I used pressure treated 2x6s for the sill plates. I cut them them to length, drilled holes for the anchor bolts and test fit them on the grade beam before framing the wall.
Once I had the sill plates down and confirmed everything was square I removed the sill plates and temporarily attached them to their corresponding top plates to mark the stud locations.
Framing the back wall was simple as there were no windows or other openings. I used 2×6 studs locating them on 16” centers. I saw one living roof build where they put their studs on 12” centers. I thought that was a little overbuilt as a 2×6 stud can support a lot of weight. A grade #2 southern yellow pine 2×6 can theoretically support around 1400 lbs. However, the longer a board the more susceptible to bending or buckling under load. A singular 8 foot long 2×6 would fail under just a fraction of those 1400 lbs.
To mitigate bending I attached plywood sheathing to the exterior. The plywood ties the stud to its surrounding stud and provides a lot of lateral strength. Later I’ll infill the stud bays with cob to further stiffen the stud.
The front wall will have a door and three windows. I made headers using two 2x8s fastened together for the windows and door. I’ll add a third 2×8 to each header after the frame is raised. I used a combination of nails and screws on a situational basis. Nails are cheaper and have more sheer strength but my cordless drill is lot more convenient than the nail gun. I also like to use screws on the window sills because when we pack the front wall with light clay straw we can easily remove the sills temporarily to get better access to the wall cavity.
The front wall was significantly heavier than the back wall so I called in some extra reinforcements to raise it. Our next door neighbors came over and with the help of Jean and my Dad the wall went onto the anchor bolts without too much trouble. I’m glad I test fit the sill plates beforehand because it was a tight fit..
Per the advice of my neighbor I built the side walls in place rather than on the ground. I put down the sill plate and then secured the end studs to the back and front wall followed by top plate. Then I was able to attach all of the intermediary studs through the top plate and toe nailing into the sill plate.
The back and side walls are sheathed in ¾ inch ground contact plywood. I fastened the plywood down every foot with a combination of screws and nails. We’ll be packing the walls with cob so the screws won’t pull out like the nails under the force of the packing effort.
The 4×8 sheets weight around 70 pounds so I made couple of little temporary supports to keep the upper pieces in place until I was able to get a few screws in.
The front wall is the only non-bermed side and I’m planning to infill it with light clay straw. As light clay straw isn’t as resistant to heat transfer as other insulation materials it really benefits from a wall slightly thicker than a 5 1/2” 2×6 so I built a double wall making the front wall 10 1/2” thick. Finished out with plaster it will be close to a foot thick.
Double walls are also nice because they prevent thermal bridging where heat passes from the interior to the exterior via the studs. Double walls have a gap between the two walls that can filled with insulation stopping the effect.
I poured a little addition to the grade beam in the front using rebar to tie the new section into the old section. Then I built the wall in place using 2x4s. It’s non load bearing so I didn’t use headers for the doors or windows and located the studs on 24 inch centers rather than 16 inch. The second wall is bolted down to the grade beam and attached to load bearing wall with strips of plywood.
The cottage has three 20’ long continuous beams running the across the structure and having a generous(~5 foot in front) overhang. On both ends of the building there are partial beams level with the wall top plate and supported by knee braces.
Each beam consists three 2×12 boards fastened together. I built each continuous beam on the wall rather than trying to do it on the ground and lift the completed beam into place. I made some scaffolding from forms board on one end of the building and moved up the 2x12s individually. Then I proceeded to glue clamp, screw and nail the pieces together.
I fastened the two outer continuous beams to the wall top plate with heavy duty brackets and lag screws. The middle beam sits about12 inches higher then the top plate. I built a knee wall between the two outer beams and used a farm jack to hoist middle beam into position. Surprisingly, it worked.
For the partial beams on the outside edges I created a pocket in the wall framing and keyed the partial beam to fit. It was kind of a pain to get the partials fitted, aligned and leveled. With the plywood decking on now I can tell that one of the partial beams angles up slightly. In retrospect, I think it would have been better to run a continuous beam.
The span between beams is less than 5 feet so I calculated that 2×6 rafters on 16 inch centers would be fine for a lightweight living roof. So, I had 30 rafters to make, each with 3 birds mouth cuts.
I got an assembly line going after a while. I would process four 2x6s at a time, crowning them, clamping them together and then making measurements marks for all of them at the same time. Then I would draw out the birds mouth joint for each board and then cut the four consecutively. This way I had to really focus on making sure the measurements were correct less often before going into birds mouth cutting mode.
Laying Out the Rafters
My construction projects really seem to slow down once I leave the ground. The walls went up so quickly but once I started on the roof things seem to slow to a crawl. For example to mark out rafter locations I climbed up on to each beam, straddled it, hooked the end of my tape measure on the end of the beam and scooted backwards marking off rafter locations on 16 inch centers. Sometimes my tape measure would slip off the end of the beam and I would have scoot forwards again to rehook it and then scoot back. So, I had to do this routine a total of 5 times for the three beams and two side walls.
Paul came out to help me put up the rafters and my Dad worked as ground crew feeding us rafters as needed. This saved me a lot of climbing up and down ladders. We worked our way from the front of the cottage to the back using hurricane ties on the top beam and the wall and toe nailing the rafter to the middle beam. Later I put in blocking between the rafters on the top beam and the wall and 2×6 sub-fascia on the rafter tails.
Beam Rafter Design Considerations
From a structural stand point the beams are weakest link in the design. A typical is designed to support a dead load of 20 lbs per square foot. That includes the weight of the rafters, plywood sheathing and roofing. A cubic foot of completely saturated soil weighs in at around 110 lbs. So if you put 6 inches of soil on the roof you’re looking at a possible dead load of ~75 lbs per square foot.
The beams span 11’ 4” (or 12’ for simplicity) from the back wall to the front and are ~5’ apart. So each beams have a (12’ * 5’) 60 square feet of roof area. If the roof weight 75 lbs per square foot each beam needs to support 4500 lbs.
Timbertoolbox has a really good beam loading calculator that I found really useful when designing the timber frame cabin. I put in the numbers for the beam and it failed at 6400 lbs.
Each rafter supports an area that is roughly 16” x 5’ which is little less than 7 square feet. If the roof weighs 75 lbs per square foot each unsupported section of rafter needs to hold 525 lbs. Running the numbers a rafter fails at over 1200 lbs.
Please note, that I am not a structural engineer. I just wanted to explain my thought process and rationale for sizing the rafters and beams the way I did.
The roof sheathing like the wall sheathing is 3/4” ground contact plywood and still very heavy. To get it on the roof I used a ladder as a ramp. I managed to get about 3 full panels up on my own and then fortunately Paul showed up to help with the rest.
I attached scrap boards to the sub-fascia to keep the first run of plywood from sliding off. We fastened the sheathing to the rafters with 2 inch screws. There’s also a 1/8 inch expansion gap between each sheet although with a living roof I don’t know how necessary that will be. Excess plywood was cut off in place.
The last step was putting roof underlayment on to provide some protection until the roof is finished. I used nails with caps and drip edge as the underlayment may be left exposed for a while.