceptional Support

While yurt construction is relatively easy, there are several critical considerations to be factored into your design.  Yurts appear to be flimsy structures, supposedly nothing more than a slightly more rigid form of tent.  Yet, these innovative buildings have been the mainstay of tribes of the Himalayas and northern Middle East for tens of centuries.  One of the most vital considerations is the counter-play of the outward pressure of the roof rafter system against the vertical walls, along with the gravitational downward pull on the roof truss chords.

In flexible wall yurts – the most popular designs – the outward stress is counterbalanced by a line of aircraft wire strung through the vertical wall lattice, and tensioned using a turnbuckle system.  The rafters notch into this cable, pressing outward in a uniform manner.  Because of the circular shape, every resting point of the heels of the truss chords applies equal pressure, and thus, each truss offsets each other truss.  At the apex of each chord, the rafter ring bears the weight and pressure of the upper end of the truss, equally and counterbalancing each other truss.  This design is simple, but its simplicity means that there is a maximum diameter of yurt that can be built.

Solid wall yurts have both drawbacks and advantages over the lattice wall design.  While they offer greater protection from exterior elements, the ability to be designed taller than lattice wall units, the flexibility to incorporate conventional doors or windows and the capacity for greater insulation, they also have the drawback of being built in a multitude of wall segments, as opposed to the continuous wall format of lattice walls. Solid wall yurt systems require, because of the individual wall panels, supplementary structural support.

When designing my solid wall yurt, I incorporated not one, but four rafter support concepts.

The first was the conventional aircraft cable, strung through the heels of the rafter chords (reinforced with a metal ring, to prevent cutting through the wood of the rafter.  Secondly, I used hurricane ties on each chord heel.  Thirdly, I nailed steel mending plates at the top and bottom of each joined segment.

The fourth concept provides unique structural support for the walls, offers exceptional wind resistance, and ensures that the rafters do not place excessive force against the top of the walls, causing them to bow outward.

My yurt is twenty-eight feet in diameter.  Using forty-four two-foot wide panels, I placed each panel at an eight-degree angle to the adjacent panel (resulting in 352 degrees of curve, rather than the full 360). By using 2 by 6 studs and cutting an eight degree  “V” shaped 1.5 inch wide, I was able to generate 12 top plate segments from each eight foot length of wood.  Each piece has two arms extending from the apex of the “V”, with each arm twelve inches long. 

These pieces are nailed on top of two adjacent panels, with the “V” placed precisely where the two pieces meet, and extending one foot into each panel.  They are secured with three nails in each arm.  As added reinforcement, I used the same system as the bottom plate for the walls.

This unique top and bottom plate system has worked exceptionally well, with the yurt enduring wind gusts of 115 kph (73 mph) without any problems. In fact, in 2011, shear wind toppled a tree with a trunk diameter of fifteen inches, less than 100 feet from the yurt.  The yurt barely quivered!  2010 winter snow loads failed to bend or bow any of the truss chords.   Although I clearly have implemented more structural reinforcement than is normally required, the strength of the design provides comfort and reassurance in the harshest weather.

Build A Yurt Rafter Ring, Version Two

There are several designs of yurt rafter rings, each serving a particular purpose, and working best in specific environments.  Snow load, wind and even humidity play a role in determining the most appropriate design.  For the majority of yurt applications, the laminated design that I described in a previous article is the most effective.  However, the design described in his article is suitable for smaller yurts (less than 32 feet diameter) and in low humidity locations.  While it will withstand moderate snow loads, it is less structurally stable than the laminated version.

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.

Yurt Trusses Withstand Heavy Snow

One of the concerns that we had when we constructed our yurt was that the winter snow load might be too stressful for the design if the roof. In our part of the continent, we get a fair amount of snow and lake effect snow (120 cm per winter, or 47.5 inches). While this is less than the mountainous areas or the eastern seaboard often gets, our snow stays from November until late April.
The roof trusses are designed with two-by-fours installed 24 inches on centre at the wall top plate, merging to 1.5 inches at the apex of the yurt dome. Most of the conventional, commercial yurts have a similar truss distribution, and claim that they are sufficiently solid to withstand normal snow loads. However, those manufacturers also offer wind and heavy snow load reinforcement options. Our design has an additional drawback: it is designed with a 28 degree slope, instead of the 40-45 degree slope that is needed to ensure that snow slides off the roof.
In order to distribute the weight of the snow, we installed collar ties at the seven-foot point on each truss (our yurt has a 28-foot diameter), with hurricane ties at the wall plates. To ensure lateral and diagonal stability, we used a 3/8 inch aircraft cable, adjustable through use of a turnbuckle, looped through the ends of each truss. Three by six inch reinforcing plates are affixed within two inches of the top and bottom of each joined wall section.
During the fierce winds that we encountered in late October, there was absolutely no movement of the yurt, providing us with some sense of security that the structure was sound. However, lateral wind is not comparable to vertical pressure of weight, so we have had to wait until the snow arrived to test our design.
As of January 26, we have received 10 centimetres more than the entire seasonal average of snow for our area, so measuring the impact of the snow load for a typical season is possible.
Not only has the roof assembly withstood the entire load, but it has not sagged more than 1 centimetre (1/2 inch), yet the snow depth is almost 8 inches on the lower third of the roof. To further test the strength and stability of the trusses, I climbed on the roof and put my entire weight on the mid-point between collar tie and wall plates, and collar tie and dome. No sag was noticed.
Given the additional reinforcing measures that we incorporated into the design, this durability is to be expected. However, it is welcome to see that theory and practice meet, when it comes to the strength of our design. Consequently, I have no hesitation in recommending a similar layout if you are contemplating construction of your own yurt.