Roofing a Tiny House

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After doing some research when we first started building, we agreed on metal roofing. Metal is easily installed, completely recyclable, long lasting, and it can be used for rain water harvesting without concerns over carcinogens. We found a local company that would supply 26 gauge galvanized steel roofing for a reasonable price as well as a “drip edge” flashing that would serve our runoff, as well as our facia needs.

After placing the 1/2 ‘’ OSB over the roof framing, we installed 1-1/2’’ polyisocyanurate (Often called “polyiso” for short, or generically “foam board”.) insulation. Polyiso is commonly regarded as the most eco-friendly option amongst the foam choices. Installation is simple as the boards are the same dimensions as the sheathing and our roof dimensions (8.5×24) with no peaks or valleys made the entire roofing portion very simple.

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Over the polyiso, we placed a layer of water proofing superior to felt paper. The product we used is manufacture by Grace for prevention of ice dams in metal roofs near the toe of slopes. We elected to cover the entire roof in the product due to its reputation as a water barrier. The underside of the product is an asphalt-like layer that will adhere to anything and forms a strong bond to the polyiso.1017141340

Placing the drip edge and metal roofing panels proved very easy and was finished in only a few hours. As with everything else, the panels were screwed on, with considerations for wind and highway travel. The panels went together seamlessly and we have the added insurance of the weather barrier beneath.

Our roof is designed to be impermeable. We have no vents, chimneys or etc. and we designed it this way for simplicity and peace of mind. Houses must however be given opportunity to expel moisture that builds up at the roof interface. We will achieve this with soffits (vents) installed in the underside of our south, east, and west roof overhangs (the roof is tilted to the north).

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Sheathing: The First Walls on Our Tiny House

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Sheathing provides lateral rigidity and a weather barrier to the exterior of a home. It also provides a clean surface over which to apply siding. We used ½’’ OSB (Oriented Strand Board, I had always just called it particle board) over the walls. We also had about 180 square feet of MDF siding that we used on the north face of the house as sheathing. We were given this siding as a bonus when we purchased our floor joist lumber from a gentleman on Craigslist and as it was stained and generally unattractive (albeit unused), we decided to use it as sheathing.

Sheathing is generally not a complex process. I was tipped that we should allot a 1/8’’ spacing between sheets as OSB can expand/contract with weather. Due to our ample window coverage, we were not able to use precisely 24’’ OC spacing for all studs and therefore were required to make many more cuts of our sheathing that is generally required. This still did not greatly increase the difficulty of a fairly simple process.

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Sheathing is also the time to find out just how square your framing job turned out. I walked around the house and checked myself with a framing square and carpenter’s level before I began sheathing. I did find a few minor adjustments to make while placing the OSB (OSB and plywood are made by machine and are near perfectly square. Assuming your framing is correct and there’s a problem with the sheathing is like blaming your calculator for errors).

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Framing for a Tiny House

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As with the floor, we decided to frame the walls with 2x6s for added rigidity and space for insulation. The 24-inch spacing allows less thermal bridging (conduction through the studs) than 16-inch spacing as is necessary with traditional 2×4 framing. The increased spacing also reduces the weight difference that might otherwise pose a problem for those building on a trailer.

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Framing is actually simpler than I expected. I decided early on that screwing the studs together would be necessitated by the torsion stresses the house would face in highway travel. We plan on being more sedentary than most tiny-housers, but we’re building with other future owners of the home in mind.

Before we began designing our framing diagrams we purchased all of our windows. Most of the windows came from the Habitat for Humanity’s ReStore. They have some great building materials far cheaper than even Craigslist usually offers. Our design is intended to utilize passive solar for chilly high elevation winters with our south-facing wall containing 4 windows (approximately 30% glass by surface area). The sliding glass door that we purchased, as well as one of our larger windows were both already jambed and we were able to simply use those dimensions for framing design. For the remaining 6 windows we added 1 inch on all sides to a lot for framing in our designs.

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The walls came together more quickly than I had anticipated. Using a cheap (I believe it cost me $100-$120) battery-powered saw and drill set, the walls were up in a week. I was continually waiting on my batteries to charge and I would strongly recommend a traditional corded circular saw or miter saw for this part of the project. I ended up buying one after framing and utilized it for sheathing.

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Our roof framing is quite simple and consists merely of 2x4s placed across the 8’ wall span at 24’’ OC. Upon sheathing the roof, this is more than enough capacity for me to wall on without any noticeable “give”. The roof overhangs the wall framing by 3’’ on both sides and 1’ at each end providing rain protection at the entry way and over our utility space.

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Mobile Foundations – Solid Beginings

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Wow, are we behind. The walls and roof are now framed and I’ve begun the process of sheathing. We have a storm moving through (former tropical storm Simon) and I have a day away from building to give some updates.

We initially planned to place our floor joists at the same level as the trailer frame allowing for ample loft space (while keeping under the 13’6” road limit). That plan quickly dissolved as we saw the complexities involved in that plan and we began laying our boards over the frame.

We employed a simple framing plan with the boards laying on their sides extending out over the metal trailer frame. The main trailer i-beams are spaced at 6′ and we want a full 8′, so we’ll have a bit of a cantilever. To support the cantilever and allow for additional space for insulation as well as sewer plumbing we used 2x6s for the joists. At the wheel wells, rather than having a step, we elected to lay the OSB directly over the gap in framing. We will add additional support in the walls to accommodate this weak point.

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We rented a pneumatic stapler and air compressor to apply flashing to the underside of the sub-floor. The thick aluminum will provide a weather, pest and radiant barrier. The flashing is sold in 75′ rolls at widths of 10 or 14 inches and is easy to work with. This is a worthwhile step and we used it as our sole underlayment for the sub-floor. Make sure to buy plenty of foil tape for all of your seams.

A mentioned, we decided to place our sewer plumbing in the sub-floor. This will be very convenient later when we work on the walk-in shower and the kitchen sink, which sits on a peninsula. Using PVC primer and glue, we shouldn’t have to worry about leakage any time soon.

As for insulation, we went with cellulose. Both of the big box stores supply this recycled newspaper as a blown-in insulation option. We placed it by hand since we didn’t have a very large area. This is a very inexpensive (roughly $100 for a 170 sq. ft. floor, 5.5 inches deep), green (85% recycled content), and effective (R-3.7/inch) insulation. It turned out to be alot of work breaking this stuff up and placing by hand, not to mention the sun reflecting off the aluminum flashing.

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To finish up the sub-floor, we placed 5/8” OSB. Our 24” spacing proved true and the placement went without issue. The floor feels very solid and I can’t tell where the studs/cavities are while walking.