How to Build an Addition on a Pier Foundation

Written by bob haring
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How to Build an Addition on a Pier Foundation
(Hemera Technologies/ Images)

If you are building in an area where the soil is not conducive to basements or grade-level concrete slabs, a pier and beam foundation can be added onto your existing structure. Masonry piers are placed around the perimeter of the house and in internal spaces to support joists. The raised floors allow for a crawl space, providing access under the house as well as ventilation. In areas prone to flooding, the raised floors also protect the house against water damage.

Skill level:
Moderately Challenging

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Things you need

  • Tape measure
  • Wood stakes
  • Builder's twine
  • Shovel
  • Posthole digger
  • Gravel
  • Hand tamper
  • Concrete
  • Level
  • Reinforcing bar (optional)
  • Pre-cast concrete piers (optional)
  • Metal termite shield
  • Thin foam insulation waterproofing
  • Level
  • Framing square
  • 6x6 inch sill beams
  • Anchor bolts or Simpson brackets
  • 2x6 inch floor joists
  • Metal joist fasteners
  • 3/4-inch pressure-treated plywood

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  1. 1

    Lay out the addition using a tape measure, stakes and builder's twine to mark the perimeters. Make sure corners are square. Measure from corner to corner; if those distances are the same, the addition is square.

  2. 2

    Place stakes at all corners, and inside the foundation at 8 to 12 foot intervals.

  3. 3

    Dig footing holes with a posthole digger or shovel at pier locations. Make holes 16 inches square and 8 inches deeper than the bottom of the pier. Plan for piers to be 18 to 24 inches above ground level.

  4. 4

    Fill the footing holes with gravel compacted with a hand tamper or concrete to make a solid base. Make sure the top of each footing is level and all are the same depth.

  5. 5

    Set piers on the footings. Use precast concrete piers, concrete piers poured in place in round forms with reinforcing bars or stacked concrete blocks filled with concrete and reinforcing bar. All need at the top metal holders called Simpson brackets or upright bolts to attach sill beams. Use long boards and a level to make sure the tops of the piers or the brackets are level and corners are square. Make double piers or install double brackets at corners.

  6. 6

    Install a sill beam atop all piers, either in holes for the upright bolts or in the Simpson brackets which hold beams upright with nails in the sides. Connect beams at corners with nails or long lag bolts. Make sure all corners are square and beams are level. If using bolts, put a metal termite shield and thin foam insulation water barrier under the beam. Fasten beams at corners with nails or long lag bolts. Make sure all corners are square and beams are level.

  7. 7

    Set 2x6 inch joists atop the sill beams. Use metal joist fasteners, which nail to the beam and the sides of the joist. Space joists 24 inches apart. At centre piers, overlap joist ends on top of the sill beam and nail them together.

  8. 8

    Nail 3/4-inch pressure-treated plywood subflooring over the joists, to the outside perimeter of the walls. Build and raise walls with 2x4 inch base plates, top plates and 8-foot studs to frame the outside of the addition.

Tips and warnings

  • Use pressure-treated lumber if possible.
  • Add a screen around the foundation once the addition is complete. Latticeboard is often used for this; it blocks underneath the house while allowing air circulation.
  • Build a crawl space entry in any foundation screening.

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