A cupped wood floor is no stranger to me. Perhaps as much as 80% of floors I see when I’m out surveying, has at least some cupping in them. Even if only very shallow. Sometimes no solution is necessary, sometimes it can flatten out on its own. However, sometimes something has to be done about it. The first question that comes to most homeowners’ minds is whether or not cupped floors can be sanded and refinished.
Cupped wood flooring can be sanded and refinished. However, due to the nature of cupped floors, it’s important to try to normalize the moisture levels in the environment first. Cupped floors that have been sanded and refinished before trying to normalize moisture levels may end up crowning.
Should you sand and finish your cupped wood flooring?
As I mentioned earlier, I see a lot of cupped wood floors. The majority of the time it has been safe for me to sand and refinish them. These days I try to talk my customers into finding the cause and fixing it before we move forward.
I suppose this is a great time to recommend my guide to cupped wood flooring blog post. I highly recommend reading this if you want to pinpoint the cause of your cupping, resolve the issue, and best practices for getting the most out of your floors.
In the vast majority of cases, these floors have changed into this cupped position over time. This means that while there appears to be a change in moisture in the subfloor beneath the blocks, the floor is in quite a stable condition.
Sanding and refinishing these cupped floors will get them flat and smooth and will look great, without repercussions. Therefore, sand away.
Theoretically, though, wood flooring expands when wet and contracts when dry. This means that if we sand the floor and the conditions that led to the cupping return to normal, (again, highly recommend you read this post on cupping!) the now-flat floor could “uncup” and end up “crowning”
What is crowning?
Wood floors that have crowned have the opposite problem of cupping. The center of the board is higher than the edges, giving it a cambered, dome-like appearance.
While this can also be caused by abnormal moisture levels, in this instance it’s caused by sanding the floor while it was cupped, and then the moisture returning to normal.
Sanding off the high peaked edges of the boards, while they were cupped, means that the sides of the boards are thinner. So if the underside dries out, and those edges go down, it will have this crowned effect.
Sanding cupped engineered flooring
Sanding engineered wood flooring that has a cupping issue presents a new problem of its own. The hardwood surface may get completely removed on those raised edges if the veneer of the flooring is too thin.
That would mean the floor needs to be replaced. Not good.
This is a rare situation though. One of the many benefits of engineered wood flooring is that it is more stable against these kinds of pressures. The ply base of engineered wood flooring doesn’t expand and contract in the same way as solid wood flooring.
To have that bad cupping, and that thin a veneer, as I say, is rare. It’s more likely that this kind of flooring with too much moisture is going to buckle, which is a completely different story. That has to be fixed before sanding.
How to sand cupped wood flooring
You’ve decided that the cupping is mild enough to sand the floor, so you have rented the sanding equipment and you’re ready to start grinding down those ridges. The question is, how should you approach this unique situation?
Floors like this will not turn out well without using a big sander
Leveling hardwood flooring involves cross-cutting the floor. That’s sanding the floor at 30-45 degrees to the direction of the boards. However, on this occasion, it may be better to start the process going with the grain of the boards.
Try positioning the peaks of those ridges in the center of the drum on each pass to focus the machines on those raised areas. It’s unlikely that these ridges will be completely removed with a single pass of the big machine. That’s ok and even preferred.
If you have narrow boards and there are several ‘ridges’ beneath the drum at any one time, then it may be more important to pay attention to the level of the machine. That means, paying more attention to the positioning of the wheels on the floor than the drum.
I’m making it sound more complicated than it really is. Just make sure the drum is level rather than tilted over because one of the wheels is running on a ridge.
Start with 36 or 40 Grit
So bringing it back to the simple. First, do a straight cut to bring down the ridges a bit, then the next pass on the diagonal. It’s likely both of these may need to be 36/40 grit and perhaps even the third pass!
Often uneven flooring like this can take several passes with the rough grit to get flat. If you’re concerned about taking off too much wood, then your goal should be to get to the point where the whole floor has been sanded (including the lower center of the boards) but the center of the boards is slightly darker than the edges.
This indicates it needs to come down a tiny bit more to get them back to clear bare wood. If you start climbing the grits from this point, it means that those darker patches will come out at a higher grit and closer to the end of the sanding process. I never know when I’m going too far with the details on these posts LOL
Also, sometimes I really don’t know if I’m talking to no one! Please let me know that writing this was worth it in the comment section below!
Hi thanks for the info…. I put down the floor 2 year after a flood and it cupped right away it’s now 6 months
If the floor is sanded and they may be more moisture that means it can crown ?that’s crazy .. Not sure what to do this will be 1st heating
Hello Ben,
I’m about to start a floor refinishing project. It’s my first but I’ve got a fair amount of finish carpentry experience. Has to lend some benefit. This house renovation project I’m doing presently has had roof leaks unattended for 20 years. The flooring is the thinnest hardwood flooring I’ve ever seen, about 3/8″. Lots of cupping. Thankfully, it has never been refinished.
I don’t expect it to look beautiful. I expect it to look better. Less bad. And, if I’m diligent it could even – gasp – come out pretty good.
My intention is renting a flat rather than disc sander from our local Home Despot. I may have to have them order a coarser grit paper for me. When I first went in to inquire they start with #80 grit.
I wish I’d read this article before starting to sand an old pine floor in an unheated room! Once we move in, the heating will come on and I guess I’m in for a nasty surprise. Fortunately, the crowning wasn’t bad for a 100 year old pine floor, so I still have hope.
I’m mainly commenting to say that your articles are read and enjoyed by me at least. There’s no other reliable resource for us DIYers. I also bought your course/ebook, and it’s given me far better results than I expected from these intimidating machines. Not finished yet, and I’ve sent you a couple of questions.
So please keep posting Ben. I have around six old pine floors to sand over the next few years as we renovate the house.
Chuffed to have a comment on here already 🙂 Yes, don’t worry about the floor too much until you’re in there and the heating is on. Glad you’re enjoying the video course!