An Easy to Build Post and Beam Foundation
A post and beam
(sometimes called post and pier or pier and beam) foundation is much
easier to build and less costly than the more common
perimeter concrete foundation. It is best for building sites with a low
likelihood of earthquake or hurricane force winds because the house is
not as deeply or as heavily embedded into the ground. For this reasons
some building departments may not allow these foundations or require an
engineering review. But, for cabins and small houses in many places a
post and pier foundation will serve as well and
last as long as standard concrete foundations. With modern materials
such as
pressure treated wood and preformed metal brackets and straps this old
style foundation is both easier to build and stronger than ever before.
In this type of foundation you have spot footings of gravel or concrete
under wood posts or concrete piers which support beams. These, in turn,
support the floor platform above. The major decision to make is the
type and depth of your footings and the type of posts or piers you will
use.
    Leveling
with Water
 PT Wood Post |
Mild Climate Footings In mild winter climates (12" or less frost depth) and well drained non-clay soil, there is little chance of freezing soil lifting the footings and piers. In this case you can dig down into the soil and either pour a concrete footing or fill the footing hole with clean crushed rock. The size of this footing hole should be a 16" diameter circle or square for gravel or rocky soils and a 24" circle or square for soils with mostly loam, loose sand or gravel. The footing depth should be 1/2 the width or more. This foundation is used in the Little House Plans kit. Make sure the bottom of the footing rests on undisturbed soil free of organic material. Don't build on fill or soil that has been dumped on the site (unless it is uncompressible material such as crushed rock). Your footings will spread out the total weight of the building over the bearing soil. Good solid material under these footings is essential. When in doubt, make the footings larger thus reducing the load per square foot (think snowshoes).Note: This simple Little House foundation has been built even in quite cold climates with well drained soil that does not expand when freezing (such as clay) see this owner built home story. Cold Climate Footings In colder climate areas with clay type soils you need the same footing but it will be at the bottom of a deeper hole so that the bottom of the footing sits on soil that is below frost depth. Coming up from the footing will be either a pressure treated wood post or a concrete pier made out of mortared blocks or a poured cardboard tube. Pour a concrete footing first with a cross of 1/2" (#4) rebar to reinforce and tie into the pier. Set these bars on rocks or broken brick so that they sit 3" up from the bottom of the pour. Wire a section of rebar vertical and this will tie into another bar running up the core of the concrete pier. Overlap sections of bar 12" to 15". The center of your tube or block pier will be poured with concrete and tie the footing and beam straps or brackets together with a reinforced concrete pier. The tube pier is shown at left. A secondary advantage of this concrete pier is that it has much better anchorage against uplift and wind forces. These foundation details are part of the following small house building plans:
Block Piers You can also build your piers from 8 x 8 or 12 x 12 concrete blocks. For short piers use the smaller blocks, for taller ones go larger. Tube piers come in various diameters as well and larger ones should be used on the downhill side of a sloping site where they will stand higher. A safe rule of thumb is that a pier or post should not exceed 12 (for concrete) or 20 (wood) times its width in unsupported height. Confirm this with your building inspector or a local engineer. When using concrete blocks for your piers, the footings must all be level in order for the piers to be level under the beams. If you have to change footing levels they must be in 8" increments. With poured tubes you can cut them off level prior to filling so the footings can be at different levels. Check for level using an inexpensive water level made from clear tubing. Run this from footing to footing or from tube to tube using the shortest pier as the master. Setting
the Tops of the Piers Set the height of the piers so that there is a minimum of 12" under the beams and 16" under the floor joists of the floor platform. This will provide enough space for plumbing, wiring and insulation to be worked on from below. Wood Post Foundation Piers When using wood posts get foundation grade 6x6 treated poles or posts. When using a gravel footing, nail a square of pressure treated 2x10 to the bottom to act as a foot. Use only hot dipped galvanized, "Z-max" or stainless steel nails and bolts with PT material. Don't rely only on the bolts to hold the beams. Notch the posts and then bolt or set the beams directly on top of the wood posts and connect them with appropriate metal brackets or wood plates as shown. Getting the beams level and in the right place is most easily done by clamping or lightly nailing the metal connector or posts to the beam, leveling the beam in its proper location with temporary bracing and then packing the hole with a dry soil cement mix or crushed rock. Packing the hole with solid concrete may hold too much moisture and shorten the life of wood posts. Shimming and any final adjustments can be made after the concrete has setup (24 hrs min., 4-5 days better). There are many different types of metal brackets and strap anchors. Ask locally at the lumber yard as these will vary by brand and the size of beam and post. A "Y" anchor that is nailed to both sides of the beam is sometimes more available than the Simpson hardware shown in the diagrams. Treated wood posts can be packed with crushed rock or soil cement made from 5 to 10 parts clean gravel and sand type soils (no organic material) to 1 part cement. Mix well and add only enough water to make workable. For longest service, the post holes should drain and not hold water against the posts. You can also extend service by painting the posts with asphalt roofing tar for 8" either side of the final soil line. This is where posts are most likely to experience organic attacks. These foundations can be used to build most of the cabin and small home plans from CountryPlans.com. Many of these plans also include other foundation options where post and pier is not appropriate. These detailed plans have all the needed structural information such a beam sizing, spacing of the piers and the design and insulation of the platform floor. |
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