Building a shed, cabin, home, or larger building takes a lot of planning and design work. Many people choose the design and then just pick a walled or slab foundation. It is also important to look at the ground and lay of the land upon which the structure will stand. A pier and beam foundation may be a better choice. If you’re not sure what it is or how to build it, we’re here to help.
Pier and beam foundations are designed from the floor plan of the structure. Holes are dug or drilled for concrete piers at strategic points around the perimeter and throughout the center of the building’s footprint. The piers support a beam grid which in turn supports the joists and subfloor, and the rest of the structure.
In this guide, we’ll explain what a pier and beam foundation is, how to design one, pier and beam spacing, and the advantages and disadvantages of this type of foundation. We’ll also discuss how to build the foundation, the costs, and possible problems, and repair costs. Plus, we’ll compare pier and beam with slab foundations to identify which is better for your purpose. Our goal is to provide you with the information to make an informed decision for your foundation choice.
- What Is Pier and Beam Foundation
- Pier and Beam Foundation Design and Diagram
- Pier Foundation Advantages and Disadvantages
- Pier and Beam Foundation Spacing
- How to Build a Pier and Beam Foundation
- Pier and Beam Foundation Cost
- Pier and Beam Foundation Problems
- How Much Does it Cost to Repair a Pier and Beam Foundation?
- Pier and Beam vs Concrete Slab Foundation: Which Is Better?
What Is Pier and Beam Foundation
Pier and beam foundations are considered deep foundations and have been around a long time. Our ancestors used them to support homes, barns, sheds, stores, and other light structures. They were easier, less expensive, and quicker to build than a full dug-out foundation, and used fewer materials. They were also more stable and longer-lasting than building directly on the ground. Pier and beam foundations typically have an 18” or higher crawl space under them for ease of access to plumbing and electrical connections.
In the past, holes were dug through the loose or soft, top strata of soil to a more solid layer, and filled with flat stones to the desired height above the ground to form the ‘footing’. Wooden beams were then placed and leveled on top that would support the structure, and transfer the load through the piers into the ground.
Today, the holes are often drilled and the piers made of concrete blocks, stackable prefab square or round concrete pier slabs, concrete-filled sono tubes, or other similar masonry or concrete material. The beams may be wood, concrete, or metal.
Pier and beam foundations typically were, and still are, used to raise the building above uneven ground to form a level base. They’re also used where the soil is structurally unsound, or the ground is seasonally exposed to moisture or potential flooding.
In some situations, a post is placed on top of the pier to extend it higher, and the beam placed atop and connected to it. This can help lift the structure to provide better views over obstructions and protect it from storm surges. Piers are not piles, which are driven deep into the ground and rely on soil friction to prevent sinking.
Pier and Beam Foundation Design and Diagram
Designing a pier and beam foundation usually begins with a floor plan of the structure it will support, how the structure will be used, and a look at the topography, climatic issues, and soil bearing strength. The floor plan and joist layout identify important dimensions, structural points, and potential plumbing concerns.
Depending on the structure’s dimensions and weight, the piers typically are spaced 5 to 10 feet on center apart around the perimeter. The open space inside the structure’s outer walls also needs to be supported if it has to support a floor or structural components, such as posts. Determining the pier shape – round, square, or rectangular – and dimensions based on the structures weight and dimensions is also key in the design stage.
Some people prefer square or rectangular piers at structural corners, and round ones elsewhere. Beams are typically sized to support the joist framework that will be used for the structure.
Identifying the pier sizing and spacing requires some math, which is why many people leave it for a professional to do. Typically, a structure has a live load of 40PSF and a dead load of 10 or 20PSF, for a combined load of 50PSF or 60PSF. In some regions though, ground snow loads of 70 to 90PSF replace the live load value for structural safety, which could alter the combined load to more than 100PSF. Once you identify the combined load for your area, multiply the square footage of your design by that number.
For example: a 20’x30’ cabin covers 600sqft, multiplied by a combined load of 50PSF, and the structure’s potential weight is 30,000 pounds, even though the actual empty weight is closer to 6,000 pounds.
To determine the combined load an individual pier can support, calculate the pier’s horizontal surface area in square feet, and multiply it by the soil capacity in pounds. The larger the horizontal surface and greater the soil capacity, the fewer piers required.
For example: a 10”x10” pier has a surface area of 100”, divide it by 144 (the number of inches in a square foot) and you get 0.694, which means the multiplying factor is 0.694. If the soil capacity is 3000 pounds, the pier will support 3000 x 0.694, so a load of 2,082 pounds.
With the weight of the building identified and the amount one pier can support, divide the structure’s weight by the amount a pier can support to determine the number of piers.
For example: The 20’x30’ cabin with a potential load of 30,000lbs divided by 2,082lbs results in a minimum of 15 piers being required. The spacing is commonly based on the tributary area and load being carried and is usually half the distance in all directions between one pier and the next.
For example: the 20’x30’ cabin, divided by 15 piers, means each pier has a tributary maximum of 40sqft, or nominally 8’x5’
A barn without a floor is usually perimeter supported with some intermediate piers to carry post supports for lofts or partition walls. A 10×10 shed may have three piers per side, and one in the center to support the beams which carry the floor and walls. A 20’x30’ cabin with a deck may have the perimeter walls supported every 5 to 8 feet, and a central beam also supported in that fashion, instead of creating a grid of piers and beams every 5-feet on the interior. The deck typically requires lighter support and allows greater spacing, so may not have as many piers.
Once the foundation dimensions plus the number of piers required and their placement is determined, it’s time to look at the physical location where it will be built. A simple soil test may identify the bearing strength of the ground. However, expansive soils and any active moisture zones may necessitate a deeper pier to properly anchor and support the structure, usually 5 to 6 feet deep.
Additionally, climatic issues such as frost, seasonal flooding, and wind loads may need to be considered too, as might seismic concerns. The deeper the piers the more protection the footing has from shifting too.
The design and construction of the piers are critical to the support of the structure too. The piers may rest on a poured 16”x16”x8” (or larger) concrete pad that is reinforced with rebar or on crushed stone.
Typically, the bottom of the gravel or pad sits at or below the frost line. The pier is then set or poured onto the footing pad and often reinforced with rebar. The diameter or cross-sectional dimensions of the pier range from 6” to 16” and depends on the weight of the structure being carried. Heavier loads commonly need a larger diameter pier of 12” or more.
Pro Note: Designing a pier and beam foundation for a building that will be occupied requires adherence to codes and regulations. If in doubt, have your design checked by the local building department or a Structural Engineer.
Pier Foundation Advantages and Disadvantages
Pier and beam foundations are used in all facets of the building industry, from sheds and decks, to cabins, homes, barns, garages, and even high-rise office buildings. The size and depth of the pier and its base, along with the beam’s dimensions, determine the size of the structure it can support.
For most of us, a drilled or dug hole with a sono tube filled with concrete or stacked cement pier blocks to carry a beam directly, or a post on which a beam can rest, is all we need.
There are numerous advantages to pier and beam construction:
- less expensive than most other foundations
- relatively quick and easy to build
- a remote access favorite as heavy equipment isn’t necessary
- minimal disruption to the ground and vegetation
- elevates the structure above uneven ground
- protects from termites, moisture, and flooding
- provides a crawl space or open access for plumbing and electrical installation and repair
- less ground vibration transfer
- easy to connect additions and decks
- can be constructed on most types of soil and rock formations, even under water
As with any type of construction, or anything for that matter, to counter the advantages, there are also disadvantages:
- the crawl space or area under the structure needs to be ventilated to prevent moisture damage
- unwelcome critters may take up residence in the open crawl space
- if too close to the ground, repairs are difficult
- poorly constructed and leveled piers make floor construction difficult
- floors may be springy if piers are poorly spaced
Pier and Beam Foundation Spacing
Pier and beam spacing depends on the structure’s dimensions, the live and dead load weights, pier and beam dimensions, soil bearing capacity, plumbing access, window and door openings, as well as the type of construction.
Typically, the larger the structure the more piers it will need. The greater the cross-sectional measurement of the pier and the beam dimensions, the more weight that can be supported. Pier spacing commonly ranges from 5 to 10 feet depending on all factors.
How to Build a Pier and Beam Foundation
Pier and beam foundations don’t require large equipment and can be built by one or two people. Whether using precast pier blocks, poured concrete in sono tubes, or other masonry options, the aim is to create a level base upon which to build your structure. The piers are spaced to carry the load of the structure.
The first step is to identify where the piers need to go using the floor plan. Common practice is to stake out and square the corners and then hammer in a stake at the center of where every pier will go. Spray painting circles or squares also is a good way to mark the pier locations.
The second step is to use a post hole digger, auger, shovel, or some mechanical means to dig the holes. The horizontal dimensions depend on the pier size required for the structure. The depth of the pier is based on the soil strength and frost depth. Additionally, if pouring or placing a footing, pad, or gravel, the depth and hole width may need to be adjusted.
With the hole or holes dug, place 5” to 6” of gravel in the base of the hole, or gravel plus a pier pad, and then build or pour your pier. Ensure the pier blocks, cement blocks, sono tube, or whatever are level. Use a plumb bob, string line, and-or level to make sure the center of the pier is in place.
Run a level line so the tops of the piers are as level as possible. If pouring pads or piers, use #3 or #4 rebar to reinforce the concrete. You may wish to use adjustable or fixed beam anchors at the top of poured piers too.
Once the piers are in place, backfill the holes around the piers, and tamp the fill well. The fill should be mounded around the piers too. The beams are placed on top of the piers and fastened into place.
Metal, wood, or synthetic shims, or adjustable beam mounts are often used to level the connection between pier and beam. The beams form an interconnected and level grid upon which the structure’s joists and subfloor are placed or built.
Pier and Beam Foundation Cost
Typically, a pier and beam foundation will cost between $8 and $15 a square foot, so a 2000 square foot floor will cost between $16,000 and $30,000. The size of the structure, the depth, height above ground level, and the number of piers, plus their dimensions and those of the beam, all affect the cost of the pier and beam foundation. Hiring others to do the work may be easier but it will increase the cost.
One 8” diameter 48” tall concrete pier uses 5.65 ft³ of concrete, and a 12” diameter pier the same height uses 12.6 ft³. So, you could pour almost five 8” piers with a cubic yard of concrete, but only two 12”. Adding rebar and beam brackets boost the cost too. The depth and thickness of the beam also are factors in the cost.
Fifteen 10” diameter 48 high piers needed to support a 20’x30’ cabin will require 5 cubic yards of concrete. At $200 a cubic yard in my area, the concrete alone would cost about $1,000.
Add in the sono tubes, rebar, adjustable beam brackets, and gravel, and the cost jumps to around $1,500. Lumber for triple 2×10 beams adds about $1,200 to the cost, bringing the full cost in under $3,000, or about $5.00 a square foot.
Pier and Beam Foundation Problems
As with almost any foundation, pier and beam foundations may experience problems over time, especially if they weren’t built properly. However, those built on solid ground with proper drainage and maintenance can last centuries. The main structural concerns are settling or cracking of one or more piers.
Settling can cause floors to sag or bounce, doors and windows to stick, and toys to roll. Settling, though, is an easy matter of raising the affected area and shimming it back into level or stabilizing the ground under the pier. Cracked or crumbling piers require replacement, which again, is much easier to do than with other types of foundations. Piers not repaired and left to settle can shift or tilt, which can cause beams to move, leading to more structural issues.
Most other problems with pier and beam foundations are caused by improperly enclosing the crawlspace or poor drainage around the foundation and structure – problems similar to other basement or crawlspace foundations. Poor ventilation can lead to wood rot, mold, and mildew, as can rain or snow run-off water pooling under the structure. If left open, insects and animals may make the crawl space home, which can lead to a plethora of other issues.
How Much Does it Cost to Repair a Pier and Beam Foundation?
The cost to repair a pier and beam foundation depends on what the problem is, and who does it. Stabilizing the ground before rebuilding or replacing a pier may cost the DIY little. Typically, the price of several 4x4s cut into 2’ lengths to make a crib, plus the cost of a hydraulic jack to sit in the crib to lift the structure for the repair.
Hiring the pros, however, can cost between $250 and $2,000 a pier depending on the job. A perimeter pier is usually much easier to repair than one supporting the middle of the floor. Lifting the whole structure to relevel, reset, or replace a beam involves more expertise, and frequently costs more. In most situations, expect pier and beam foundation repairs to range from $2,000 to $6,000.
Pier and Beam vs Concrete Slab Foundation: Which Is Better?
The type of foundation depends on many factors. A slab foundation is ideal for flat or nearly flat ground, while a pier and beam foundation work well on most types of terrain. Pier and beam also raise the structure above the ground and surrounding topography, plus protect it from flooding. Ground vibration transfer is also much greater with a slab than a pier and beam foundation.
A pier and beam structure rests on piers that provide a deep foundation, while a slab rests on the ground or on a gravel pad. The piers raise the floor above ground level and the slab is either level with the ground or the surface is 3” to 6” above it. Many structures on piers have wooden floors which are easier on the feet, shins, joints, and back than concrete floors. The raised floor also provides easy access to plumbing and electrical connections and makes repairs much easier too.
A 20’x30’ cabin on a 6” thick slab uses more than 11 cubic yards, at $200 a unit, that’s about $2,200 for the concrete alone. Adding in the machinery to level the pad area, the gravel base, vapor barrier, rebar, and laying in the pipes and electrical connections prior to pouring, all increase the cost. Plus, a smooth finished slab typically requires a hired crew, which easily pushes the cost over a pier foundation of the same size.
Pier and beam foundations are inexpensive and easy to build foundations which provide a solid base for many types of structures. The piers provide deep support and stability, while the interconnected beams transfer the building’s weight through the piers into the ground.
Hopefully, you have a better understanding of what a pier and beam foundation is, how to build it, and its advantages and disadvantages, and are ready to make a foundation choice.