How to Build a Tiny House – Part 2: Setting a Tiny House Foundation
Setting a tiny house foundation doesn't need to be a daunting task. These easy instructions will make the necessary steps clear and manageable.
By the Editors of Skills Institute Press
Good things do come in small packages. Just ask internationally recognized small living expert, Jay Shafer. Ranging in size from 100 to 120 square feet, Jay Shafer's DIY Book of Backyard Sheds & Tiny Houses (Skills Institute Press LLC, 2013) features beautiful small houses can be used as guest cottages, art or writing studios, home offices, craft workshops, vacation retreats or a full-time residence. Whatever your building goal, this book will teach you how to plan and build your structure from the ground up. The following excerpts from chapter four, "How to Build a Tiny House," takes you through the steps necessary for setting the foundation for a tiny house or shed.
You can purchase this book from the GRIT store: Jay Shafer’s DIY Book of Backyard Sheds & Tiny Houses.
Looking for other steps? Check them out in How to Build a Tiny House.
A Foundation on Poles
From a distance, a shed resting on wooden poles may look shaky and fragile. In reality, such a foundation can provide decades of stalwart support and withstand storms and floods.
Pole platforms cannot be used everywhere. In some localities, building codes require continuous wall foundations. Walls are also recommended in earthquake-prone regions, cold climates, and on building sites with insecure soil— such as sand or soft clay—or a slope greater than 1 in 10—1 foot of rise for every 10 horizontal feet. The techniques on these pages are designed only for fairly level sites with stable soil.
Lumber and Concrete
Buy pressure-treated poles that are reasonably straight and uniform in diameter, and get enough to space them about 8 feet apart. For a level site, they need to be long enough to extend 1 1/2 to 3 feet above ground level and at least 4 feet below it, or at least 6 inches below the frost line if it is deeper than 4 feet. Where the ground slopes gently, buy poles that are 1 foot longer. For the beams, obtain pressure-treated lumber long enough to span the rows of poles.
To anchor poles in their holes (below, Step 2), you will need concrete or a wet mixture of 1 part Portland cement to 5 parts clean soil, free of roots, leaves, and other organic matter. Prepare this mixture by combining the dry ingredients, then adding slightly less water than you would for concrete—the amount of water will vary depending on the soil
Safety: Goggles protect your eyes when you are using power tools or hammering. Wear gloves when handling pressuretreated wood; add a dust mark when cutting it.
|Anatomy of a Pole Platform A foundation 16 feet square consists of three rows of pressure-treated poles sunk into the ground at 8-foot intervals. Each pole is firmly anchored in a jacket of concrete or soil-cement mixture; the tops of the poles are sandwiched between 2-by-10 pressure- treated beams, fastened in place with 1/2-inch carriage bolts, washers, and nuts. The 2-by-6 pressure-treated floor joists, which span from the outside beams to the middle beams, are fastened to the beams with framing anchors. They are also nailed to a header joist at each end of the foundation. The outer joists are placed to coincide with the eventual location of the cabin’s walls. Sheets of plywood nailed across the joists can serve as a subfloor or as a finish floor.|
Putting up a Pole Platform
• Tape measure (50 foot)
• Plumb bob
• Power auger
• Garden spade
• Posthole digger
• Carpenter’s level
• Line level
• 2 x 4 tamper
• Electric drill with auger bit (1/2 inch)
• Circular saw
• 1 x 2s, 1 x 6s
• 2 x 4s, 2 x 8s
• Pressure-treated poles (6–8 inch thick)
• Pressure-treated 2 x 6s, 2 x 10s
• Exterior-grade plywood (3/4 inch)
• Ring-shank nails (2 inch)
|1. DIGGING THE HOLES. Lay out the site as you would for a block wall, marking the location of the poles in relation to the walls. Rent a power auger to dig each posthole to a depth of 4 feet or at least 6 inches below the frost line, whichever is deeper. You can use a one-man power auger (photograph), but a two-man version (above) is more powerful and easier to handle. With a spade and a clamshell posthole digger, widen the holes to 16 to 18 inches in diameter. Set all the poles in their holes. On a gently sloping site—one with a rise of 1 in 10 or less—you will need to dig the postholes deeper and larger than those on a level site; consult local building codes or a building professional.||2. SETTING THE POLES. Plumb the corner poles with a level and, while a helper holds them straight, brace them with 1-by-2s nailed to stakes. Measure down from the top of one corner pole the width of a beam plus 3 inches, and make a mark there. Drive a nail at the mark. Stretch a string fitted with a line level from the nail to the other corner post, and mark the second corner post at the height of the level string. Drive a nail there and tie the string to it. Align the middle pole with the string, then plumb and brace it (above). Mark the point where the string touches the pole. Prepare enough concrete or cement-soil mixture to fill in around the poles and shovel it into each hole, overfilling it slightly. Tamp the mixture with a 2-by-4, sloping the top downward from the pole to the ground. Repeat the process on the other rows of poles, then remove the strings and nails and let the concrete cure for a day.|
Working with Concrete in the Wilderness
To save you the trouble of having to mix separate ingredients for concrete—Portland cement, sand, and gravel—get the cement and sand premixed. Protect the dry ingredients by placing the bags on wooden pallets and covering them with plastic sheets or tarpaulins. To clean up your tools after a day’s work without the luxury of running water, keep a 6-gallon bucket of water on hand, and when you’re finished with a tool, place it in the bucket. Replace the water as necessary, but use it to clean out the wheelbarrow before discarding it.
|3. STARTING THE DAPS. To enable the beams to sit squarely against the poles, you’ll need to cut a notch, called a dap, on both sides of each pole near the top. With a bucksaw or pruning saw, first make a series of horizontal cuts 1 1/2 inches wide and 1/8 to 1/4 inch deep on one side of the pole (above). Work from the top of the pole to the mark you made in Step 2.||4. FINISHING THE DAPS. Place the saw on top of the pole and align the blade with the ends of the horizontal cuts on one side, then saw down through the cuts from the top of the pole to the beam mark (above). Cut daps on the same side of the remaining poles.|
|5. LOCATING THE OUTSIDE BEAMS. With a helper, set a beam against the outside of one row of poles so the bottom edge sits squarely in the daps in the posts. Holding the beam level, temporarily fasten it to each pole with a 3-inch common nail (left). Nail a beam to the outsides of each remaining row of poles in the same way. Cut the tops of the poles flush with the tops of the beams.|
|6. POSITIONING THE INSIDE BEAMS. With a helper, run a string across a row of poles on the side opposite the beam (above). Line up the string with the edge of the pole that is smallest in diameter (inset). Mark the tops of the larger poles at the string line, and cut a dap (above, Steps 3 and 4) at each mark. Temporarily nail beams to the inside of the poles as described above.|
|7. BOLTING THE BEAMS TO THE POLES. Measure and mark a point one-third of the way from the top and bottom of each beam. Install a 1/2-inch auger bit in an electric drill and bore a hole through the beams and pole at each mark (above). Insert a 1/2-inch carriage bolt about 1 inch longer than the combined thickness of the beams and pole into each hole. Tighten washers and nuts onto the bolts. As wood tends to shrink over time, check the nuts for tightness several weeks after the foundation is completed, and tighten them as necessary.|
|8. ATTACHING THE FLOOR JOISTS. Set the joists across the beams at 16-inch intervals, letting them extend beyond the outside beams by a foot and overlap at the center beams by a foot. With 3-inch common nails, fasten together the joists that overlap at the center beams. Fasten the joists to the beams with multipurpose framing anchors and the nails recommended by the manufacturer of the anchors (above). Nail header joists across the outside ends of the floor joists, and fasten them to the end of each joist with 2 1/2-inch common nails. Lay a subfloor across the joists.|
For further instructions, see How to Build a Tiny House.
Reprinted with permission from Jay Schafer's DIY Book of Backyard Sheds & Tiny Houses: Build Your Own Guest Cottage, Writing Studio, Home Office, Craft Workshop, or Personal Retreat by Jay Schafer and published by Skills Institute Press LLC and Four Lights Tiny House Company, 2013. Buy this book from our store: Jay Shafer’s DIY Book of Backyard Sheds & Tiny Houses.