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A grade beam (a.k.a. perimeter beam foundation) is a concrete foundation with rectangular cross section running around the perimeter of the house under the exterior walls. We call it a grade beam because it’s a beam, a horizontal structural member, which sits at grade, the elevation of the ground. Although not technically built with natural materials, it pairs well with many natural building methods.
An owner-builder can build a grade beam foundation (with some help) or hire any competent contractor to build it for them. This type of foundation uses less concrete than most conventional foundations. In frost free locations or places with very shallow frost depths, a grade beam can be placed on native soil. If you’re building in a climate with moderate to deep frost depths, you can still use a grade beam foundation, but consider a rubble trench below it and/or adding insulation to make it a frost protected shallow foundation.
How does a grade beam work?
Just like any other foundation, the purpose of a grade beam foundation is to transfer all of the loads from the structure to the ground. The largest of these loads is the weight of the structure and everything attached to it. Calculating the minimum required width of the grade beam for supporting this load will depend on the bearing capacity of the soil. However, it is standard practice to build foundations the same width as the wall they support. The width of the wall is usually the limiting factor for how narrow the foundation can be in wide-walled natural homes such as strawbale or cordwood. To determine the minimum width for supporting the load consult the tables in chapter 4 of the International Residential Code, or hire an engineer.
What are the advantages?
People often choose a grade beam foundation for the advantage of using less concrete than conventional foundations. This will result in less money spent on concrete and lower embodied energy in the building.
Compared to a footer and stem wall:
The most common conventional foundation in northern climates is a footer and stem wall with either a basement or crawl space. An additional advantage a grade beam has over this foundation type is the entire foundation is poured at once rather than in multiple pours. (Typically the concrete footer is poured first and after it sets up the stem wall is poured on top of it and in the case of a basement a slab is poured later.) Pouring only once saves both time and labor.
Compared to a slab on grade:
In frost-free climates, a slab on grade is the most common conventional foundation. A grade beam requires less skill to pour than a slab. This is because the top surface which must be finished is way smaller and the finish doesn’t need to be perfect, or even pretty, as it will be completely covered by the wall.
Compared to a stone or block foundation:
Stone and block foundation walls are labor intensive to build and require skill, stone foundations more so than block. A grade beam requires much less labor and skill which saves time and makes it more achievable for owner-builders.
Ease of construction and savings in time, labor and cost make grade beam foundations an attractive option. Ease of construction means that owner-builders can build their foundation themselves.
What are the disadvantages?
Grade beam foundations don’t work with basements. This is an obvious disadvantage for people who want their house to have a basement. Another is grade beam foundations require a level building site. If you’re building on a hill you’ll need to do significant earthwork before starting a grade beam foundation, or choose another foundation type which accommodates slopes. People concerned about the carbon footprint or embodied energy of their home must weigh the pro of using less concrete with the con that a grade beam foundation still consists of concrete and steel. Finally, building inspectors who aren’t familiar with this type of foundation may question it. But if you explain that it’s basically a footing without a stem wall they may accept it. If they initially don’t accept it, hiring an engineer or architect to provide plans should work — a building department will nearly always accept stamped plans.
How is a grade beam built?
The basic steps are:
- leveling the site
- setting up batter boards to achieve a square, level foundation
- creating forms to hold the concrete
- putting rebar inside the forms
- pouring the concrete
- stripping forms
This sounds easier than it is, but even for inexperienced builders it isn’t unachievable. You can use forms that are purpose manufactured, built with wood or construct forms using foam board. The latter makes a lot of sense if you’re planning to insulate the foundation with foam anyway. The foam option is great because it saves the step of stripping the forms. If you’re building in an area with moderate to deep frost depths you will have additional steps of building a rubble trench footer and/or insulating for frost protection.
A grade beam foundation may be a the best choice for your building. If you have built or are planning to build a grade beam foundation, I would love to hear about it.