Floor joists – Part I: Rim Joists

Note: This happened before the chinking post, but I got too excited about chinking! So here you go….

The setup

Staining is done. Gutters are done. Logs are dry. But the ground inside was staying moist – like, all the time. I thought after I got the roof on things would eventually dry out inside the cabin. Weirdly, they didn’t. Our water table is somewhat high, but not overly high- I dug down 4 feet and didn’t find any water. It must just be “the humid South”. I researched and found that installing a vapor barrier – basically 6 mil plastic (like really, really thick garbage bag material) held down with gravel or pins would help. At least two folks over on LHBA did this and it really helped. I can attest that is true – it is unbelievably drier inside now to the point that I’m not worried about mold on my joists anymore. Nice. It was, as usual, a lot of work.


Now, onto: “How to build a floor inside a log cabin on piers”. We had to consider:

  • how to start out level
  • what are rim joists
  • attaching to inner support piers
  • span lengths (follow the plans – no problem)
  • floor joists – 16″ on center, parallel wall to wall, all the way throughout the house

how to start out level:

I didn’t trust that my piers are level. I was sure that I didn’t set them up perfectly when I poured the concrete. Also, with all that weight on them (I seriously estimate the weight of this house at about 290 tons, compared to a standard home at around 35 tons), it’s possible the piers have settled a bit. Not by a lot – probably less than 1″. But I want the floor to be as level as possible (secretly, I want the title of “flattest floor on Earth”). My neighbor had been offering his 360 degree laser level ever since I showed him the inside last year. The laser level sits on a tripod, and spins out a laser line. You just go around and mark the line it shoots out on the wall, then line everything up.


I tried it. It was too dim to see the laser line. Even inside the dark cabin, the bright sunny day was too much for the laser. Also, the adjusting knobs to fine-tune making it level were broken or stuck- I don’t know. I resorted to leveling it using the tripod (pain in the….), and also making the laser stationary (not spin), and had my wife manually turn the dial while I drew short lines on the wall every few feet.

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But when I went to set the first few joists – nope. Something wasn’t right- my joists didn’t seem to be level, according to the line I drew on the wall. I looked online and found a guy on “This old House” saying he doesn’t trust any method of leveling a foundation – string level, laser, or water in a tube – he always double-checks one with another. I bought some clear tubing from the hardware store, and got my wife to help me. Took us a bit to figure out the water level, but the results were impressive. The lines I drew were only about 1/16″ thick, yet we could both tell a difference with the water level when it was at the bottom, middle, or top of the laser line.


Working our way around the inside of the house, we found there were 2 or 3 lines from the laser level that were off by just a bit – less than an inch, but enough that we drew new lines using the water level. I made a video and a “job aid” on leveling rim joists:

video: using a water level

job aid: leveling-rim-joists

Rim Joists:

These are bolted to the 1st row of logs and rest on the piers along with the logs. They are just 2×12’s. You simply shave off a bit of the 1st layer of logs if there are knots, and use 1/2″ x 6″ – 8″ lag bolts every 2 feet. After putting up a few joists, I found that one of the joists I put up had quite a bow in it on the end that threw off the joist next to it. The water level helped verify and set me at ease on the lines. Live and learn.


Inner support piers

There are 6 inner piers on our plans. They are 13-ish feet apart N-S, and almost 10 feet apart the other direction. A double 2×12 beam spans this area. The floor joists are attached every 16″ to them or to a rim joist. Maybe a picture will help. The yellow line represents the double 2×12, the orange line represents one (of the 96) 2×12 floor joists:


Span lengths

I found out while researching building a deck a rule of thumb on span lengths- essentially, you can span in feet 1.5 times the number of inches that your board is thick. So, a 12″ board can span 18 feet. Shorter spans are always better (and more expensive).  The longest span is 13 feet on my design. 

With the rim joists installed, I went and chinked the first row because the chinking will partially cover some of the floor. 

floor joists

There is an engineered product called an i-beam, which is basically a letter “I” where the vertical beam is a piece of OSB, and the top and bottom are a laminated plywood. They are really strong, and very straight. And are really susceptible to water damage. And more expensive. So I said no. The floor joists are hung on the rim joists or double beam with Simpson or similar hangers. The ones I bought require 12 nails each. ugh. Oh well. At least my imaginary pet elephant will be happy.

Let’s roll (muh Trump check)

yup. I’m converting that cash into lumber. I consider money pretty unstable at this point. The faster I can turn it into a house, the better off I’ll be.

Next steps

I’ll install the floor joists, and then after checking and rechecking for level, glue and nail the OSB subfloor. 

At some point, I’ll need to insulate the floor (code). But talking it over with my wife, we decided this can be done at any point. I think it would be easier to get my electrical and plumbing installed and then go back and do the floor insulation, so it’ll have to wait for now. But instead of just 1/4″ metal screen under the floor joists to hold the insulation in and the critters out, I think I’m going to copy my friend Ivan and install 1/2″ OSB to the bottom of the joists. The vapor barrier goes to the “warm side”. I found out that in the South, it’s more often warm than cold outside, so our vapor barriers should be installed towards the outside of the insulation. In the cold North, it’s often warmer inside the house (a full 6 months of winter, plus cool Spring and Fall weather), so the vapor barrier goes on top of the insulation, and under the subfloor.

Then we’ll think about windows and doors, chinking, framing the gable ends, 2nd floor, etc. All that stuff could be done at any time, but I’d really like to have a subfloor to roll my scaffolding around on while I work on the gables, getting the last of the bark off the ridge pole, cutting windows, and etc.

The plan is now for me to build the doors. Exciting and scary. I want them to be strong, but I’ve never built a door before. Up until now, anyone that wants to see the inside of our home has to crawl under the logs and stand on dirt to look up at it. In my opinion, it’ll be a milestone when we can open a door and walk in. Also makes a more secure place to store materials.

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