Floor Joists – Part III: subfloor

almost done. having a door (not installed yet) shows the scale…

In the last post, I finished the joist installation. Before installing the 4×8 3/4″ OSB panels for the subfloor, I wanted to get everything as near as possible to level.

The set up

The 2×12 double beams are made out of 20 foot long boards. They are not perfectly straight over that distance. When I installed them, I checked carefully for any crown in the boards, and placed it up. But you don’t want your joists following the crown of the beam. I used a water level to level the joists, and followed that line when installing them. The idea was to shave off any discrepancies from the 2×12 double beams after the joists were in.

I bought an electric planer, rechecked and marked any high points in the beams or joists with the water level. I found that the rim joists had quite a bit of warp to them. The rim joists are made from lumber that I got for free from a guy when I was trying to get 2×10 sleepers for the roof. He threw them in for free because he just wanted to get rid of them. Turns out he gave me 160′ of 2×12’s – enough to use as rim joists throughout the house. But they were also the most warped of my lumber – probably because they sat around at his house for a few years. But they weren’t warped too much – the worst one was probably 1/2″ out of wack. So I cut off the excess rebar on the piers, and planed everything down.

The problem with a “real log” home…

…is that logs aren’t straight. yes, it’s true. There is no reference point for laying out the floor, nothing to say, “use this as your zero and measure everything off of it.” One website said to line up your floor with the longest wall in your home….which we don’t have yet. Which is another difference between a log home and a regular home- I can install the entire finished floor right now because none of my interior walls hold up the next floor or the roof. But it’s extremely easy to mess up what should be a square home, but isn’t because of crooked logs.

So I started with the joists. When I laid them out, I started in the center of the house so a line stretched through the house would intersect the RPSL’s and GSL’s in the middle. Some folks would say just box around the RPSL’s so you don’t have to do a half joist in front of them. But it didn’t matter on my RPSL’s since they are much bigger than 16″.


I marked every 16″ across the entire house. At the corners, I made sure the space between the last joist and the rim joist was less than 16″ to meet code. It didn’t have to be perfect, but within an inch or so of all the other corners was fine.

The must-haves:

  • make sure the joists were exactly parallel with each other, across all 3 sections of joists
  • make sure all joists were exactly 16″ apart

The nice to have was:

  • that the floor joists were more or less parallel with the edges of the house.

So, really, you kind of ignore the crooked walls, and just do the best you can.

Installing the subfloor

The plans say to use 3/4″ 4×8 plywood or OSB. So I called my favorite supplier, and they delivered it the next day. I started laying it out on the joists right away.

It was nice, for the first time, to have a hard dry surface to stand on. It really is a milestone when you get a floor in. Up until now, all we’ve had for surfaces to stand on has been “the ground”. But without it being glued and nailed, we still had to watch our step to avoid falling through the joists.

Everyone loves math

Ha ha, just kidding. But I did use math to make sure my subfloor isn’t all wonky and sits on the joists properly.

First, I decided that it would be easier to start on a row of subfloor that I could lay all the way across the house. I thought about starting at one wall, and working my way towards another wall, but figured if my measurements were just a bit off, 40′ is a long way to magnify the problem. Much better to err at half that distance – so start in the middle of the house. The 8′ side of the OSB should be perpendicular to the joists, while the 4′ side should be parallel with them. They also need to be laid out like bricks so the seams of the 4′ edge don’t line up all the way across the house. How to get the OSB perpendicular? I used a 3-4-5 triangle:Capture

  1. I took a string and stretched it on one side of the RPSL/GSL combo near the middle of the house, making sure that when the OSB pattern reached the front and back of the house that I wouldn’t have a 1′ strip of OSB up against the wall. It would be ideal if the last gap was 4′ wide. Next best would be a 3′ gap. So I placed the string at a point near the middle where I could make this happen.
  2. With the string nailed temporarily in place to a joist on near the wall, I marked a spot on the string 4′ from the nail (right in the center of a joist, if you’re paying attention to the 16″ spacing).
  3. I marked another line on the same joist 3′ away from the first mark (the red lines).
  4. Starting at the nail, I used my tape measure to find at what point on the joist my 5′ mark on the tape measure would hit the center of the joist.
  5. I had to move the nail several times before I felt confident I had a perfect 3-4-5 triangle (the double arrow in the diagram represents moving the nail on the string (in green). I also used a large framing square to check several points on other joists along the string to see if the string and the joist it crossed were set at a right angle.
  6. Once I felt like I had the correct spot, I got my daughter to snap a chalk line (the coolest job in carpentry) where the string had been. The chalk line is the reference point for laying out the OSB subfloor.

Getting around the RPSL’s and GSL was my wife’s job, but cutting the OSB is my job. We make a good team!

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Getting up to the wall and making the OSB fit against the first row of chinking was also my job. I used a regular compass to trace the bumpy line of the wall onto the OSB, and then cut off and fit into place.

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Once each panel is rough-fitted, I apply construction glue and then ring-shanked nails to permanentize everything. Yes I made that word up. And THAT is when it feels like a milestone. My daughter has been waiting the entire build to ride her bike inside the house. She said, “I really like this, Daddy, it’s so solid it feels like I’m riding on a sidewalk!”


Next steps

With the subfloor installed, it’s time to figure out where the doors and windows go. Once that is done, we can get going on more chinking, at least on the outside. The inside will have to wait for the electrical to get finished.

Meanwhile, my wife is excited to put down tape where all the walls should go. She’s been waiting for that moment the entire build because with crooked and tapered logs, you don’t know exactly how much room you’ll have in the bathroom, or anywhere else, really. The logs are really big and take up a lot of space on the inside of the house.

When we place the finished floor down, I’ll have to repeat the 3-4-5 process to line them up as well.

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