Best Good Enough 12x16 Foundation?

[tensada05]

I am looking to build a very cheap off grid 12x16 hunting "Shabin" and after spending weeks browsing the forum I really can't make up my mind on the best type of foundation for me. The property is located in Northern CA and really doesn't ever freeze and no snow. Earthquakes are rare. My building site is fairly level, however all materials must be wheelbarrowed up a rather step incline. I am building it under 200 sqft so I don't need permits. My Uncle is a contractor so I can borrow almost any tool, however my knowledge is limited to 4 years high school woodshop and access to YouTube. I have a limited budget but have the whole summer to devote to this project.

I understand one of the most important parts (if not the most important part) of this project is the foundation.  However with my $$$ constraints I am looking for the simplest way that will be good enough in the short term.

I'm hesitant to make this post as this desire to purposefully not be perfect seems to go against the nature of some of the regular posters. And I very much can understand where they are coming from. But for me I want to get by with the least amount of time, money, and effort as this will ensure that something, rather than nothing happens. I will be the only one staying here so any stupid decision impacts only me. And I don't need this to last forever.

With all of this in mind however, I do value my life and don't want to be absolutely reckless. I guess I'm trying to find a middle ground but am more committed to making something happen now then being prudent.

So my first question is how foolish is it to just level out a bunch of concrete blocks and call it a day?

I did see several posters suggest a continuous concrete footing. And while I don't mind doing that to some extent, I wonder how deep and wide a continuous footing would need to be? Would it be vastly better than blocks if I made the footing say 8" wide and 8" deep and ran it around the perimeter (really wouldn't want to do more than that)?

Are there any other types of foundations that folks recommend?

[ChugiakTinkerer]

Welcome to the forum!  It sounds like you've got a handle on the pros and cons of different foundation types.  I agree that a permanent continuous foundation isn't always a necessity for a small part-time cabin.  I don't mean to denigrate your cabin, but if you look at how folks build garden sheds you may find some good starting options.  You won't get a structure any stronger or more reliable than such a shed but if you go into it understanding the limitations then you'll be better off.

The question unanswered at this point is what are your ground conditions?  If you are on clean sand & gravel then all you would need to do is level the ground and put down some pressure treated bunks.  Plenty of sheds have been built this way and are still standing.  If you want to get a little more elaborate, a permanent wood foundation could allow you to have an enclosed crawl space.  It all depends on the soil as to how much gravel footing you need under whatever base you use.

The International Residential Code gets referred to a lot because it is a collection of standard building practices that have been proven over the years to allow the construction of safe, secure houses that will last their intended lifetime.  I read your post as saying that this is how you would like to build, if budget and conditions allow.  As long as you make any compromises or deviations from the IRC prescriptions in an informed manner you should be able to satisfy yourself that what you build will indeed be safe and secure.

If I can be so bold as to suggest homework, I'd say you should satisfy yourself as to what the compressive strength of your soil is.  You generally do not want any organic material in it, so the top soil needs to be removed and then determine how much load the underlying soil can take.  There are some generalized values given to the various soil types, and you should be able to find them online.  Another thing to consider is what the surface water is going to do in a heavy rain.  Finally, what sort of wind storms might you need to anticipate.  Are there any trees knocked down in the vicinity?  If you deviate from the residential code then you'll want to design for these conditions and forces yourself.  Best way to start is to figure out what they are.

I grew up in Sacramento and spent a summer as a teen helping replace floor joists that had been eaten by termites.  I have no idea if they are a problem where you want to build, but one of the benefits of a concrete foundation is to provide a barrier to those nasty critters.  Keep that in mind as you weigh your options.

[akwoodchuck]

+1 to a gravel pad and some burly skids...thats my favorite foundation for small structures...can you get a skid steer in to the site?

[tensada05]

ChugiakTinkerer, thank you very much for your informative post!

I agree that a permanent continuous foundation isn't always a necessity

- Do you think an 8in wide and 8in deep concrete continuous foundation would be enough? Because if so, I'm happy to do that and call it a day.

I read your post as saying that this is how you would like to build, if budget and conditions allow

-Yes, that is a much more eloquent way of saying what I'm thinking.

you should satisfy yourself as to what the compressive strength of your soil is.

-I wouldn't have the slightest clue how to figure that out. It just looks like dirt to me. It seems to drain well as we just had a VERY wet spring and I never noticed a lot of water pooling. Do I need to hire someone to figure out the soil conditions?

Are there any trees knocked down in the vicinity?

- Not at all. We are on the non wind side of a hill...meaning the direction of the wind usually hits the other side of the hill, not the side I want to build on.


akwoodchuck,

Thanks for your post. Unfortunately I cannot get a skid steer up to the site.

[jsahara24]

-I wouldn't have the slightest clue how to figure that out. It just looks like dirt to me. It seems to drain well as we just had a VERY wet spring and I never noticed a lot of water pooling. Do I need to hire someone to figure out the soil conditions?

Check out this link, you can zoom to your location and create an area of interest and it will tell you "generally" what your soils are.  If you need help with the website you can PM me your location and I can pull the soils map.  Good luck. 

https://websoilsurvey.sc.egov.usda.gov/App/HomePage.htm

[tensada05]

jsahara24,

Thank you!!! That is really neat that they have that.

So now the problem is...what does this all mean?!?!

Collayomi-Aiken-Whispering complex, 30 to 50 percent slopes
Map Unit Setting
National map unit symbol: hf5l
Elevation: 1,200 to 4,600 feet
Mean annual precipitation: 30 to 65 inches
Mean annual air temperature: 50 to 61 degrees F
Frost-free period: 130 to 225 days
Farmland classification: Not prime farmland
Map Unit Composition
Collayomi and similar soils: 40 percent
Aiken and similar soils: 35 percent
Whispering and similar soils: 15 percent
Minor components: 10 percent
Estimates are based on observations, descriptions, and transects of the mapunit.
Description of Collayomi
Setting
Landform: Mountains
Landform position (two-dimensional): Backslope
Landform position (three-dimensional): Mountainflank
Down-slope shape: Concave
Across-slope shape: Concave
Parent material: Residuum weathered from andesite
Typical profile
H1 - 0 to 15 inches: very gravelly loam
H2 - 15 to 60 inches: very gravelly loam
Properties and qualities
Slope: 30 to 50 percent
Percent of area covered with surface fragments: 0.1 percent
Depth to restrictive feature: More than 80 inches
Natural drainage class: Well drained
Runoff class: High
Capacity of the most limiting layer to transmit water (Ksat): Moderately high to high (0.57 to 1.98 in/hr)
Depth to water table: More than 80 inches
Frequency of flooding: None
Frequency of ponding: None
Available water storage in profile: Low (about 3.6 inches)
Interpretive groups
Land capability classification (irrigated): None specified
Land capability classification (nonirrigated): 6e
Hydrologic Soil Group: B
Hydric soil rating: No
Description of Aiken
Setting
Landform: Mountains
Landform position (two-dimensional): Backslope
Landform position (three-dimensional): Mountainflank
Down-slope shape: Concave
Across-slope shape: Convex
Parent material: Residuum weathered from andesite
Typical profile
H1 - 0 to 5 inches: loam
H2 - 5 to 20 inches: clay loam
H3 - 20 to 61 inches: clay
H4 - 61 to 74 inches: cobbly clay
Properties and qualities
Slope: 30 to 50 percent
Depth to restrictive feature: More than 80 inches
Natural drainage class: Well drained
Runoff class: High
Capacity of the most limiting layer to transmit water (Ksat): Moderately high (0.20 to 0.57 in/hr)
Depth to water table: More than 80 inches
Frequency of flooding: None
Frequency of ponding: None
Available water storage in profile: High (about 9.7 inches)
Interpretive groups
Land capability classification (irrigated): None specified
Land capability classification (nonirrigated): 6e
Hydrologic Soil Group: C
Hydric soil rating: No
Description of Whispering
Setting
Landform: Mountains
Landform position (two-dimensional): Backslope
Landform position (three-dimensional): Mountainflank
Down-slope shape: Convex
Across-slope shape: Convex
Parent material: Residuum weathered from andesite
Typical profile
H1 - 0 to 5 inches: gravelly loam
H2 - 5 to 15 inches: gravelly loam
H3 - 15 to 26 inches: very gravelly clay loam
H4 - 26 to 36 inches: unweathered bedrock
Properties and qualities
Slope: 30 to 50 percent
Depth to restrictive feature: 26 to 30 inches to lithic bedrock
Natural drainage class: Well drained
Runoff class: High
Capacity of the most limiting layer to transmit water (Ksat): Very low to moderately low (0.00 to 0.06 in/hr)
Depth to water table: More than 80 inches
Frequency of flooding: None
Frequency of ponding: None
Available water storage in profile: Very low (about 3.0 inches)
Interpretive groups
Land capability classification (irrigated): None specified
Land capability classification (nonirrigated): 6e
Hydrologic Soil Group: C
Hydric soil rating: No
Minor Components
Gentler slopes
Percent of map unit: 5 percent
Hydric soil rating: No
Unnamed
Percent of map unit: 5 percent
Hydric soil rating: No

I couldn't figure out how to insert a picture, so below is a link to the snapshot of the "Engineering Properties Report"
http://imgur.com/1WkMj51

Thanks so much guys!

[akwoodchuck]

akwoodchuck,

Thanks for your post. Unfortunately I cannot get a skid steer up to the site.

Dang. That must be quite the spot....well something else that works good for sheds and such is just plain old concrete deck blocks with adjustable saddles....cut out a square of sod, tamp down the dirt, and lay in a couple of 4x beams....we have a 12x12 guest cabin built like this, and I'm putting in a new greenhouse the same way...

[MountainDon]

I am building it under 200 sqft so I don't need permits.

Where I am located a careful reading of the building permit information reveals the following...

"A building permit shall not be required for the following:
1. One story detached accessory buildings used as tool and storage sheds, playhouses, and similar uses, provided the floor area does not exceed 200 square feet."

Speaking with the building officials here has clarified that to mean that any such buildings are accessory to an existing building that was built with permits and to code; in other words a permitted home must be on the lot before adding any storage structures...

...and, the building is not used for residential purposes, not even temporary; in other words no beds, no kitchens or cooking facilities.

Some jurisdictions may not have as strict interpretation or enforcement as I do here. Of course there are a few cabins and sheds in our woods that I know were built without permits and certainly not to code. However, if there was ever a complaint lodged about a non permitted, non conforming structure the officials do have to enforce whatever laws are on the local books.  It might be best to check that 200 sq ft limit carefully if for no other reason than to be absolutely sure you will be legal or not. Even in regulation heavy California there are homes I know of that just sort of developed quietly w/o any official blessings.

Good luck.

[jsahara24]

jsahara24,

Thank you!!! That is really neat that they have that.

Collayomi-Aiken-Whispering complex, 30 to 50 percent slopes
Collayomi and similar soils: 40 percent
Aiken and similar soils: 35 percent
Whispering and similar soils: 15 percent
Minor components: 10 percent
Estimates are based on observations, descriptions, and transects of the mapunit.
Description of Collayomi
Typical profile
H1 - 0 to 15 inches: very gravelly loam
H2 - 15 to 60 inches: very gravelly loam
Properties and qualities
Slope: 30 to 50 percent
Percent of area covered with surface fragments: 0.1 percent
Depth to restrictive feature: More than 80 inches
Natural drainage class: Well drained
Runoff class: High
Capacity of the most limiting layer to transmit water (Ksat): Moderately high to high (0.57 to 1.98 in/hr)
Depth to water table: More than 80 inches

Description of Aiken
Typical profile
H1 - 0 to 5 inches: loam
H2 - 5 to 20 inches: clay loam
H3 - 20 to 61 inches: clay
H4 - 61 to 74 inches: cobbly clay
Properties and qualities
Slope: 30 to 50 percent
Depth to restrictive feature: More than 80 inches
Natural drainage class: Well drained
Runoff class: High
Capacity of the most limiting layer to transmit water (Ksat): Moderately high (0.20 to 0.57 in/hr)
Depth to water table: More than 80 inches

Description of Whispering
Typical profile
H1 - 0 to 5 inches: gravelly loam
H2 - 5 to 15 inches: gravelly loam
H3 - 15 to 26 inches: very gravelly clay loam
H4 - 26 to 36 inches: unweathered bedrock
Properties and qualities
Slope: 30 to 50 percent
Depth to restrictive feature: 26 to 30 inches to lithic bedrock
Natural drainage class: Well drained
Runoff class: High
Capacity of the most limiting layer to transmit water (Ksat): Very low to moderately low (0.00 to 0.06 in/hr)
Depth to water table: More than 80 inches

Above is the information that is most useful from the soil report.  You will see your soils are typically made of 3 components.

Collayomi is a gravelly loam, loam is a mix of sand/silt/clay with clay being the minor component.  It is very well drained.

Aiken is a clay loam, which means its a mix of sand/silt/clay with the major component being clay.  It is surprisingly well drained for having such a significant clay component.  This tells me there is a fairly high sand component to offset the clay. 

If you look up soil bearing capacities you will find bedrock is of course the best around 6000 psf, sand/gravel comes in around 5000 psf, sand/silt/clay/gravel mix generally comes in around 3000 psf and clay/silt the worst around 2000 psf.  My best guess from the information I have is your soils are likely around 3000 psf.

I would advise you to dig a couple holes where you plan to build to get a feel for what your soils look like in the exact area.  Then you can make an assumption of your bearing capacity, generally with a safety factor applied, and size your footers based on the weight of your building. 

Hopefully this points you in the right direction. 

[ChugiakTinkerer]

@tensada05

The footprint of your foundation needs to be big enough that it distributes the load from the cabin into the ground, without any settling.  If you know the exact compressive strength of the soil that would be awesome, but using a conservative estimate allows you to plan and build without investing in geotechnical sampling and lab tests.  From the analysis that jsahara24 pointed to, I'd go with a 2,000 psf (pounds per square foot) soil strength.  Figure out the weight of your cabin and you know the minimum size of your foundation.

Engineering practice has one calculate the load for each floor and the load for the roof.  The IRC requires load calculations to use a minimum 40 psf live load for living areas, with a minimum 10 psf dead load.  For the roof, if you are in an area that sees snow then there will be a ground snow load for the area.  Far enough south and that will be zero.  For the floor, 50 psf load times 192 sf equals 9,600 lbs.  For the roof, allow some overhang so you might have a 14x18 roof footprint, with a minimum 20 psf live load and 10 psf dead load.  That's 30 psf over 252 sf which comes out to 7,560 pounds.  Add them together and get 17,160 lbs.  That's probably way more load than a hunting cabin will ever see, but if you're concerned about stashing your hoard of lead in the cabin, bump the load up appropriately.

So a design load of 17,160 lbs on soil that has a compressive strength of 2,000 psf means that the foundation needs a footprint of at least 8.58 square feet.  You could probably get that from nine cement block piers like shown in akwoodchuck's photo.  But that assumes the load is always evenly distributed.  Turns out not to be the case, so you'd want to make sure to have more or bigger blocks to accommodate uneven loading.  Or go with skids on gravel.  If you use a pressure treated 6x6 as a skid, you get 7.33 square feet of footprint per skid.  Railroad ties give you even more.  Better yet, if you put skids or piers on a layer of packed gravel, you essentially are increasing the area of your foundation. 

If you make the gravel footing extend beyond the width of a pier or skid by a distance equal to the depth of the gravel, you will get maximal load distribution.  For that 6x6 skid, a gravel footing 14" wide and 4" deep will spread the load over an area of about 18 square feet.  With two skids you have a total of 36 square feet of load-bearing support, which should be ample for your cabin even with a freak snowfall. 

[Bob S.]

I think You will be far better off with a gravel pad and Skids. Usually when you poor concrete they class it as a permanent structure  and  the 200 sg. ft. rule will not apply. Plus if push gets to shove you can move it. Bob 

[tensada05]

Wow, you all have been so helpful explaining all of this, thank you very much! I am very grateful!

akwoodchuck,

Yup, I like it and it's really the only place on the property to put what I want. Thanks for the photo, it helps to visualize everything.

MountainDon,

the building is not used for residential purposes, not even temporary; in other words no beds, no kitchens or cooking facilities.

Yes, you're right, that's why it is a shed for those who ask and a cabin to me. Luckily the nearest neighbor is a good distance and wouldn't be able to see the structure. But I have heard of the county flying drones to catch unpermitted structures, so definitely a good point and something to keep in mind.

jsahara24 and ChugiakTinkerer,

Thank you both so very much for taking the time to break down all that info. It has helped tremendously and has given me a good amount to think about. 

Bob S,

I think You will be far better off with a gravel pad and Skids. Usually when you poor concrete they class it as a permanent structure  and  the 200 sg. ft. rule will not apply. Plus if push gets to shove you can move it.

That's an excellent point and I think that's where I'm leaning.


Again, thank you all! I will update the forum when I start in a couple of weeks!

[flyingvan]

I've considered just building a pond and a boat