How to Build a Tiny House – Part 3: Masonry Foundation Piers for Stronger Supports
Use masonry foundation piers for a stronger, more durable foundation. Learn how with the following instructions.
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," walks you through the installation of masonry foundation piers.
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.
Masonry Piers for Stronger Supports
Piers make a more durable foundation than wooden poles, and are almost as easy to put up. You can make them in three different ways: by pouring concrete into cylindrical fiber forms; by stacking and mortaring masonry-block piers and filling the cores of the blocks with concrete; or by setting precast piers of solid concrete.
Dealing With a Slope
As with pole platforms, piers can be used on gently sloping sites, provided the piers and their footing holes are deeper and wider than on a level site; consult a building professional. But if the slope is greater than 1 in 10—1 foot of rise in every 10 horizontal feet—you will have to build a continuous-wall foundation.
All three types of piers rest on footings—solid concrete bases that are wide and thick enough to support the structures above them. In general, a footing should be as thick as the width of a pier, and twice as wide—a pier 8 inches wide, for example, would require a footing 8 inches thick and 16 inches wide. Dig the holes so the bottom of the footing is 1 foot below ground, or 6 inches below the frost line, whichever is deeper.
Fiber tubes 8 inches in diameter and 10 feet long make quick work of casting cylindrical concrete piers; store them upright and keep them perfectly dry until they are filled. Cylindrical piers can be built to any height, making it easy to set the tops of the piers to the same level.
These blocks come in a wide range of sizes; single-core, 8- by-12- by- 12-inch blocks make particularly sturdy piers (see below). Level the footings for block piers with a water level in the same way you level forms (Step 3), but mark the height on the reinforcing bars (rebars), then measure down from the marks to set the height of the concrete for the footings.
Although simple to install, these piers (see below) are heavy and awkward to move and set into position. To level their footings, drive stakes into the footing holes, mark stakes with a water level as you would for concrete forms, pour concrete up to the marks, and remove the stakes before the concrete begins to harden. Because they generally come no more than 18 inches high, they are used mainly on level sites that have a shallow frost line.
|1. Casting the Footings.
Locate and dig the footing-and-pier holes as for a pole platform, excavating each hole 16 inches wide to the required depth.
At the center of each hole, drive a length of 1/2 inch rebar into the ground until the top of the rod is about 6 inches lower than the planned height of the pier above ground (right).
Fill the first 8 inches of the holes with concrete, creating footings. Allow the concrete to cure for at least one day.
|2. Positioning the Forms.
Slide a form over each rebar so it rests on the footing. Fill the hole around the form with earth, tamping it down firmly. After every 6 inches of fill, check that the form is vertical with a carpenter’s level (right).
|3. Leveling the Forms.
Mark the planned height of the piers on a corner form.
Have a helper hold one end of a water level or a transparent hose filled almost completely with water at the height of the mark on the corner form.
Hold the other end against the next form in the row (above) and mark a line at the level of the water.
Repeat the process to mark each form.
Cut the forms at the marked heights with a hacksaw.
|4. Embedding the Anchors in Concrete.
With a helper and a string, go from form to form to align and mark the position of a beam anchor on the front and back of each form, ensuring that each beam—3 inches wide for doubled 2-by-10s or 2-by-12s, or 4 1/2 inches for a triple beam—will be centered on the pier.
Mix enough concrete to fill the forms, then shovel it into each form in the row, using a length of rebar to pack the concrete down with every foot you add.
As you fill each form, level the concrete off with the top of the form using a 2-by-4, then following the marks on the form, push an anchor that is as long as the width of the beams to be installed (Step 5) into the concrete down to the embedment line marked on it. Embed anchors in concrete in the remaining forms in the same way.
Double-check the alignment of the anchors with the string, adjusting them to line up as needed.
Allow the concrete to cure for five to seven days.
| 5. Attaching the Beams.
Make a beam for each row of piers: For a prefab cabin, nail pairs of 2-by-10s or 2-by-12s together with 3-inch galvanized common nails. Drive three nails in a row about 1 inch from each end and stagger nails along both edges at 10-inch intervals. For an A-frame, make triple beams.
For a vapor barrier, set asphalt shingles on the piers, or cut 6-mil polyethylene sheeting into pieces to wrap around the bottom of the beam at each pier, and staple the pieces to the beam.
Set the beam into its anchor, then check whether it is level. If not, shim the low end by placing galvanized washers between the beam and pier.
Fasten the beam to each anchor by nailing into each hole (above).
Fasten joists to the beams as for a pole foundation (Step 8), then lay a subfloor across the joists.
Block and Precast Piers
Cast a footing for each pier as for concrete piers (Step 1), but level the footings by marking the rebars with a water level as for concrete forms.
To keep the heights of the piers consistent, make a story pole: On a piece of lumber, mark lines at intervals equal to the height of a block plus a 3/8-inch mortar joint.
Trowel a bed of mortar on the footing and lay a concrete block on the mortar, centering the block around the rebar.
Add blocks and mortar to build up the pier, checking each course for level and plumb with a carpenter’s level (right), and for height with the story pole. Build each remaining pier in the same way, running a string between adjacent piers to keep them all consistent and in alignment.
Fill the cores of the piers with grout—concrete thinned with water so it can be poured—then embed concrete anchors in the piers and attach beams as for concrete piers (Steps 4 and 5).
Cast a footing for each pier as for concrete piers (Step 1), but proceed immediately, before the concrete cures.
With a helper, press a precast pier into the wet concrete (left).
With a carpenter’s level, ensure that the pier is level.
Position each remaining pier in the same way, running a string between adjacent ones to keep them aligned and level.
Install vapor barriers and attach beams to the piers’ built-in anchors as you would for concrete piers (Step 5), nailing exterior-grade plywood spacers at 24-inch intervals between the boards making up the beam, if necessary, to ensure the beam sits snugly in the anchors.
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.