Footings above frost depth? Really? 1

[ACtrafficengr]

I have a mason telling me that 2 1/2 feet (0.75m)4 is deep enough for for footings supporting an enclosed porch. Frost depth is ~4 ft(1.2m) in our area. The soil appears to be silty sand, well above the water table. The porch open underneath and supported by masonry columns, not on an enclosed heated basement, if that makes a difference.

Is he trying to blow smoke up my posterior, or am I missing something?

Thanks.

My glass has a v/c ratio of 0.5

Maybe the tyranny of Murphy is the penalty for hubris. -

http://xkcd.com/319/

 

[WARose]

I would consider the source.....a mason tell you this and you trust his judgement? Will you also be asking him to check your calculations?

Check the building code and/or ask a local code official.

 

[bridgebuster]

You can build a shallow foundation if you provide free draining material around and under AND provide insulation. You would have to design the insulation thickness.

Sometimes craftsmen are misinformed & sometimes they do know what they're talking about.

 

[oldestguy]

Hold on there. Just what do you mean by "frost depth". My bet is you never dug to 4 feet and find it is not frozen there and fro9zen above. It's a very general term. So, what is the percent of silt and clay in your "silty sand" (P-200 sieve size)?
that's what you need to decide anything. What have you observed for heaving of walkways, fence posts and the like? Quite likely your mason has experience in your area. We here in Wisconsin have codes with frost dept of 4 feet. It is a catch all that hopefully prevents problems, that's all. So go to LaCrosse Wisconsin and run a boring down in winter and measure temperature. I did that before Christmas at a site where there had been a gas explosion. At 8 feet below grade it was below freezing, yet there was no heaving of roads, walks, etc. So what is the frost depth there and what does it mean? Almost nothing. Following a building code may or may not prevent problems later. You can set a post to 4 feet and with the right moisture, soil type and weather, that post can be jacked up by frost heave. So can your footing at 4 feet. I'd dig down about a foot, place closed cell sheet of 3" Styrofoam, replace soil and set the footing below that a convenient depth. Extend that foam out 4 feet in all directions from each foundation.