20x30 cabin need foundation advice

[atomicskier]

Hi Everyone. I'm going to be building a 20x30 single story cabin in the wyoming grasslands. we've got a 40" frost line and I'm stuck on what to do for foundation. The plans I've purchased do not include any foundation work (they assume screw piles). I've got a quote for $8900 for a standard foundation with crawl space but am thinking I can do concrete piers myself for less than half of that if I can figure it out. I'm having a hard time determining size - the county planners don't care, there's no inspection and they say "ranchers hate it when we tell them what to do, so we don't." maybe some of you can help.

for a basic 20x30 building I'm hoping to run three rows of piers. I'm not sure how far apart each should be or what diameter. can you suggest? the cabin will be basic stick construction, no bricks.
How does one typically connect to the piers, J bolts or more of a traditional deck post holder? Our cabin will be subject to regular gusty winds, does this change my approach?
When using piers to get a crawl space will I be better raising the pier up above ground and building right on top, or having the pier only come up to the top of soil and using a post to elevate up to the joists for crawlspace?

is there a link anyone can point me to for putting a cabin on piers? everything I find is decks and such.
thanks!
Mark

[JRR]

With that frost depth and high winds, my first thought would be a full, connected, basement.  Concrete or block perimeter walls, rigid foam exterior insulation under shoveled earth backfill.  Well drained footings.  Would that work?

... and welcome!

[alex trent]

I will give you my experience building a 22x27 cabin on a slope using piers and posts.  Been up almost year and everything looks good. Got  a lot of the advice on how to do it (well) on here.

You will get a lot of opinions on piers vs, crawl space, slabs, etc. and most of them will be negative about piers.  There are some very good points in those arguments, so do not discount Them. I built june on piers mainly because of the slope and while I could have done a "walk out" with a foundation, and even gained some advantages with that, I do not like them...so personal preference plays  a part as much as construction details. Also, this was a lot easier on the environment than digging up the land to level it for the building. That also is important to me.  Piers and posts are sound building and millions own them existb for various reasons.  That they are not in the perceptive code is not a good reason not to do it...but there may be others.

Spacing of the piers depends on two main things  1) the support you need in the soil 2) the support you need for the structure above.

#2 is easy...size and weight which are interrelated and easy to calculate determine what you need to hold it all up. I'd venture a guess that with three rows and 30 feet, a pier every 6 feet is fine.  That is three rows of six..18 total.  You will need to size your beams to support the building and something on the order of three 2x10's will likely be plenty for each beam. I used two, but I have a lightweight cabin and no snow load at all.

#1...depends on the load bearing capacity of the soil.  Lower bearing soil means a bigger base on the pier and maybe more piers. Try to find out what your is. Lots of reference to that and maybe you can get a local who can tell you what kind of soil it is.  My guess is you are 1,500 psf or a bit better. You could be 1,000 or 3,000.  If you have 18 piers and each one is 2 sq.ft at base, you have 36 sq ft. and at 1,500 lbs/sq ft have the ability to support 54,000 lbs total. or 90 lbs per sq foot which is within the typical figures for loading on a house your size.

You just have to make sure you site has good drainage and you are not sitting on a spring or other anomaly.

Concrete all the way up or post on top.

Concrete all the way up is much simpler to make strong..especially since you have wind. The connection between the bier (concrete) and the post is a vulnerable one and is subject to "bending' in the wind.  You can deal with this, but if you do concrete all the way you do not have to.

The lower to the ground you make the tops of your piers, the more stable the structure will be.  at a couple of feet, you have no problem.

My build is on here..Mtn Cabin in Nicaragua. Deal with a lot of this stuff with advice from people on here.

[ColchesterCabin]

my build is 16x24 and here is how mine is laid out. Now I am not sure on the engineering with the spanning distances vs the size lumber but there is a lot of threads on this site if you are willing to research I think in my thread Don_P or MountainDon talk about the reference on page 2 of my thread I think.



Hope I was helpful. Good luck!

[atomicskier]

thanks for that! what diameter piers did you use? how are you attaching to them?

[atomicskier]

JRR, I'd LOVE a basement, but it pushes us up and up. crawl space foundation looks to be $8900, full basement would be closer to $12k. I would love the storage and it's not that much deeper, but the excavation and extra concrete work are just not in the budget. I have high hopes of getting the foundation and shell done for $20k. if I can get the foundation done under $5k that gives me a lot more room to work toward finishing the inside.

[alex trent]

My concrete piers were 12x12 inches square.   Tapered out to 18x18 at the bottom for a 2.25 sq ft footprint..  4 feet deep.  My soil is a lot of clay so I dug and did not need a form in the ground.  Built a small form for the one foot that came above ground.  Dug by hand so cost is concrete and rebar and a bit of wood for the forms.

My 6x6 posts were attached to the piers with Simpson connectors....ABU types ..you can see them on their site and a lot of other stuff too.

http://www.strongtie.com/products/category_list.html?source=home

This way is definitely less than 5K

[MountainDon]

  Piers and posts ......  That they are not in the perceptive code is not a good reason not to do it...

If one subscribes to conspiracy theory's then I suppose one would believe that. There are those who believe that everything in the codes stems from the basis of somebody making money on some product.  If one believes that the building codes have lots of good structural theory and practice behind the rules, then one will believe that there are sound reasons that codes do not recognize pier foundations.

If one has spoken with a very experienced licensed structural engineer as I have, then they may also be believers in the basic structural "advice" that can be found in the codes (BTW, the most commonly used code is the IRC and there is a thread on it in a board under the General board. )  There are many users of piers who do not understand all the implications inherent in such things and forge ahead simply because they see many examples of piers here and there.

I do not believe a building as large as a 20 x 30 should be built on a pier foundation, unless one has the blessings of a structural engineer. Soils vary too much to base the decision of what to use as a foundation strictly on the price tag.

For anyone insisting on using piers for a large building I would suggest that the best such foundation is likely one based on the use of the Bigfoot Footing System. Google it. As impressive as it looks in the larger diameter columns the company will not, can not, issue certification to a user that the system would be passed by an engineer. Why? Local conditions vary widely. 

(I have had correspondence with one of the principles of Bigfoot and once summer is over and I'm back to a regular schedule, I plan on posting some of what I learned.) Some of the opinions that will show up on this sunject are from DIY builders who have built but one structure. Some voices will have years of experience behind them. In the end it is the DIY'er who makes the decisions that may later affect the stability and safety of what they build. Nobody else will care much after the topic is buried under new musings.

[alex trent]

I asked this once before and never got an answer.

What does getting a local engineer or a soils guy (or both) cost?  If someone has local experience and training, that would seem to solve the problem of having expert advice.  Is it really that much, or do those guys not readily exist.

[alex trent]

There are many users of piers who do not understand all the implications such things and forge ahead simply because they see many examples of piers here and there.

Empirical evidence is really useful. I knew smoking was bad for me before they put the warnings on the packs.  Many of those that waited died.

[MountainDon]

What does getting a local engineer or a soils guy (or both) cost?

Like soils, that can vary a lot from location to location. Does the engineer have to travel a distance to the site? Is he going to be asked to approve a entire plan set? Is it a simple plan or does it involve complexities? It could cost into the thousands or perhaps $300 -$500.     The thing about the IRC is that it contains engineered solutions to many of the problems that are encountered in design and construction. And they are free for the reading. Add to that the use of engineered trusses and joists where the engineering comes with the purchase of the product and it seems to me that most DIY builders can avoid the cost of hiring their own engineer.  I have searched for a one size fits all solution to the question of using piers and have yet to find one. Many of us who build on piers have success because we have been issued some good luck.

[alex trent]

Seems to me that $500, or even $1,000 is a small price topaz for the peace of mind this brings. And not a big investment in a 30K dwelling. If you do not need for permitting and you really want a site/soils review and not a full sign off in writing on a plan you have done, should be a lot cheaper then the full boat treatment, but just as useful.

I sure would have done it. You never know when your issue of good luck will run out.

[CjAl]

mountaindon. for someone who is so set against piers like i have seen you say in MANY threads. why is your place on wood posts?

[MountainDon]

Because I am more knowledgable today than I was when I started out on our adventure. Someplace here there is an "explanation". Right now I'm out the door to the cabin; no time to find it.



EDIT: corrected spelling error

[CjAl]

btw. my 20x32 is on three beams, each beam has 5piers. 8" tube wherr it comes out of the ground, 3' deep. they bell out to 18"-24" round and about 10" thick footings on compacted sand mostly. three pieces of #3 rebar in each tube and footing. then i used the asjustable post brackets that use a J bolt.

i have been thinking after its dried in and instead of simply skirting the bottom i may dig a trench around it and pour a footing and frame up a wall from it to the rim joists

[Squirl]

This is a very common topic.  I personally don't mind piers on smaller, or thinner structures.  I've seen enough of the math posted by people more knowledgeable than I, and I was uncomfortable with the risk on my 20x30.  Sure it is very low risk of failure in comparison to other methods, but it wasn't worth it to me.  I am in a slightly different boat, I consider my 20x30 a house.  I want it to last and hold value over my lifetime, which will hopefully be a long time.  I will likely be dependent on this structure many years down the road, maybe when I am no longer physically able to make many repairs.  A few hundred dollars extra and an extra week or two of work, on something that I was going to spend 30k and a year building was well worth it in my analysis.

It is unfortunate that the other set of plans did not include various foundation options, the 20x30 plans I purchased from countryplans included what you are looking for.

Cost is the primary argument I have read from most, but not all, people over the years.  When all the factors are weighed in, there is very little cost difference between it and other foundation options.  Most of the price you have been quoted is labor and knowledge.  Information is power, power is money. The materials are cheap.  Concrete is one of the cheapest building materials.  Sure setting up forms and pouring a concrete foundation can be difficult and intimidating for the average DIY, but concrete blocks are a lot easier.  If you are not able to reuse the forms on a pour, a block wall comes out a few hundred dollars cheaper. 

I built a full crawlspace out of block with a 5 ft depth on a 20x30.  Cost about $3100.  Fully code compliant.  The only thing stronger would have been a solid concrete pour. 

Weekend Mini-excavator rental $600 delivered. Really easy to learn.  Took about 10 minutes. If you grew up with video games it is just two joysticks.
Footing Forms and rebar $150.
3 yards of concrete delivered $300.
Concrete block and Mortar delivered $1500.
Sill plates and J bolts $150.
Center beam $150
Weekend rental of walk behind loader to backfill inside and out $250.

By the time you add in the piers themselves, the girders, the connectors, and the skirting, the costs come pretty close.  A full foundation is a bit more work though.

There are also Permanent Wood Foundations, which cost a slight amount more in materials, but come together like framing a wall.  It is a fast way to put in a full crawlspace foundation.  I suspect it would be very functional in the cold arid grasslands of the west.

There are also frost protected shallow foundations, as long as the structure will have a small amount of heat through the winter (passive solar?), they can be fast, the same cost, and less work.

In addition to piers, there are a whole lot of options open the average do it yourselfer. Most have very similar costs, don't be intimidated by the quoted cost from a contractor.


BTW 

[Squirl]

I asked this once before and never got an answer.

What does getting a local engineer or a soils guy (or both) cost?  If someone has local experience and training, that would seem to solve the problem of having expert advice.  Is it really that much, or do those guys not readily exist.

It depends.  Some county extensions or services may have low cost soil testing.  My recollection was the soil and water extension in my county charged $150 a few years ago when I checked.  Septic is highly dependent upon soils, and in my state an engineer is usually required to do the approval.   Soil analysis was simply included with my septic testing.

[UK4X4]

My soil report with two off 10ft test holes was 1375 usd- required by county

I designed the foundation including rebar etc alongside the builder and months of study- the foundation is a crawl space 4ft interior height ridgid slab fully rebarred and in a single poor - basicly my mountain can move and my house will stay in one peice

It cost 2700usd for the engineer to get him to sign off on the design.

Mind you he had to drive 5 hrs to get to the plot and that includes an inspection of the rebar before the poor, so another long day and fuel

I think I have the most expencive built on here so far and I have'nt even dug a hole yet !

Here's my take on foundations, you can't just look at whats simple and cheap- you have to concider your plot and your conditions- what works in one place does not mean it will work with yours

Find out what your soils are- min supporting load
If you are in a large flat valley - maybe just from this site will work, if your on a mountainside like me, then maybe a site specific soil report is advisable
http://websoilsurvey.nrcs.usda.gov/app/HomePage.htm
Their report gave me a pretty good representation to what the soil guy gave me.
The site specifc soil report outlined specific requirements for a few suitable foundations..............

Look at your snow load
Look at your frost depth

Work out the weight of your house then the loading on the beams/walls

I have 90# snow load- even on a 10/12 slippy metal roof I get 76#

This adds up weight fast.............I have a worked example somewhere, that I need to post. as for my area by the time I've used 36" diameter BIGfoots at 6 ft spacing - its going to be easier and cheaper to do a foundation like Squirls.

Start from the roof down- know your loads and conditions then look at a foundation thats suitable

[Steve_B]

[quote\]

There are also Permanent Wood Foundations, which cost a slight amount more in materials, but come together like framing a wall.  It is a fast way to put in a full crawlspace foundation.  I suspect it would be very functional in the cold arid grasslands of the west.


[/quote]


Can you give some details what this is and how it works? Maybe ballpark cost as well?

[Squirl]

Permanent Wood Foundations are, like the name implies, made of wood.  They are pressure treated to the level of fresh water dock pilings. 

There are two main types.  The first are those with evenly backfilled soil on both sides.



The second are ones with a poured slab on the interior.



The purpose of both of these is to keep the weight of the outside soil from kicking in the bottom of the wood wall.

The charts and specs of the requirements can be found here.
http://publicecodes.cyberregs.com/icod/irc/2009/icod_irc_2009_4_par082.htm

I was going to do it for my foundation and looked into it over a year ago.  The prices are based purely on memory.  You would have to call around to local lumber yards and they would probably have to order the lumber stamped PWF.

Since there is an 18" minimum requirement for a crawl space, I wanted my studs to extend at least 2 ft above grade.  So I asked for a 2x6x12 and I would cut them in half with the cut end up out of the soil.  Since I planned on just trenching and evenly back filling, unbalanced fill height was unimportant.  My recollection was that each plywood sheet was $50 and the 2x6s were $25.  So for the wood it would have cost around $1900.  The stone and equipment would all have cost the same, but I wouldn't have needed to order the 3 yards of concrete.  So it would have cost almost the same as my block foundation but would have been a bit faster.

The reason I didn't go with it was because drainage is important with a wood foundation.  Although additional drainage it is not required by code, it is still highly recommended.  Below 12-24", my land doesn't drain.  It is a "fragipan layer of shale clay".  So when I say doesn't drain, I mean almost at all.  It failed for conventional septic.  Since there is almost no slope to my land, the is no way to install a french drain to daylight, and since my land doesn't drain a drywell would be useless.  I had to scrap the idea of a PWF for concrete because it can sit in water better, and I am in the wet northeast.

If you do research it locally, would you please post the prices they quote you for PWF lumber?

[Arky217]

I think the biggest factor on whether or not piers would be a viable choice is the ability of the ground to support the weight of the house. Piers are only a fraction of the square footage of a perimeter footing.

Take my house for example; it sits on 36 piers. The piers are 10" in diameter but tapered outward at the bottom to around 16" to 18" diameter. That's approx. 1.5 sq.ft. per pier or 54 sq. ft. for the entire house.

If I had used a typical stem wall with center support on a 16" wide footing for my 24' x 48' house, the footage would be approx. 245 sq. ft. or more than 4 times the piers footage.

I didn't have any qualms about the soil under my house supporting the weight, however, since it was on a mountain ridge and is basically nearly solid rock.
I went 18" to 20" deep (frost level is 6") and after the first 3 or 4 inches of dirt, I pretty much had to bust out the rest of the hole with a rock bar.

Here's a picture of how it looked before setting the end sills and floor joists.
(The posts are cedar, the rest is SYP; all milled with a chainsaw. Everything is tied together with 1/2" bolts and 1/4" angle iron.)

http://www.diychatroom.com/members/arky217-86953/albums/all-pictures/5437-1-18-12-031-600-x-450/

Arky

[Don_P]

This is why we have prescriptive codes. The reason pier foundations require competent design is only partly with their decreased ability to resist vertical loads. Lateral loads must also be resisted, this is where the walls and continuous footing of conventional foundations come into play.

Construction is all about figuring out the loads and providing adequate resistance. If you cannot do this, then stick to prescriptive solutions, or hire an engineer. A fine house on an inferior foundation is only as good as that foundation. With all the respect it is due, if you get advice from the internet that cannot do more than blow sunshine up your backside, it is incompetent design. Incompetent people overestimate their abilities and do not understand the nature of what they are doing. We are getting a number of one house builders giving expert advice here, go carefully.

[alex trent]

I would agree to go carefully, not only on this but on other stuff as well. Carefully is not the same as do not do it. This seems to be one area where small investment in an engineer is well worth it. Im am one of those one home builders, but I did my homework and have a sound build.

Soil bearing capacity being the #1 concern...and I believe it is a great part of it, should be easy to figure if you get a reliable soil test...which it seems you can do at a reasonable cost. A good investment for a pier build.

I could not do that down here.

The estimating guides for soils list from 2,000 to 5,000 psf for 'typical" soils. Assuming you have good drainage and no seasonal spring underneath.  Mine is sandy clay..supposed to be 2,000 psf.

My 21 piers @2,25 sq ft/ pier are 47 sq. feet. At 2,000 psf, that is 94,000 lbs. soil bearing support capacity.

My house is about 20,000 lbs. dead weight (calculated from materials) and the allowance (mostly wind) for live load is 30 psf (90 mph wind) or another 18,000 lbs. Total is 38,000 lbs.  So even at 1,000 psf soil bearing capacity (half of what we determine it is) this is OK, ....a good margin.

Given not having a soil test, had I not had this margin, i would have done more piers or bigger footings, given the fact that this is based on estimation, not a test.

A point to consider is that not all your piers near the same mount of weight. Ones at the ends bear quite a bit more...figures are published and on here I think. So you do need some margin for that. Fifty percent comes to mind, but that may not be accurate.

A little extra footing makes a lot of difference. My piers, if carried down at 12x12 would have only been 1 total of 21 sq. ft...not 47 sq. ft. as was done by tapering them to 18x18 at the bottom.EZ to do.

[Don_P]

Alex, go to the link on postframe design I posted last week. Read the section on unconstrained posts. You overguessed the windward load, totally neglected the leeward load and applied the resulting horizontal load to the vertical load bearing capacity of the soil. That 18,000 lb horizontal load, or whatever it actually is, is trying to slide the posts off the piers or tip the posts or piers over... laterally.

Steve gave an important caveat, subject to frequent gusty winds. This should be designed by an engineer or it needs to follow the methods known to work outlined in the building codes. Personally, I do it prescriptively and put that money into structure. If you build conventionally we can give sound advice and back it up by referring back to the codebook and standard references.

Provide those for pier and beam and you'll have something to offer.

[alex trent]

Provide those for pier and beam and you'll have something to offer.

I do offer something...how to calculate the required pier size for soil bearing capacity for pier foundations. Yes, there are a lot of other items in the "pier equation", but this is the one at hand in this post.

You are talking about something quite differen in your last response...lateral loads  As a matter of fact, I did not neglect anything in the wind calculations....at least nothing of critical importance on either lateral or downward loads. I researched this a lot and there is a lot of information available . I posted a couple of links on the site and you commented on how valuable one was. I followed the basic tenants of this for lateral load. This is in addition to the empirical evidence I see in dozens of buildings built this way and withstanding the elements for decades.

Here is what I was referencing in he post in question....it is directly relevant to the original subject.

There is a vertical component of wind (puts downward load on the structure). This is what the 18,000 lbs in my calculations is. It is likely high....but that is OK. It is actually the highest i could find for my roof type..most a quite a bit lower. We are talking about soil bearing capacity here, so that is what I addressed.

As you know, wind loads on roofs is a complicated subject..but it does not differ on pier or full foundation buildings.  On the opposite side of the side of the gable roof from the side with the load is a suction...that also can be calculated and varies with  lot of factors. That is all boiled down in the tables (code, if you will) for roof construction and securing the roof. This is a roof issue, not a soil load bearing issue..as the force is up. The roof will go, way before the building is lifted (as least in my case).

The figure I used is for the max downward force of a 90 mile an hour wind 90 degrees to my 4/12 roof.  There is some leeway in that, as if it is exceeded momentarily it puts load on the piers which can be tolerated. In my case, this is a very unlikely occurrence...plus i have an about double to that as a safety factor.

As for lateral loads...windward and leeward, I did not neglect them in my calculations (remember, the above is just for downward loads). Once you get above the posts, the lateral forces are essentially the same a in a full foundation building, so the issue is to be sure the pier and post and post and beam connections hold up to the vertical and lateral forces. Those also exceed the calculated forces for my building...most by a factor of two.

No, I cannot take you to a "perscriptive" chart for the connections...but there is a lot of other information to use.  Yes be nice if there were a chart.  I will say this...if a person is not willing to do a lot of homework and is not able to synthesize a lot of information, get an engineer.  Or maybe get one anyway...I looked but could not down here.  For a $2,000 fee (maybe a lot less0 and a $40,000 house seems like a good investment.

I have two people who are going to build houses based on my design. Both in the immediate area. They like the design as it fits into the topography and the environment.  They also really like the idea that it does not require big machinery and excavation. I have discussed the concerns about this type of structure with them...both will do somewhat bigger ones.  I am not sure if they fully get the issues but i will be sure they do before starting. They have a lot of money and connections in NICA so i believe we can likely get an engineer to look at all this and give us an opinion and specs.  I will post what i find out. should be interesting and maybe a start at some type of reference.