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Question DetailsAsked on 2/6/2015

What is a frost line and what is its relevance when building/digging?

I am basically asking if there is a standard depth you dig when building a foundation for a structure and what does the frost line have to do with it?

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There are quite a few quirks between different building codes and also regarding which frost penetration charts you go with which I will not go into, but generally foundations for occupied structures (as opposed to detached sheds or fully detached garages), including footings for decks and porches rigidly attached to a house, have to go below the maximum expected frost penetration depth. In areas without significant frost typically 1 or 2 feet minimum for crawlspaces depending on local codes to provide lateral foundation stability, though many warmer areas allow concrete slab-on-grade foundations - basically just a few inches excavation past topsoil and house sits on a flat concrete slab. So - typically 1-2 feet depth in the warm states for concrete or block wall foundations, and up to about 6 feet in some colder mountainous areas and northern tier states. In the coldest areas, and Alaska and Canada, the required depth is typically set shallower for ordinarily heated foundations (house with heated basement) - typically 3-5 feet depending on code, but unheated foundations for attached decks and porches have to go to full expected frost depth. Generally foundations set into sound bedrock do not have to go deeper than sound rock regardless of frost depth, and can also be shallower if tested by a geotechnical engineer and proven to be free-draining non-frost susceptible soil like a coarse clean sand or clean gravel - meaning the in-place ground is of a type not expected to frost heave during the winter. That is why you go below frost depth (other than to prevent pipes freezing, too) - the ground "swells" with moisture freezing in it so can lift and damage structures, and also foundations firmly in contact with the ground (especially post-type poured-in-place footings like piers for decks) can progressively "frost jack" out of the ground over time, as much as a foot or more per year. In severe jacking conditions, 6-10 foot burial depth power poles can jack clear out of the ground in a few years.

Your local building official can provide requirements for your area - typically posted on their website, and porch/deck footing depth typically is on a deck construction FAQ's sheet. If trying to avoid digging deep deck foundations for a deck/porch which meets the house, you typically have to have a special design by a structural engineer for the deck-house interface to avoid the full-depth foundation requirement, so it is commonly easiest to just support the deck on posts both on the outside and the inside edges not connecting it to the house at all, or only with cables or "slide-bolt" connections for sway bracing but free to move up and down.

Other factors affecting foundation depth is if you have expansive/shrinking soils where you typically go deeper to provide foundation bearing below the zone that shrinks and expands through the year, if in a floodplain area where a deep foundation or basement can cause your house to float out of the ground as water levels rise (so in many floodplain areas basements are very rare), if bearing capacity of the soil is poor (like in soft clays and silts) you have to go deeper with foundation walls or build on pilings to get adequate load bearing capacity, and the foundation has to be set below the organic/topsoil layer which can be just inches to a foot in some areas but in swampy or peaty areas may be a number of feet deep so you either have to put your foundation below it or excavate the substandard material and replace it with structural fill.

The architect or engineer who does your construction plans will know how deep you need to go - based on building code, foundation type, and typically a site inspection and maybe a couple of shallow borings or a test pit by a geotechnical engineer at site selection time.

Of course, water and sewer lines have to be below frost depth everywhere outside the heated house envelope, to avoid freezing. Leach fields are also best put below frost line,though some allowance is commonly given for the fact they self-heat to a certain extent due to biodegradation - but that can be an issue at times.

Answered 4 years ago by LCD

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