Post Foundations and Floors

[daverave]

Dear Forum,

I want to build a 18'X28' cabin on my land that has about a 17 degree slope. I'll use a post foundation but the question is this: After I have the posts situated in the ground I could 1) Attach the floor joists directly to these posts or 2) Place beams on the posts and build the floor on the beams. To me it seems the former method is easier plus the floor is not elevated quite as high. I've seen it done both ways. Are there advantages of one over the other?

I'd like the floor to be at about 2' above ground at the highest part of the slope so that the deck on the other side will be about 9-10 ft above ground.

Dave

[poppy]

  to the forum Dave.

Not sure how you would mount joists directly to the posts; sounds like there would be a lot of posts. 

When cabin builders want to lower the floor level on a post foundation, they normally mount the joists inside the beams rather than on top.
I think you still need beams either way.

Others here can be more specific.

[MountainDon]

Dave, an 18 foot span is pushing things for just two beams, one under each long wall. The 18 feet could be spanned with Doug Fir or Southern Yellow Pine, number 1 or select structural grade. Even then some folks would find too much spring to the floor. They would have to be 2x12's. The required length and grade would be more expensive and more difficult to find in some places. An alternate would be engineered joists.

If you are planning a normal gable roof the side walls will be supporting the entire weight of the main floor, walls, upper floor and roof, plus snow loads, if applicable. The whole house including contents in other words would be supported by the long sides. This is why they have to be beams, not just a section of 2x nailed to the posts. The two beams should be located directly under the side walls for something this size.

The beams sitting on top of the piers is much stronger than relying on nails or bolts thru 2x material into the posts. People have done that very thing but it is not at all recommended. In my opinion that is a disaster waiting to happen. There would likely not be any sudden collapse, but over time the results would be less than satisfactory.

With a central beam added to the two outside beams the simplest method of installing the joists is to place them on top of the beams. 2x8's would make a sturdy floor done like this. You could likely get away with using 10 footers with an overlap over the center beam. The joists could also be placed in between the beams using steel joist hangers. That means more trimming to length.

There are minimum clearances between the ground and the beams, and another minimum clearance or the joists. Sorry, I don't have the numbers at hand, maybe later. Also if any wood is too close to ground it must be PT according to code and best building practices. Again, I forget the number so rather than guess I'll leave that for later.

You could also consider having the ground graded to be more level if you didn't want to have one end so high.

hope that helps.

[rocking23nf]

my land is sloped and i was told if I used PT lumber i had to have 6 inchs clearance to the ground.

[daverave]

In reponse to Mountain Don and rest,

Your answers were just what I needed. Now, let me ask a few specific questions based on your answers.

I'm thinking of the following foundation design: I'll use 5, 6X6 PT SYP foundation posts spaced 7 feet apart in order to span the 28 foot length. Three rows of these, spaced at 9 feet would give me the 18 foot width. So,  I have 15 foundation posts total for the floor, more if I put on a deck.

I could make the big beams from 2X8s - nail three together, staggered, and end up with a 28 foot beam 4 1/2 inches wide. These can be attached using steel connectors. Now I hang 2X8s, either at 16" or 24" O.C. between these beams using joist hangers. The beams become, in effect, the rim joists.

The posts would be placed on 18" concrete footings, down 48". I'm in Central WI near La Crosse with a frost depth of about 42". The roof will be gables but I really want to use a steel roof for longevity and because it's so much lighter than wood+asphalt shingles.

How does that sound to everyone?

Dave

[MountainDon]

How did you determine the size of the beams; three laminated 2x8's? I ask as some of John's plans use 6x12 beams down each long side at a maximum spacing of 7'6", if I recall correctly.

And when you state you want to use a steel roof because its lighter than wood+asphalt shingles, do you mean steel on horizontal purlins without any roof sheathing? That is done but it raises the question of ensuring water vapor does not condense on the underside and drip into the insulation. Your local building department may have information on what is required there. Steel over sheathing is also a quieter roof when it rains. Just something to think about. I do like metal roofs myself but have done all three over 7/16 sheathing.

[daverave]

MountainDon,

The 3 laminated 2X8s were a substitute for a long 6X6 beam. The calculator on this site is for joists and rafters, so I had to guess that 3 2X8s across 7' would be stiff enough. Perhaps 4, 2X8s which would provide that 6" width would be better. How would you suggest determining the correct width and thickness? You mentioned the  beams John uses, the 6X12s. Maybe 2, 6X6 beams each 14', connected at the center would be OK.

On the steel roof, I did mean steel roof on purlins. However, I think putting steel on 7/16 sheathing would be lighter than asphalt shingles on 7/16 sheathing. The asphalt shingles, I guessing, weigh 120 lbs or so per 100 SF and I'm guessing the steel (29 gauge) is lighter. I didn't think about the possibility of dripping condensation.

There's a book, "How to Build your Dream Cabin in the Woods" by a biologist/hunter/writer. His cabin is built with 2Xs just nailed to railroad ties with steel roof on purlins. Of course he reports success with this, but I'm glad I'm checking around.

Dave

[MountainDon]

I'm not a certified engineer, okay.

The calculator for joists and raters can not be used for calculating beam loads.

The beam size determination requires a look at the total structure plus environmental issues like snow load. A "trace load" calculation has to be done. Whether there are only two side beams or a 3 and ourth center beams(s) the load of the entire cabin/house is on the side beams. The central beam(s) will take some of the interior load, but not much, if any, o the actual structure. So generally speaking one half of the roof load, including snow, is transferred down to the side walls. In a buildings with no center beams all the floor loads, furniture, people, framing materials, drywall, appliances, water beds, is transferred out to the beams; for calculations it is assumed that one half goes to each beam. The weight of the windows and the exterior siding has to be included as well, of course. If there is a porch hung on one side then one half of the porch weight, plus party people load, is added to that one beam. So it can get complicated.

So my thinking is that if John uses a built up 6x12 under each side wall on his 20 foot wide plans, then they would be adequate for an 18 foot wide. The central beam will take some (1/2?) of the floor load (floor materials, plus furnishings and people); I don't know how much. A 6x12 in the center would likely be larger than necessary, but I'd be guessing at what size to go down to. 6x10? 4x8?


On the roofing, we used 26 gauge metal. It resists hail damage (dimpling) better. We get some hail, especially in the mountains, so that was a consideration.


I don't recall if you mentioned this to be a single story, loft or no loft, 1 1/2 story or what? I was wondering about the ceiling joists. Are you planning on having the 18 foot span open inside with no load bearing interior walls? The joist calculator can be used for whatever is to be used.

[daverave]

MtnDon,

I found plans for an "Alaskan Trapper" type cabin by Sheldon Designs, but I haven't bought them yet. I know they call out for a slab foundation but I've really got to use a post and beam one  because of cost and the steep slope I'm on (about 17 degrees) I think what I might do is sort of synthesize the designs of Sheldon and Raabe. If I buy John's plans for a similar sized cabin, I can use his foundation design and place Sheldon's simple cabin design with extended porch roof on that foundation. That way the engineering for roof, walls and foundation is done for me, pretty much. What do you of that approach? Sheldon's cabin is very simple, no interior walls except for a bathroom. I'd like to build a 10' long loft though over the bath/kitchen area.

What do you mean by 6X12?  4 laminated 2X12s?


Dave

[MountainDon]

That sounds like an idea that would work. John's 20x30 has three foundation plan sheets;
• A wood floor on a post & beam foundation
• A wood floor on a concrete crawlspace wall
• A concrete slab foundation

http://www.jshow.com/y2k/listings/46.html

You could price out the pier and beam vs concrete crawlspace. Dry stack block with concrete and rebar fillers and SBC (surface bonding cement) has been used by at least two of our members.

[Don_P]

For many situations the girder and header charts in the codebook will work, and don't require an engineer. Tables R502.5(1) & (2);
https://www2.iccsafe.org/states/Virginia/Residential/PDFs/Chapter%205_Floors.pdf

(To answer your ? MD, yes the center girder takes half the floor load IF the floor joists are lapped, the outsides take 1/4 the floor load, don't forget to look up. If a continuous joist is used the center takes 5/8 of the load and each end takes 3/16).

[daverave]

Don_P and MtnDon,

I took a look at the girder and beam span chart, Chapter 5 of Virginia Building Code. A conservative design would use 50 PSF (I'm in WI) so, since my cabin will be 18 feet wide and each span from foundation post to foundation post will be 7 feet, and using "Roof, Ceiling and one center bearing floor", the chart says: 3-2X10s. Would you both say I'm using the chart correctly, even though your own preferences may be different (4-2X8s would also work)? The joists will be 9 feet long, from beam to beam and for those the joist calculator works.

Dave

[MountainDon]

It looks like you are reading the chart correctly.   

Three 2x10 or four 2x8 would work. I might choose the four 2x8 if my 6x6 piers were rough cut 6x6. The lamination of the four 2x8 would be the same width as the rough 6x6.