Drop a marble to the floor of an old house and it’ll probably roll a bit until it gets stuck in a cupped floorboard or a valley where a whole section of floor has sunk. The reasons for this are legion. The joists that the floor and subfloor rest on can twist, bow, and shrink. The beams supporting the joists, the posts under the beams, and the foundations anchoring the posts can settle. What’s even worse is that electricians and plumbers, amateur and pro, seem to have little compunction about chopping away at beams and joists, notching them at the edges, and drilling
large holes to accommodate the passage of small wires. Then there’s water, probably the most destructive substance a house ordinarily encounters. Leaking radiators and loose toilet gaskets (a $7 ring of wax that can produce tens of thousands of dollars in damage if it’s not kept in good shape) keep subfloors and joists wet, introducing rot and, sometimes, the insects that rotten wood beckons to.
Our old house suffers from all these things except insects. Its original finish floor (birch or maple planks painted red) was a rolling field of little hills and valleys, high near the front of the house and 2 ½” lower in the centre (near the leaky bathroom toilet, of course). We want, eventually, to install solid or engineered wood flooring but, as with all renovation, “want” is preceded by “must.”
The must in this case is making the floor flat so the new flooring won’t look wavy, particularly considering that we’re changing the floor plan to produce longer sight lines and a feeling of openness. A distinction has to be made between level and flat, though. The flooring material you put down doesn’t give a damn about level. You can put a perfect floor on a surface that slopes 15 degrees; what matters is that it’s flat. Lay a long straightedge on it, peer under the straightedge, and see no light. That’s flat. Put a level on it and watch the bubble settle in the middle. That’s level.
Since we had to tear up the floor and subfloor, (the progress of that “peeling” shown below) we thought we might as well go for broke and make the whole thing flat and level. But how to do that, considering the joists, beams, and all the rest were anything but flat and level?
We consulted our great structural engineer, Jimmy Vathis, about the floor and he hemmed and hawed for a minute because he wanted to save us money and grief. At which point, Patricia asked him, flat out, “What would you do if it were your house?”
“Sister the joists,” he said, without hesitation.
Sisterhood, in this case, really is powerful. Sistering usually involves taking a new piece of lumber the same size as the original joist and gluing and screwing the old and new together. The new, sistered joist may also be levelled and placed in careful relation to the other sisters so their tops form a plane that’s both level and flat, a democratic sorority of infrastructure for a new floor.
What about floor levelling compound? No. It’s very heavy, the subfloor in many old houses is in bad shape with large gaps between boards, and you can’t drive nails into levelling compound when you install the finish floor. Shimming? Well, I suppose, but the thought of cutting dozens of 16-foot wood strips to bring each joist up to the proper height at the proper angle is a recipe for madness.
Sistering the joists was the answer I was sure we’d hear from Jimmy, and the one I most dreaded hearing. It meant that, working alone, I would have to glue and screw 16-foot 2”x 8”s to the old joists while making sure they were level and all positioned at exactly the same height to produce a flat plane. There was a modest saving grace, though. Since the original joists were all strong and in good condition, the new, sistered joists only had to contact them for 4 inches. That meant I could do the sistering with smaller, and less expensive, dimensional lumber.
The first one took me four hours to put in. But before we got to the point of gluin’ it up, I had to do some careful measuring. I bought a DeWalt laser level and set it up in the middle of the floor, attached to a post. It projected a red line around the walls about two feet above the joists. Then I taped an ordinary bubble level to a yardstick so I could hold the yardstick perfectly plumb as it sat on a joist and measure its height. From there, it was a matter of hopping from joist to joist and finding the highest spot in the entire floor. Why? So the new, sistered joists could be set to exactly the same height. Flat and level? Easy.
Not. It’s still a 120-year-old house, and those joists are twisted and tilted, when means that attaching new 2”x8”s and expecting them to sit square to the face of the old joists is just not going to happen. I did the best I could, using lots of nails, lots of screws, and sometimes filling gaps with shims. I also discovered that the space between joists is anything but the modern standard of 16”. The spacing varies from 21” to 26”. That’s important because the OSB subfloor I’m putting down over them is rated 24” OC. Which is to say that it’s only acceptable to my engineer and to building codes if the maximum space from the centre of one joist to the next is no more than 24”. So, I sometimes had to sister on both sides of the original joist to decrease the distant between them. And before I’m done, I’ll have to hang some entirely new joists using steel joist hangers that can be nailed into the beams that cross the joists at right angles every 12 to 16 feet.
One of the worst problems of working alone in this situation is just that, working alone. I made a jig to hold the new joist in place (from under the floor) while I ran upstairs to make small adjustments in the height and slope of the joist. It was inefficient, tiring, time consuming, and generally crazy-making. Finally, I decided to rip a flat edge, using a Makita track saw and guide (though the factory edge of a 4 foot or longer panel and a regular circular saw would also do a good job) on a 12-foot 2”x 8” and use that as a guide.
Here are the first steps to achieving flat and level:
• Once the high point of the floor is determined, attach sisters at the proper height to joists separated by a distance equal to the length of the straightedge
• Clamp the straight edge across the ends of the newly sistered joists
• Install new sisters between them by pushing the new lumber firmly to the bottom of the straight edge. That way, you don’t have to measure (thereby introducing errors) to have the sisters all at the same height
• Move straight edges once new sisters are glued and screwed, and repeat.
This method has saved me a bunch of time and hassle. Carpenters avoid measuring when they can because it takes time and introduces error.
After doing that for a bunch of joists and laying down a couple of OSB 4 x 8s, I was worried about how wavy (not flat) the floor would be. It’s not dead flat, but the greatest error I could find was about 0.075” of light shining underneath my long straight edge. Good enough.
By the way, this long-delayed blog is flying under a WordPress search category I’m using for the first time: Construction. Long time comin’.