We spent all summer in 2018 building this roof. It is quite simple in purpose, design, and functionality. I believe it to be the most important piece of the log home building envelope. It’s also the most labor-intensive task associated with the build so far. I’m throwing inexperience under the bus.
The roof joists are 4″ x 12″ Douglas Fir, grade 1, rough sawn lumber. There are 17 joists per roof panel, spaced at 3-foot centers. The total roof length from gable to gable is 51 feet. From the roof ridge to the eave, each joist measures approximately 28 feet in length. They were cut and delivered slightly longer. Each of the joist tails were cut in-place using a string line for a precise fit. The length of the ridge pole and eaves enable a roof over-hang of 8 feet at the gables, and 5 feet at the eaves. This allows for plenty of moisture and UV protection for the underlying wall logs. For a log structure it’s all about keeping the logs dry. With this design, the logs will last for hundreds of years. It’s a log structure with a big sombrero.
Before the joists were anchored to the superstructure, the top cap logs in the wall that are parallel to the ridge pole were planed at a 6:12 roof pitch. This is normally performed at a log mill before delivery, and takes less than a minute. Unfortunately with this style of butt and pass construction, the final rotational placement of the log is not determined until it’s placed in the structure and “field fitted”. The planing was accomplished by temporarily fixing a sled to the logs that would match the finished 6:12 pitch. A plunge router was then mounted to a frame and the logs were planed by passing the frame over the rails. Each 51-foot cap log took about 2 days to complete.
After the cap logs were planed, we were ready to anchor the joists. The 3-foot joist spacing was marked on the cap logs and ridge pole. The outside joists at the gables were squared by measuring and matching equal diagonals from the outside ridge edge to the diagonal corner of the outside eave edge. I’m not a fan of Pythagoras’s world philosophy but he was a brilliant mathematician, and his contributions to math are immense. Most carpenters use the Pythagorean Theorem every time they square any type of framing and probably don’t realize it.
The joists were temporarily mounted to the ridge pole and cap logs using Simpson ties. These were later removed after the permanent rebar connection was in place. The tops of all the joists were planed with a string-line. Due to the variability of the rough sawn joist thickness and the variability of the ridge pole surface (taper, knots, grain, etc.), lots of notching was necessary to achieve a planed top surface. After planing, the joists were attached to the ridge pole and cap logs with a 1/2″ rebar pin driven through the center of each joist and half way into the underlying ridge pole or cap log. The rebar was advanced with a 70-pound demo hammer in like manner as the log wall construction.
After the joists were anchored permanently, the temporary ties were removed and 4″ x 12″ Douglas Fir blocking was installed between the joists over the approximate center line of the cap logs. The blocking helps keep the joists true, and also provides an environmental barrier, essentially completing the top of the wall. The blocking was secured using 8″ Timberlok wood screws.
The next blog topic will be the beetle kill pine tongue and groove roof decking. Until then,