This post describes the construction of the south-facing cathedral roof over the single story portion of the house. A prior post described the first cathedral ceiling/roof but left the story untold beyond the first layer of sheathing. Here the entire procedure is described except for installation of the final metal cladding. Reminder: click on any photo to enlarge it for detail.
Roof Trusses
The actual R-value of a roof or wall is not only about the R-value of the insulation but also the amount of thermal bridging through the structural members, especially through solid 2x lumber. Because trusses minimize through-and-through structural members, they have less thermal bridging than solid rafters like 2 x 12s (manufactured I-beam rafters have even less). The roof trusses that we used for the second story cathedral ceiling were 16" tall, i.e., they accommodated 16" of insulation that would provide R-50+ before thermal bridging.
After the trusses were installed, I wondered how expensive it would have been to have increased the height of the trusses to 18" as a way of compensating for at least some of the thermal bridging. When ordering the new set of trusses for the first story roof, I had them quoted both ways -- 16" and 18". I was pleasantly surprised that the difference in price was less than $4 per truss so, naturally, I bought the taller trusses. Of course, the additional 2" will increase the cost of insulation by 10% but we will be using inexpensive rice hulls and, after buying a trailer-load, will have a surplus anyway.
Roof Slope Issues
Truss Installation
The trusses were supported on one end by the top of the exterior wall and on the other end by a ledger comprising two back-to-back 2 x 6s fastened to the second story wall with nails and construction screws. After the layout lines were drawn on the ledger and top plate of the wall, installing the trusses becomes realistically a two-person operation. One person lines one end of a truss with the layout line and with the outer edge of the top plate of the wall and the other person lines the other end with the layout line on the ledger. We used both nails and construction screws to fasten the trusses. The latter, angled up through the top plate and into the bottom chord of the truss, is a code-compliant alternative to steel rafter ties. In addition to nailing, the other end was secured with one set of construction screws up through the bottom of the ledger and another set near the top of the truss through blocking in the wall.
Air-Sealing
First layer of sheathing. All cracks are air-sealed with flashing tape |
Traditionally, a leaky area for air infiltration/exfiltration is the space between the double top plates which, if it is addressed at all, is typically caulked interiorly. I as the nearby photo shows, I used tape on it, albeit duct tape instead of flashing tape because it is cheaper and fit better. Duct tape is not nearly as sticky as flashing tape but will be held in place by the sheathing should the stickiness wane. And, with my being the belt and suspender type, it will get caulked on the inside as well.
Mini-Attic
The next job was to add another layer of sheathing to create
We used OSB for the second layer of sheathing over the 2 x 4s for a couple of reasons -- in order to save cost and because, with the mini-attic below, trapping moisture will not be an issue. If I had it to do over again, however, I would have installed 2 x 4 blocks in the gap between the long 2 x 4s and the second story wall. I would have aligned them with the long 2 x 4s and made them short enough not to interfere with air movement but long enough not to split when the OSB was nailed to them. Without them, the OSB was too springy as it rested on the ends of the long 2 x 4s then bridged the 21" gap before resting on the 2 x 4 next to the wall. As it was, I had to use metal strapping to stiffen the junction between OSB sheets but could do nothing about supporting the sheet between edges.
Temporary Protection
It will be several months before all of the roofs are ready for installation of the metal roof so temporary protection for the sheathing was mandatory. I used battened-
The mini-attic completed: the subfacia and the second layer of sheathing are installed and protected by 30# felt paper |
Masonite protects the felt while working on the wall |
As will be explained in the next post, using 30# felt as a temporary covering proved to be a bad idea. Despite conscientious fastening with roofing nails, several areas peeled away with the first heavy wind. Augmenting with roofing cement was only marginally better -- several pieces even then lifted off as if the cement did not adhere well to the felt at least at high summer temperatures. Also the felt that stayed put badly wrinkled but I am not sure why -- whether it was from getting wet or from thermal expansion or both. Some of the wrinkles were so severe that I was afraid they might keep the metal roofing from laying flat as it should for good appearance. So I punted and added 6 mil plastic over the felt until it could be removed and replaced in conjunction with installing the metal roof.
I had a surplus of Craigslist Masonite so I screwed down a few sheets next to the second story wall so the felt will be protected while we build the overhang for the second story windows and install the windows.
No comments:
Post a Comment
As a do-it-selfer-in-training, I welcome your comments.