What is a Pier Foundation?

Building a house, cabin, shed or other structure usually requires a foundation of some sort. If a basement isn’t necessary and the ground is wet, sloped, unlevel, or even flat, builders may choose a pier foundation. “What is a pier foundation?” you ask.

A pier foundation is a set of wood, masonry, metal, stone, or concrete piers or posts used to support a structure. The piers transfer the weight of the structure and all applied loads to solid ground. They may be set into the ground to rest on solid ground strata or sit on bedrock.

In this guide, we’ll explain what a pier foundation is, the different types, and their advantages and disadvantages. We’ll look at building code requirements, depth piers need to be, their spacing, costs, and how long they typically last. We’ll also identify problems with pier foundations and compare pier vs slab foundations. Our aim is to provide you with a solid footing when choosing a foundation for your building project.

What Is a Pier Foundation?

A pier or post foundation is used to support buildings on uneven or unstable ground. Piers can be wood, stone, metal, masonry, or concrete. They are typically cylindrical in shape but can be square or rectangular too. Their purpose is to transfer the structure’s load to solid ground or soil strata and provide a level base upon which to build.

A pier foundation often has a crawl space between the ground and floor joists. However, they can also be used to support a poured footing and below-grade concrete foundation.

A pier foundation commonly has a series of equally spaced piers around the structure’s perimeter. Plus, other piers strategically placed inside the footprint to support structural bearing elements. The piers may have a concrete cap to broaden support for heavier buildings or have a beam bracket to carry a grid of beams for lighter structures. Pier foundations are commonly considered deep-bearing foundations and are used for shallow depths up to 20 feet.

Piers may sit on bedrock, be set into the ground on stable soil, supported by a poured or precast footing, or rest on a pile cap supported by driven piles. The diameter and length or height of the pier depends on how deep the bearing strata are, and how high above the ground it needs to reach to be level with other piers. It also depends on the weight of the occupied structure it will support, and how far apart the piers need to be. Today, pier holes are often drilled to the required depth, whereas in the past the holes were hand-dug.

Pier Foundation Types

Pier foundations have been used for millennia and many practices are still similar. However, the mass of the structure being supported, location, ground composition, and the lay of the land often determine the type of pier foundation.

Piers may be timber, stone, masonry, concrete, steel, or combinations of those materials. They are either dug or drilled, or even placed if exposed bedrock is present. They usually stand or reach above ground level to form a level base upon which to build a structure. A series of piers may be used to support exposed columns, footings, walls, or beams.

Timber piers are commonly cedar or treated wood and may be fully exposed, sit on bedrock, or be placed in a dug or drilled hole. The hole may be filled with dirt and compacted around the pier or set in gravel or concrete. The pier or post may also rest on a large flat stone or a poured or precast footing. A key problem is decay due to moisture and insects such as termites.

Stone piers have been in use for almost as long as wooden piers and typically are comprised of flat rocks or quarried blocks. They may rest on exposed bedrock, a poured or cast footing, or into a dug hole so they sit on hard or solid strata.

Helical or helix piers have been in use for almost 190 years. They have a straight metal shaft with helical or screw-like blades and are twisted or turned into the ground like a giant screw. The blades distribute the structural load to the bearing ground. A U-shaped bracket welded or affixed to the top carries structural members supporting the building.

Masonry piers have been in use for millennia too. They are often used when the solid strata are 15 to 20 feet below grade but are also used on exposed bedrock or for shallower depths too. An advantage of masonry piers is the high load capacity, uniformity, and comparatively low weight of the parts. Masonry piers may be square, rectangular, circular, hexagonal, or other shapes.

Concrete piers, similar to masonry piers, have a similar load capacity but with a smaller diameter. Most are cylindrical, but may also be square, rectangular, or other shapes to suit structural requirements. Some different types of concrete piers for use with various ground strata may be drilled or dug.

• End-bearing straight smooth shaft piers, whether dug or drilled, rely on solid stable strata to carry the structure.
• Straight with grooved sides for side/friction bearing piers rely on the use of the surrounding overburden strata to provide frictional support with potentially no end support by a solid stratum.
• Straight with grooved sides plus end bearing support loads with both frictional side support and end bearing on solid strata.
• Belled or under-reamed piers have a conical-shaped base to better distribute end-bearing loads.
• Drilled concrete caisson piers are usually cylindrical and are engineered to carry greater axial loads. They may be belled or under-reamed at the base, have a metal casing filled with concrete, or concrete with a steel core within a metal shell or casing.

Pier Foundation Advantages

Pier foundations are found in all types of terrain, including coastal areas, prairies, hills, mountains, wetlands, cities, and homesteads. They are used for sheds, barns, single and multi-family dwellings, high-rise and low-rise commercial structures, and even factories and other industrial buildings too. Anywhere there is uneven or unstable ground, and even in places where other foundation types could be used, you’ll find pier foundations as they offer many advantages.

Disadvantages of Pier Foundation

Pier foundations have many uses and advantages, but, as with all things, there are also disadvantages or situations where an alternative foundation would be better suited.

Building Code Pier Foundation Requirements

The International Residential Code (IRC) for 2021 – Chapter 4 – identifies that a foundation must be designed and engineered to support the required structural loads. In addition, it must factor in surface and subsurface drainage, soil conditions, slopes, and pier depths. It also addresses requirements for brackets and fasteners and how to minimize damage due to moisture, pests, and decay in crawl spaces and basements.

Table R403.1(1,2&3) addresses the footing size required for different soil load-bearing abilities based on ground and snow load for different types of structures. Pier foundations for light construction commonly have a crawl space, so refer to those sections.

Section R403.1.3.6 of the IRC (2021) identifies that stud-framed dwellings of three-story or fewer for one or two families may be constructed on pier foundations, provided all factors and requirements are met, which include approved engineered drawings.

My local code for Pier Type Foundations identifies that the piers must be designed to support all loads applied to the structure. It addresses the maximum pier spacing for 1-story structures as 12ft and that they must be placed to support principal bearing members.

Pier height also must not be greater than 3X the smallest dimension at the pier base. If concrete blocks are used to form the piers, their cores must be vertical and their long dimensions parallel with the long dimension of the structure.

Pro Note: Always check with your local building code and building department for specific requirements and information for your location before building.

How Deep Do Pier Foundations Have to Be?

The depth for pier foundations commonly depends on how deep it is necessary to go to reach stable bearing strata or bedrock. The depth also depends on frost levels typically found in the building area.

The maximum depth of frost in the U.S. is 100” with the exception of Alaska, which has areas of permafrost of unknown depths. If bedrock is exposed or closer to the surface than the frost line, that is as deep as is necessary. Pier depths for most light frame construction will be between 3 and 6.5 feet depending on loads, frost, soil conditions, and surface and subsurface drainage.

Pier Foundation Cost?

A pier foundation typically is less expensive than a poured footing and wall foundation. The cost of a pier foundation depends on the depth they must be to reach stable ground strata and to be below the local frost line. Plus, how far above ground they need to be to form the desired elevation level.

It also depends on their diameter or dimensions, materials used, rebar, brackets, and the number of piers required to support the structure and all pertinent loads properly. Hiring the pros or doing the task yourself are also factors that affect the costs of a pier foundation.

A DIY pier foundation for a 600 square foot cabin with fifteen 10” diameter four-foot-high piers with triple 2×10 beams and adjustable brackets will cost around $3000. So, $5.00 per square foot depending on equipment rentals and delivery charges. Hiring the pros can double or triple the cost. The larger the build, the greater the individual costs too, regardless of who does the work. Expect to pay between $5 and $15 per square foot for a pier foundation.

How Long Do Pier Foundations Last?

There are many factors that affect the longevity of any foundation type, with maintenance being vital to any foundation’s lifespan. The soil composition, settling, pier materials used, moisture, freeze-thaw effects, and appropriate sizing can all impact longevity. However, with regular inspection and proper maintenance, the lifespan of a pier foundation can easily be hundreds of years or more.

There are Roman aqueducts still standing on their piers with little intervention over the millennia, so they can last a long time. One key benefit of pier foundations is that they are exposed above ground making maintenance potentially easier. Piers can also be individually repaired or even replaced without negatively affecting the structure it supports.

What Is Better Pier or Slab Foundation?

Both pier and slab foundations are good choices for light frame buildings depending on the topography and the purpose they are to serve. If you don’t require a basement or crawl space and the ground is flat and well-drained, a slab foundation works well. However, access to electrical and plumbing for repairs may be more difficult. Additionally, since the slab rests on the ground, it is more susceptible to ground vibration, frost movement, settling, and flooding.

A pier foundation can be used on uneven ground, slopes, flat, or wet ground in accessible or inaccessible locations. The piers usually rest on solid ground strata below the frost line, or on bedrock, so are less affected by ground vibrations or frost.

The structure is typically 18” or more above ground level, so there is a crawl space providing access for electrical and plumbing repairs. Piers also raise the building above the ground, protecting it from insects, termites, rodents, snakes, drainage concerns, runoff, and flooding.

A slab foundation uses more concrete than a pier foundation of the same dimensions, but the slab is usually faster to compete. Plus, once the concrete slab is hard, the structure can be erected, whereas the pier foundation still requires the beam grid and subfloor installation.

The cost of getting both the slab and the pier foundation to the point where it is ready for the structure to be framed is approximately the same. Both pier and slab foundations can be completed by a DIYer depending on their skill set, but pier foundations tend to be easier.

How Far Apart Are Foundation Piers?

The spacing of a pier foundation depends on many factors. The dimensions of the structure’s footprint, design elements, its load variables, depth to solid ground strata, frost depths, diameter and depth of the piers, type of construction, plumbing and electrical requirements, and even openings for windows and doors.

Additionally, the greater the cross-sectional dimension of the pier, the more weight it can support so spacing can fluctuate. Typically, pier spacing for most light framing structures is centered between 4 and 12 feet, depending on the variables.

Pier Foundation Problems

Pier foundations can last for centuries or longer with proper maintenance. However, they are still susceptible to certain problems.

• Moisture: Whether open or enclosed, pier foundations require airflow to prevent mold, mildew, and rot caused by condensation and ground moisture.
• Settling: Individual piers can settle causing windows and doors to stick or floors to sag or bounce.
• Insects, rodents, and other pests: The crawlspace can become home to unwanted guests that can damage structural components or pollute the air quality within the structure.
• Poor construction: Piers can tip, tilt, crack, crumble, or rot if improperly installed or poorly constructed.

Conclusion

A pier foundation is an inexpensive foundation used to support light-frame construction as well as heavy high-rise buildings. Piers of wood, metal, stone, masonry, concrete, or mixtures of materials transfer the structural loads to load-bearing strata or bedrock. They often elevate structures above local topography to provide a level base and protect it from run-off, insects, ground vibrations, and flooding.

Hopefully, you have a better understanding of what a pier foundation is, and how it may benefit your next building project.