I am building a gambrel roof cabin that is 24 X 32. I was thinking of 20 piers. 5 along the 32' length and 4 across the 24' width. Is this enough?
Should I put more ? if so placement or spacing suggestions.
Welcome to the forum!
With that many piers you may as well put in a continuous perimeter foundation. Beyond that, it's hard to make any recommendations without knowing what your soil conditions are, ground snow load, etc. For a structure as large as you're describing, I suspect you are going to invest a lot of money. You'll probably want it to last, and the best way to ensure that is to build a foundation in accordance with residential codes. If you don't have to deal with a building code inspector and are interested in learning more about post and pier construction you should search this forum using the keyword piers. Here are a few threads with more info...
http://countryplans.com/smf/index.php?topic=14659.0
http://countryplans.com/smf/index.php?topic=14630.0
http://countryplans.com/smf/index.php?topic=14593.0
http://countryplans.com/smf/index.php?topic=14470.msg189287#msg189287
Post and pier foundations are useful for small structures or ones that aren't full time residences. If you go down the pier route, you'll need to have enough footprint to support the design weight of the structure. Assuming a second story and no snow load, your design load will probably be in excess of 100,000 lbs. If you are in decent soils that can support 2,000 lbs per square foot then you'll need at least 50 square feet of pier footprint. You can go with many smaller piers or fewer larger piers. Just remember that every post will require diagonal bracing. The more posts, the more bracing. At some point you may be better off moving to a continuous foundation such as poured concrete, masonry blocks, or permanent wood.
Piers seem more economical for a cabin. I am looking at 18" round footings and 10" sonotube piers. For 20 footings that gives me 35.34 ft^2 for bearing area. A 32 X 24 cabin is 768 ft^2. At 100# per square foot that gives me a soil bearing pressure of 2173" /ft^2. Sound reasonable?
I'm not a fan of piers myself, but this build seems to be about the best of them I've seen on this forum.
http://countryplans.com/smf/index.php?topic=12576.0
Seesm like that pier foundation would withstand a nuclear blast. Had something much less involved in mind.
With that amount of concrete you could pour yourself a continuous footer....slap a treated stem wall on top (material you'd have to buy anyway if you're skirting it), and voila! Not a bad payoff for two days of work.... [cool]
A treated stemwall doesn't have to be on concrete, a gravel filled trench would suffice... a permanent wood foundation.
A post frame building is another way to achieve a braced structure. This is an old sketch of a smaller structure but the idea is the same. The perimeter posts extend unbroken from footing to plate. They are braced by the wall sheathing.
(http://timbertoolbox.com/sketches/1624barn.jpg)
It still goes back to what are you building and where? If snow load is NOT a concern, you still need to allow for 50 lbs per square foot per level of living space, plus 30 lbs per square foot for the roof. If you are building in an area that has a high ground snow load you would need to allow for yet more weight on the roof. You can see what your snow load is at http://snowload.atcouncil.org/
Allowing 12" eave and rake overhang for the roof, your roof area is 26' x 34', or 884 s.f. The minimum design load for the roof is 26,520 lbs. Your 100 psf for the two floors comes out to 76,800 lbs. Add them together and you get 103,320 lbs. Divide that by 35.34 s.f. and your average load on the soil will be 2923 psf. But houses don't distribute load evenly. The middle piers may only be supporting the ground floor. The side piers then take up more of the load, and you may end up with upwards of 5,000 psf on an individual pier. If your soil is good for 2,000 psf do you think your footings will be adequate?
In the original post, you ask if 20 piers is enough. With the size of pier you want to use, the answer is no. I would look into a permanent wood foundation to get a continuous perimeter foundation that will provide support along its entirety and give the bracing needed to prevent racking from winds or ground movement. It would definitely be faster than pouring concrete and wouldn't surprise me if it comes out less expensive as well.
Upstate NY. Frost penetration of 48". Snow load of 40 PSF.I was looking at one floor and a half loft. A gambrel roof with no overhang. Soil is mostly clay with channers (flat rocks) all over the place. I can't put a shovel in 8" without hitting a rock. You never see wood foundations around here. Piers, perimeter block or poured concrete. Have to go down below the frost line.
Will the loft be free-span or will you have load bearing support under it? Same with the roof. If the side walls are bearing most of the load you will want to consider a bigger pier footprint. Even with rocks in the ground, if you have a lot of clay the soil strength will be on the low side.
The way to look at load on a given pier is to add up the loads from the tributary area of each surface the pier supports. It sounds like your loft is free span and you do not have a structural ridge. That is the scenario that would yield max load, well also if your floor is free span but it sounds like you'll have center support. A BOTE estimate for the max load will be on the pier that is supporting the floor, the loft, and the roof and not on a corner. Your floor tributary area will be 8' x 4' @ 50 psf for a total of 1600 lbs. Your loft tributary area will be 8' x 12' @ 50 psf for a total of 4,800 lbs. Your roof tributary area will be 8' x 12' @ 50 psf (40 snow load and 10 psf dead load) for another total of 4,800 lbs. Add those up and you're looking at 11,200 lbs. Assuming a 2,000 psf soil strength, you should have a pier footprint of 5.6 s.f. That equates to a circle with diameter of 32".
If you want to go with piers, you may need to have a structural ridge, where you have the center of the roof being supported by walls or posts that provide a direct line of support all the way down to the ground. That will reduce the tributary load from the loft and floor significantly.
Quote from: Ricky J on April 06, 2017, 11:43:02 AM
Upstate NY. Frost penetration of 48". Snow load of 40 PSF.I was looking at one floor and a half loft. A gambrel roof with no overhang. Soil is mostly clay with channers (flat rocks) all over the place. I can't put a shovel in 8" without hitting a rock. You never see wood foundations around here. Piers, perimeter block or poured concrete. Have to go down below the frost line.
Another upstater here, although "upstate" can mean many different things depending on where you are in the state. You soil sounds like mine.
I personally haven't seen any PWD foundations, but one guy here has done one in upstate NY and it appears from his thread that is is working well.
http://countryplans.com/smf/index.php?topic=10671.msg139962#msg139962
Also this one, but his photos have disappeared (but his build came out great).
http://countryplans.com/smf/index.php?topic=10791.msg140128#msg140128
Both these guys are in the Adirondacks, which is different than, say, the Southern Tier.
I was considering a PWD for part of my cabin, but now am thinking something different (need to get a post up soon).
If you're set on piers you may want to look into "bigfoots", basically a spread footer that connects directly into sonotubes. I'm building in upstate/central NY, I can't figure out where that line is, and most of the cabins up there use piers against the recommendation of most on this forum. Our soils are sand/gravel which is going to give you pretty much the highest soil bearing you can get except for bedrock, however bedrock has its own issues. Clay would be the worst and likely around 2000 psf.
Its doubtful you will put in a continuous perimeter foundation and regret it, however I am sure many have regretted piers. I understand cost and ability is a concern which is why many lean toward piers. Good luck with either route and keep us up to date!
All of NYS is under the IRC code which does not allow pier foundations.
Quote from: NathanS on April 06, 2017, 04:42:02 PM
All of NYS is under the IRC code which does not allow pier foundations.
...unless under a design stamped by a PE
Nathan: great build in your main thread. Lots of great stuff I can use as I *finally* get going on my cabin this summer.
Thanks neighbor. :)
I was told by the local building inspector piers are acceptable. I am in Cattaraugus county, the south western part of the state. I was thinking of the 18" diameter bigfoot and 10" sonotubes. I see a lot of pier foundations on cabins around here, so I doubt they are not legal. I was told under 1500ft^2 no stamped plans. If you look at this link, you will see a bunch of their big barns on piers. http://buffaloshed.com/html/barns.html. NAthan, where in the IRC code say no piers? I see many images of building with piers on their site.
https://basc.pnnl.gov/images/typical-installation-double-section-modular-home-pier-and-ground-anchor-foundation
https://basc.pnnl.gov/images/polyethylene-completely-covers-floor-crawlspace-and-attached-walls-and-piers-well
Also virtually all of the state cabins at Allegany State Park near here are built on piers.
https://parks.ny.gov/PhotoGallery/Default.aspx?gal=Cabins%20and%20Cottages
Quote from: NathanS on April 06, 2017, 06:59:05 PM
Thanks neighbor. :)
Your welcome.
But actually, Ricky sounds like an even closer neighbor. I am in Cattaraugus Co.
Yes, piers can work well in CattCo soil. Check out my shed thread in my sig. But I didn't need to get a building permit for that.
And I'm spending this coming weekend in one of those Allegany State Park cabins on piers. They probably are either PE-stamped, or built a long time ago and were grandfathered in.
Ricky: Stop in at the ASP summit cabins this weekend if you want to say hi. Got a big group there for trail work. We're grilling Saturday evening! (will be out in the woods during the day).
That's interesting the code officers allow piers - that is really all that matters.
https://codes.iccsafe.org/public/document/code/446/7992266
This is the foundation section for IRC/NYS - no provisions for pier foundations. If you go to 403.1 it says the footing must be fully continuous.
With piers you are relying on the soil to brace your structure horizontally. Over here in Chenango and Otsego there are virtually no old buildings on piers, and the stuff that is 30-40 years old has moved a lot.
That's sure the case here. No old buildings on piers. When people build retaining walls, the soil quickly reaches the top of it---soil on slope is always flowing downhill. Two of the things I've learned since haunting this site are, once people decide to go with piers, there's no changing their minds. The other thing is, there are quite a few of us (myself included) with bad experiences with piers. Saving a nice house with a failing foundation is no fun. It will be fine in the short term. My feeling is, I want the structures I put my heart and soul into to still be around in 100 years...
I think the UBC designs stuff for a 30 year lifespan. Depending on a whole bunch of variables, a pier foundation might last that long. On bedrock, even longer. There are other disadvantages besides outright failure though---earth/wood contact being one. Airflow underneath a house really drives up heating costs. All your plumbing has to be carefully freeze protected. There have been breaches in home security by people coming up through the floor. Homes in the wildland interface are difficult to insure, and difficult to protect from fires, trapping heat under a structure with plenty of airflow to cause problems from a small ember
Southerntier I am in Lyndon (as I call it a suburb of Franklinville) 3 miles past Case lake. There is a cabin larger than what I am proposing on 6 X 6 wooden piers across the street from the lake up on a hill.
Thanks for the input. I am rethinking my choice. If I put up a perimeter wall I can go with 2 X 10's and still have a stiff floor If I run support up the middle parallel with the long axis. Next question. I must go down 4' so I am under the frost line. That means 8 courses of block. Should I just excavate the perimeter and then backfill? Open up the interior as a crawl space? kind of a shallow basement?
Is radon an issue?
No radon, a little cow manure next door, but no radon.
Going down 4' you are beginning to see why a basement is common up north and a crawlspace is more common in the south where the frost depth is much shallower. The logic was, if you're down 4' what is 2 more feet, project 2' up and you have a useable full basement. As time has moved forward there are a couple more options, a frost protected shallow foundation is one, it allows for a shallower footing by insulating from the frost. A gravel trench and wood foundation is another, a precast foundation on a gravel footing is a third.
Without radon I'd go the basement route. Great storage, a place that stays cool in the summer and freeze protected in the winter. Plumbing, mechanical and some electrical will be accessible. You might be able to put HVAC and water heater freeing up interior space.
The problem with a basement is cost. Need to do another 3 (or more) courses of block, extra excavation costs, extra cost for the concrete slab, sump pump, drainage etc.
Got this from our friends at Green Building Adviser.
QuoteQuotes taken from the book: "From The Ground Up," by John Cole and Charles Wing
"A full basement foundation is essentially a large and expensive concrete-lined well that we try to keep dry."
"It is difficult for me to comprehend how any harmonious relationship with the natural world of your site could lead you toward a full cellar. One does not follow the other."
"There are no two ways about it – I agree with Rex Roberts – the cellar is second-class space. It's naturally dark and damp, and to achieve anywhere near the same livability as above-ground space, it will end up costing as much or more per square foot."
http://www.greenbuildingadvisor.com/blogs/dept/musings/building-unvented-crawl-space
Pretty much why I don't put a whole lot of stock in GBA, they paint with a mighty broad brush.
With a frost protected foundation you can put your footings closer to grade than frost depth, check out section 403.3
https://codes.iccsafe.org/public/document/code/594/10050982
Right now in this county there probably isn't a spot 4' down that isn't below the water table. That is one reason to think about sticking with a crawl space or slab.
Actually was in a totally different state today and someone was talking to me about how they had 1-2" of water in their basement because their sump pump went during a rain storm, and that this time they were going to install a backup sump system. That's an expensive storage space especially after throwing out their storage and needing to buy new stuff to store. Don't forget the dehumidifier and the fact that it does add to heating and cooling loads.
Non architect that I am, I also think that spaces should be designed for purpose and not as a catch all. I can see a lot of reasons why not to have a basement. On the same note, if you have mostly flat land and a steep spot, the steep spot could be the perfect spot for a house with two floors with both floors being accessible from ground level. Can't beat that and now the step spot that was unusable is utilized.
Foundation is probably about the hardest thing to design because every building site is going to be different.
Most, if not all homes around here have basements, and rarely get water in them. Just have to be designed correctly and water accounted for. I have never more than a trickle come in under snow melt or really heavy rain and my basement is from the 1940's. That being said, I don't want one for a seasonal cabin. Too much could go wrong when I am not there.