Wed. May 29th, 2024

Replacing Hardwood (Part 6): Subfloor

Now that our floor is good and shimmed (referred to in our house as the sub-subfloor), it’s time to work on the subfloor.

The subfloor is a layer of wood boards or plywood that sits between the joists of the house and the finished hardwood. Our house currently has its original subfloor: wide wooden planks that were screwed into the joists of the house.

I went back and forth about whether or not to keep the original subfloor (and just sand it down to get it flat and level), but the deciding factor was that these boards would be parallel to the installed hardwood. Ideally, the boards should be perpendicular to the hardwood, so this wouldn’t have worked. We also could have ripped up this subfloor and simply installed the new plywood directly on the joists, but this would have involved a lot of work, so we decided to just put a new subfloor on top.

Any book or website about installing wood floors offers one consistent piece of advice: the finished hardwood floor is only as good as the subfloor.

Flat v. Level

All subfloors should be flat and level. These two things are different: flat means that there are no peaks and valleys and level means, well, level.

Our floor is relatively level. Other than a slight slope towards the front of the house, which we fixed with our shimming job, we don’t have a major level problem. Compared to a new house, it’s defintiely not level. But considering our house is 100+ years old, it’s alright.

The major problem with our floor is in its flatness. There are several peaks and valleys throughout the room. Because of this, we decided to install a new subfloor on top of the original subfloor to help with this unevenness issue.

OSB v. Plywood

Most contractors use OSB (oriented strand board) plywood to lay the subfloor. It’s much cheaper than plywood ($15 per 4’x8′ sheet compared to $31), which can really add up when you’re covering a large space.

Plywood, on the other hand, is much stronger than OSB and doesn’t have the same swelling issues when in contact with moisture. Most of my reliable resources on the subject recommended real plywood, so that’s what we got.

Tonge and Groove

Plywood sheets are available in tongue-and-groove. This feature is great for flooring because it helps reinforce the strength of the subfloor when minimizing those wonky areas. It’s more expensive than regular plywood sheets, but again, advice pointed towards t&g, so that’s what we got.


Before we bought our plywood, I measured the room to make sure we’d be buying the right amount. At $30/sheet, I didn’t want 5 extra pieces laying around that I didn’t need. The sheets are available in 4’x8′, but this actually means 4′ including the tongue. This is important because when they snap together, the tongue will fit into the adjacent piece and you essentially lose the width. It’s not a huge deal, but if you’re total width is a multiple of 4, it could mean an extra row of plywood.

Our house is about 17′. Four rows of 4’x8′ plywood gets us to 16′ – so we were 1′ short of being able to get away with 4 sheets across. Annoyingly, we couldn’t just tack on an extra foot at the end because that 1′ piece of plywood wouldn’t be strong enough to support any weight. So, I had to make a plan on how to distribute the boards. We also had some challenges with the location of the load-bearing wall, since we also didn’t want to have small 1′ pieces on either side of the wall. It was a challenging puzzle, but I finally came up with a plan:


Because our house isn’t a perfect square, we spent a lot of time trying to figure out what our straight line would be. If we measured out from our exterior wall to determine a straight line, it didn’t line up with the window or the other walls and looked really crooked. So, we needed to figure out what looked right, instead of what was right. We did most of the work during our hearth installation, and determined that the original subfloor boards were this straight line. 

As we covered the floor, I wanted to make sure that we knew where those floorboards were since they would soon disappear. In the main area, I was able to mark the walls on either end, which we will eventually snap a chalk line from to give up our starting point for the hardwood.

I also made marks along the way to make sure that our future chalk line will be “straight.”


The first piece was installed in the vestibule. Mostly because it was a tricky spot and I like to get the worst over first, but also because it was exactly the width of one sheet, and I wanted to confirm this before moving on to the others (in case we needed to adjust the plan).

Before we could install the plywood, we had to cut away the door trim to make way for the hardwood. By removing the bottom of the trim, we can now install the hardwood slightly under the trim so that there’s no gap. I used a scrap of the subfloor and a piece of our hardwood as a guide, making sure to put the hardwood bottom side up to protect the top.

Because of the doorjamb, we had to install the first piece in 2 pieces. It took about an hour or two to measure and make all the cuts (including a hole for the vent) and to clean the sub-subfloor thoroughly. Then we screwed the subfloor directly into the joists. Alex went to the basement to confirm that the screws were hitting the joists and not coming out just beside them. I think I had a failure rate of around 20%, which wasn’t bad.

Main Area

After the vestibule was done, it was on to bigger and better things.

The first half of the room was pretty easy because we installed entire sheets of plywood. The second half of the room was less so because they required cuts.

Each piece was also securely fastened using 3½” wood screws. I did most of the screwing while Alex stood in the basement to make sure I hit all the joists. My back and legs were pretty sore from this, so Alex took over for a while.

Alex was also a superstar at cutting the plywood.

To install the subfloor near the stairs, I had to cut away the bottom part of the stair post so the hardwood can slide underneath it. It was particularly challenging because it’s solid wood and wasn’t easy to cut through. Result: wood massacre.

It took us longer than it should have because I spent a lot of time cleaning all the years of crap that had accumulated in the gaps of the original subfloor. I cleaned it by taking an old nail and scraping out the dirt, dog hair and other random stuff I’d rather not think about. I think each piece took us an average of 1 hour to install and we had 16 pieces of plywood total…

By the end of the weekend we were sore and tired, but managed to get almost all of the subfloor installed!

The only part left was the area in front of the kitchen, which would have to wait.

The following weekend we didn’t get much work done because I ran a marathon. That’s right. A MARATHON.

Here I am at around 41.1km. Almost at the finish line!

But then I came home and insisted that we finish the subfloor (because I’m clearly crazy). We were just so close and I have a schedule I’m trying to keep up with! I couldn’t move much, so Alex did most of it on his own while I sat and watched.

Next step: Painting our walls so we can install the floors! These are exciting times!!

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