About three or four months ago, here on our blog, we wrote an article about building footings for decks in our Washington DC area. Some of the requirements change in different areas and there’s a difference in the frost line depth from area to area. Generally when you go to the north, the frost lines are a little bit deeper and a bit more shallow further to the south. The trend basically just follows the climate zones of the United States.
In the picture below, you can see the exposed portion of a footing above ground. This particular footing happens to be at the area under the outer edge away from the exterior building facade. Some decks are built with the ledger board that connects the deck back to the structural vertical building facade. Not all portions of all building facades are structural and or have a structural bearing capacity to support a deck. In this particular case though based on the type of original construction, the masonry facade of this building has a structural load capability, but it’s not as strong as other types of similar construction. For example, because this is a skin or face brick, the historic brickwork of Washington DC, generally build with a double wythe brick above grade will often have a higher structural bearing capacity or ability to support a deck or other additional or adjacent elements.
At another footing at another post area of an adjacent bearing point, you can see that the outer post is better centered on the footing below. Also, there is an additional footing where the post is set closer back to the building. This is the type of location where when a deck is mounted or supported with an exterior ledger connected through a structural transference methodology, it’s not always required to have a post or footing close to the building as done here in this example.
In the Washington DC area, the frost line is approximately 30 inches below grade. When soil freezes, it expands and can exert significant upward pressure on structures, a natural condition or effect known as frost heave. Frosty occurs because Frozen materials generally expand. Frozen soils, for expand sample can expand up to 7 to 9% their volumetric size. If a building isn’t protected from this amount of movement, it can be a significant structural shift. Freezing water has enormous strength and or power to move heavy objects, even buildings. By placing footings below the frost line, they remain bearing on unfrozen soil throughout the year, preventing the cyclical freezing and thawing process from affecting the deck’s stability.
Nonetheless though, if you are going to install a footing at any point which is then supporting a structural part of the load path of the deck above, that particular putting must be on good bearing, down to the frost line and on compacted subsoils. Here though, you can see there is significant movement in the soil because there are large cracks that run throughout the surface of the soil where the soil has expanded, likely related to cycles of hydration and evaporation.
Footings installed above the frost line are susceptible to movement as the ground freezes and thaws seasonally. This movement can cause the deck to shift, leading to uneven surfaces, misaligned joints, and potential structural damage over time. By contrast, footings placed below the frost line provide a stable base that resists these forces, ensuring the deck remains level and secure regardless of temperature fluctuations. This load path stability is particularly important for maintaining continuous bearing, preventing structural shifting and or destabilization, and preserving the deck’s overall structural integrity.
The phenomenon of stable underground temperatures is rooted in the earth’s geothermal properties and its interaction with solar radiation. At a depth, typically ranging from 4 to 6 feet below the surface, the ground temperature remains nearly constant year-round. In the Washington DC area (amd at many places on Earth), this consistent temperature is around 54 to 56°F .
This stability occurs due to the earth’s thermal mass and insulating properties. The upper layers of soil are influenced by daily and seasonal temperature fluctuations, but these effects diminish with depth. Soil and rock have low thermal conductivity, meaning they transfer heat slowly. As a result, the impact of surface temperature changes becomes negligible at greater depths.
The frost line, typically around 30 inches deep in the DC area, marks the maximum depth to which groundwater in soil is expected to freeze. Below this line, several factors contribute to maintaining unfrozen conditions:
- Thermal inertia: The large mass of earth below the frost line acts as a heat sink, storing warmth from summer months and releasing it slowly during winter.
- Geothermal heat: A small but constant flow of heat from the earth’s core provides a baseline warming effect.
- Insulation: The layers of soil above act as insulation, slowing heat loss to the surface.
- Groundwater movement: Subsurface water flow, even minimal, can transfer heat and help maintain temperatures above freezing.
The depth at which temperature stabilizes (the zone of thermal neutrality) is deeper than the frost line. This zone experiences minimal annual temperature variation, typically less than 1°F. In building science, understanding this zone is important for geothermal heat pump systems and earth-sheltered construction.
For deck footings, while they don’t need to extend to the zone of complete thermal neutrality, placing them below the frost line is sufficient to avoid the damaging effects of freeze-thaw cycles. At this depth, the soil temperature remains above freezing due to the combined effects of earth’s thermal mass, geothermal heat, and insulation from overlying layers.
This stable environment below the frost line is why many underground structures, like basements and foundations, maintain relatively constant temperatures year-round. It’s also why deep caves, and spring houses in historic times but to a lesser extent, exhibit steady temperatures regardless of surface conditions. In the context of building science, this knowledge informs not only foundation design but also influences strategies for energy-efficient construction and geothermal energy systems.
Use a contractor who understands and cares about doing things right. Always, feel free to reach out to us here at Dupont Decks and Patios. We are happy to help with almost all steps of the deck building and design process. Let us know about your ideas and talk to us if you have questions about possibilities . You can call us at (202) 774-9128. You can find us online at https://dupontdeckspatiosdc.com and you can email us there as well at https://dupontdeckspatiosdc.com/contact-us
