Seattle frostline

[kebwi]

I'm curious if any other Seattle builders used a footer or sonotube piles or anything else like that? I was planning on the original Pompeii directions: dug just deep enough to level the ground, couple inches of pebbles, rebar/mesh slab (still unsure about the mesh). I liked the simplicity. Grrr.

I believe the Seattle frostline is 18", which would be two or three blocks deep.

Is it necessary? :-/

Thanks.

[dmun]

Re: Seattle frostline

A well drained bed of crushed stone, then a slab, should be fine if you chimney won't stand much taller than you. Well drained is the key: water in contact with the foundation is what causes frost heave. If you're building in the low spot in your yard where the water runs when it rains, you will need footings, or if you are building a tall chimney.

Sonotubes under the slab is the worst of both worlds, unless you use the special flared footing they sell for them. It just gives something for the ice lenses to grab on to.

[ThisOldGarageNJ]

Re: Seattle frostline

Hi Keb,
I built mine on a 12" slab using rebar and wire, over about 4-6 inches of gravel, I also have an 18" frostline (i believe) of course its too early to tell as I havent been thru a winter yet, but I dont forsee any problems as I have a 10 year old 8x8 shed built on a 6x6 pressure treated rr tie foundation and that hasnt budged in all that time,,
Good luck and start digging,

[kebwi]

Re: Seattle frostline

Holy smokes, your foundation is a foot thick? May I ask why?

Thanks for the input guys.

[kebwi]

Re: Seattle frostline

@dmun, I was referring to Neil2's "pile" comments in this thread:

http://www.fornobravo.com/forum/f6/c...ions-6530.html

[dmun]

Re: Seattle frostline

@dmun, I was referring to Neil2's "pile" comments in this thread:http://www.fornobravo.com/forum/f6/c...ions-6530.html (Cold climate foundations)

The quote in question:

Easy to do. I just dug down 4 feet with an ordinary hand held post hole digger. Put in a couple of 3/8 rebar, filled with concrete to about 6 inches of the surface. A second pour, from there up to the bottom of the structural slab I used 7 inch diameter "sonotube" cardboard tube forms - available at any builder supply.

If I understand correctly, Neil2 did put proper footings at the bottoms of the sonotubes. Most people just dig and fill, and it's not a cure for frost heave, it can even make the situation worse. The tube makers do make a flared plastic bottom for the tubes that provide proper footings in one pour, or you can pour your own footings.

But really, in most cases, its' not necessary.

[ThisOldGarageNJ]

Re: Seattle frostline

Holy smokes, your foundation is a foot thick? May I ask why?

I framed it out with 2 x 12's and poured on top of the rock, I just wanted to make sure it wasnt gonna move, It might have been easier to just pour footings, but his is another thing more experience has taught me

Mark

[papavino]

Re: Seattle frostline

keb,

I just poured some pea gravel (about a yard) and then built a 6" slab, which is probably overkill.

[kebwi]

Re: Seattle frostline

Thanks guys.

[Wiley]

Re: Seattle frostline

Well as you don't need a building permit I ask a question, perhaps a few:
How long have you lived in Seattle? In all the time you have lived in Seattle have you ever found that your yard froze long and hard enough that you had problems with your sidewalk heaving? Your driveway? I live further north than you and perhaps more maritime than some parts of Seattle, (seeing as how I'm on an island surrounded by water to the west less than 2000ft and perhaps a bit more to the east). But inspite of once every 7 years or so it freezing such that shallow ponds freeze over enough to skate on, I really think the 18 inches is major overkill and perhaps simply a COA on the part of the building department. But then I think the IBC is inane, one building code for all climes, gov't at its worst.

Just my opinion, and no I made no such thickness for my WFO (which wasn't inspected) or either of my shops or my home (which were).

Bests,
Wiley

[Neil2]

Re: Seattle frostline

Mine is built on straight sontubes down 4 feet. I flared out the bottom somewhat. Been through several winters here on Vancouver Island with no problem.

My second choice would be to build it on a 6 inch heavily reinforced slab with no below frost line footings.

I think the frost heaving issue is overstated.

[kebwi]

Re: Seattle frostline

You know, Wiley is my last name. :-) Anyway, I've only lived in Seattle two years and owned a house here one year, with accompanying winter, so the answer is, I have no idea. Maybe it'll be fine with slab-on-pebbles then.

I'm still open to further input of course.

Thanks.

[Neil2]

Re: Seattle frostline

Frost heaving can be a problem with decks mostly. In that type of construction, each support is independent so you can get uneven movement. Frost heaving is mostly a problem in clay/till or silt/till soils. It is not a problem in granular soils or in heavily compacted soils like the natural "hard pan" common in the Seattle area.

With any structure you should plan for some movement, either by settlement or by frost heaving. A reinforced slab will move as one unit. (Unless it tilts so much that your pizzas slide out this is not a problem.)

[Johnbear]

Re: Seattle frostline

I dont forsee any problems as I have a 10 year old 8x8 shed built on a 6x6 pressure treated rr tie foundation and that hasnt budged in all that time,,









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