If you want the floor above to look and feel good — you need to start with the floor below.
The subfloor.
It’s easy to obsess over the surface; think about the finish, the grain direction, and colour variations of your floorboards. But what about that layer sitting directly underneath? The one that has such a massive impact on how the floor performs — but rarely gets any of the attention?
Today, the spotlight is on subfloors. Especially for those working with quality timbers like oak hardwood — where air dried oak beams demand a properly prepared base to perform at their best.
What you’ll discover:
- What Is a Subfloor?
- The #1 Enemy of Every Subfloor
- What Makes a Subfloor Great?
- Air Dried Oak Beams and Subfloor Compatibility
- Before You Lay The First Board
- Wrapping Up
What Is a Subfloor?
So what is a subfloor?
In the simplest terms, it’s the layer that exists between the floor joists and the surface people actually walk on. Think of it like the stage that everything else is built upon.
Typically speaking, there are two main types of subfloor used in new construction:
- Plywood — by far the most popular subfloor choice when working with timber framed buildings. It’s strong. Stable. And handles screw fixings and other fasteners extremely well.
- Concrete — found in many newer builds. Ground floor extensions, and slab constructions typically use a concrete base.
Irrespective of which type you’re working with, both need to be prepared properly before flooring is laid on top. Regardless of how “good” the subfloor looks at first glance.
The #1 Enemy of Every Subfloor
Moisture.
Period.
Moisture is the leading cause of flooring failures in homes and businesses across the country — totaling an estimated $2.4 billion per year in repair and mitigation costs alone. Suddenly that number doesn’t seem so big after you understand the kind of damage moisture wreaks directly below a floor.
Wood planks cup, buckle, and warp. Glue loses its bond. Mold starts to grow — threatening the structure of your entire building. And what was once a beautiful floor installation turns into an expensive repair job in just months.
Here’s the tricky part.
Most of these issues originate underneath the floor boards. Somewhere you can’t see until the damage is done.
With wood flooring in particular, the optimal wood moisture content for timber sits anywhere between 6–9%. When acclimating material before installation, the subfloor reading should be within 4% of the timber’s moisture content. Any larger of a discrepancy and there’s a real risk of movement, gapping, and general warping as the wood tries to find balance.
That’s why it always gets tested first. No matter what.
What Makes a Subfloor Great?
Truthfully? Four things.
4 things that will set a flooring installation up to look and perform exactly as expected.
Flat
Floor = level. Subfloor = flat. They are not the same thing.
A level subfloor can still have dips and peaks all over its surface. Floors need to be installed on surfaces that have minimal variation in height. As outlined by the National Wood Flooring Association, subfloors should never have changes in height exceeding 3/16 of an inch over a 10 foot area. Go past that threshold and the door opens for movement issues, squeaking, gaps, and surface instability in the finished flooring.
Dry
…it’s the moisture issue again. If the subfloor is not bone dry before installation begins there will be problems. Full stop.
For plywood, a pin-type moisture meter should be run across at least 20 different areas of every 1,000 square feet. If working over concrete, an in-situ probe test is required. One test can take up to 24 hours to complete. Don’t rush it. Because that’s almost always how expensive problems are born.
Structurally Sound
The subfloor should be solid all the way through. No soft spots. No damage. No areas that wiggle or feel loose underfoot. Little quirks on the subfloor translate to major issues on the surface.
Walk it. Listen and feel for creaking or bouncing as you go. Solid wood subfloors should never flex beneath your feet. Address these problem areas before installing anything on top.
Clean
Leftover adhesive, dust, debris — everything matters when layering one surface on top of another. If the subfloor has any lingering residue it will prevent the finished floor from sitting correctly. Take the time to sweep, scrape, and clear it before installation starts.
That’s all folks. 4 things. Keep track of these 4 things and the floor is ready to be laid.
Air Dried Oak Beams and Subfloor Compatibility
Air dried oak has a charm that simply can’t be replicated by kiln dried timber. Because it’s dried slowly over time, rather than being force-air dried in a manufacturing facility.
The results are gorgeous.
Higher density wood that’s more stable throughout. Richer looking grains. Deeper colour variations. And a finished floor that only gets better with age.
As long as the subfloor gets proper attention.
Because air dried oak beams tend to have higher moisture content when they arrive on-site.
It’s natural. Expected, even.
But. it does mean extra care is needed when preparing the subfloor they’ll be installed over.
- Acclimation Is Key. Store air dried oak beams in the room they’ll be installed in, prior to installation beginning. That way they have time to naturally acclimate to their surroundings — minimising the risk of movement or gapping once they’re laid.
- Invest In a Moisture Barrier. Given the inherently higher moisture content of air dried timber, using a proper vapour barrier between the subfloor and the oak beams is a smart idea. Especially when installing over concrete.
- Don’t Neglect The Subfloor. Air dried oak holds screw fixings exceptionally well. But for it to perform that way long-term, it needs an equally solid subfloor to sit on.
Easy enough when you know what to look for.
Before You Lay The First Board
Let’s take a second to review. Here’s what needs to happen — every single time — to check a subfloor is ready for installation:
- Walk the entirety of the subfloor slowly. Listen for any squeaks and feel for bounce as you go.
- Run a long straight edge, or laser level, across the floor to identify dips and raised spots.
- Grab a moisture meter and test readings in at least 20 areas per 1,000 sq.ft.
- Remember: plywood = pin type moisture meter. Concrete = in-situ probe test.
- Look for any dark staining or visible signs of rot and damp.
- Clear the floor of any old adhesive, debris, or surface residue.
Skip one of the steps above. And the risk of ruining the floor above increases significantly.
Wrapping Up
So there you have it. Everything needed to understand subfloors.
Remember: the flooring people see has zero effect on how a floor performs if the subfloor is lacking.
Don’t settle for fancier flooring — level up the subfloor it’s being installed on top of.
Sounds pretty straightforward when you put it that way, right?