Cabin design: Foundation and 1st floor

This is only a few hours of work with a lot of analysis that still needs to be done. I’m not an engineer so you’ll need to run these designs past one if you want to use them to build your own cabin. I’m not building a “Tiny House” per-se, since some of those are half this size, but it is a “Small House” or small cabin at 16 x 16 feet and two floors.

I’ve added copyrights and disclaimers all over the designs (this is very unusual for me since most content on this blog is in the public domain) only because I do not under any circumstances want to be sued! If you do find these plans useful or if you have improvements, I’d love to hear about them. Post in the comments or drop me a line via email.

The Foundation

I’m thinking the structure should be built on a pier foundation since it will be the most flexible if the land I choose is uneven. And since a slab on grade would be too expensive and a full basement would be both too complicated and expensive. Now there are local code variations when it comes to pier foundations, so it’s best to look those up on your local county and state websites or call the local building department.

Here in New York, most pier foundations are built with a concrete footing under treated lumber. The depth is usually 4 feet + 6 inches or so of gravel to allow for drainage, but it may change if your frost line is higher or deeper. Even though this isn’t the standard for all locations, it’s best to make this the minimum since it will account for the frost line in most cold climates in the Northeast.

Note: There is a 1″ standoff anchor connecting the post to the concrete pier since this will prevent premature decay of the wood. Yes, even treated lumber can decay if it’s kept in contact with water for too long so a standoff anchor is not only part of most building codes, it’s also a good idea.

There will be up-thrust down-thrust (see Wally’s comment below) as water takes more volume as it freezes and you want to make sure the concrete is well below the frost line to prevent this from happening.

I don’t know if the rest of this is 100% sound so please consider carefully before blindly copying (disclaimers notwithstanding). I’m thinking of using double 2 x 12 treated lumber as beams since I’m hoping to avoid a solid beam (extra weight/cost), but if this isn’t enough, I’ll need to go with a solid beam anyway.

When using 4 x 4 posts, generally a 10″ diameter concrete pier would be the minimum, however for 6 x 6 posts, you’ll need at least 14″. I’m going to go with 6 x 6 posts with 18″ for insurance since I do not want my cabin to move once it’s built. If 6 x 6 isn’t strong enough, I’ll have to bump that up to 8 x 8 posts with 20″ diameter concrete pier.

I’ve seen a lot of examples out there that show posts connected to beams via carriage bolts alone and I think this is a very poor choice from an engineering perspective. The entire load is then depending on the shear strength of the bolts. A better option would be to notch the posts in, rest the beams on the notch and then attach them via carriage bolts since that would ensure the post is bearing the load and not the bolts; their job should be just to keep the beams attached to the post, not to bear weight.

Notching the post in and setting your beams is better than having the carriage bolts carry the weight of the beams alone.

You’ll also need support braces between posts since the span is over 4 feet (it will be 8 feet specifically) with 9 posts total since this is a 16 x 16 cabin. As mentioned above, if this isn’t good enough, I’ll have to go with a solid beam and 16 post layout every 4 feet. It will mostly depend on soil conditions.

This center post arrangement adds a great deal of stability to the structure. Naturally, the posts in the corner can only have one brace. This too must be notched as mentioned above and not merely rest on the carriage bolts.

Here’s the post and beam layout. Note, even though it looks like I’ve wrapped the beams around the post, they are to be notched. I’m just putting them outside the post for emphasis.

Posts beams and joists laid out. Note the measurements are for mostly 24 on center.

I went with 24″ on center to allow for more insulation and I think I can get away with it since the joists will be 2 x 12 (just like the beams), I’ll be using 23/32 or at least 5/8 plywood for the subfloor with hardwood flooring on top. This should also keep deflection down even with 24″ joists.

The 1st Floor

After going over some of the ideas from my last post on designing the bathroom, I realized that it would be best to put most of the plumbing inside a shared non-load-bearing wall with the kitchen. This would simplify both plumbing and electrical runs and I wouldn’t need to knock out holes in critical structure.

All the windows are 24″ wide (height to determined) and the window on the staircase will be at the same height off the landing as the rest of the windows are off the floor.

Lot of stuff happening here. The bathroom can be elevated a foot off the floor giving a 7 foot clearance to the ceiling if the rest of the floor is 8 feet (96″). This elevation will allow me to run plumbing underneath the bathroom without cutting into the load bearing joists below. It will also alow me to reach the shower lines without too much trouble. The small square in the middle is actually the load bearing column continuing from the floor to the ceiling. The 2nd floor joists will also be tied to this post.

The sink location is variable, but I want it near a window so I can look outside while washing the dishes. The landing and stairs heading toward the front door can house a closet so I’ll have plenty of storage space.

The one thing I really wanted to make sure was to have all “walking areas” at least 36″ wide. This is just a personal preference since I like to walk up and down a lot, especially when I’m thinking, and I want to make sure there’s nothing in my way. The copious amount of windows is to also make sure I don’t feel cramped. I figure there’s plenty of room for a convertible couch and maybe a couple of futons on the first floor alone in case I have folks drop by to stay overnight.

Finally, here’s the layout and foundation superimposed.

Note how 24″ on center joists and 24″ wide windows go well together. Also note, that if I do need to cut under the floor, I don’t need to cut any joists to get to the plumbing wall since they’re between joists, not on or across them.

The second floor will more or less be completely open. I don’t want to have too many walls in between main areas (in fact, I’d rather not have any interior walls at all) and notice the only wall on the 1st floor is to separate the bathroom and to provide plumbing and electrical space. I think the 2nd floor won’t extend the full length of the cabin and instead will open up to the 1st floor via a balcony facing the front door. This will make the place feel much larger and also let me see who’s at the door without coming downstairs.

I’ll come up with something on it later.

Enjoy!

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