Hide Subfloor Hump Under Planks (Underlayment Shim)
A hump beneath plank flooring should be corrected at the subfloor, not hidden with extra underlayment. First, map the raised area and find out whether it comes from a surface defect, loose panel, moisture, or framing. Then follow the flooring maker’s flatness rules, repair the cause, and check the floor again before installation.
A raised ridge can look small before the planks go down. Once flooring spans it, though, the spot may cause rocking, gaps, noise, or stress on joints. It is tempting to add a soft layer beneath the planks, but that does not make the base flat. It can also conflict with the flooring instructions.
I start by finding out what is high and why. A sound, stable surface hump may need careful removal. A loose or moving ridge calls for a different repair. The right fix depends on the subfloor material and the flooring system you plan to install.
Survey the Hump and Identify Its Cause
A survey shows the hump’s height, length, and likely source before you remove or cover anything. Check the floor with the flooring removed where needed, and use a straightedge and feeler gauge to map the area. Also look for movement, moisture damage, loose panels, and changes under load.
Begin with the flooring manufacturer’s instructions. They explain how flat the base must be and how to check it. “Flat” means the surface does not have sharp highs or lows beyond the allowed limit; it does not mean perfectly level. A floor may slope gently and still be flat enough for the product.
Place a long straightedge across the ridge in several directions. Check the gap beneath it with a feeler gauge, which is a thin metal strip used to measure small spaces. Mark where the straightedge rocks or where the gap exceeds the product’s limit. Use the maker’s specified straightedge length and tolerance when available.
A commonly published resilient-plank requirement is 3/16 inch over 10 feet or 1/8 inch over 6 feet. Use those numbers only when the chosen flooring’s current instructions state them. There is no single flatness tolerance for every plank product, and building codes do not set one universal floor-plank flatness rule.
Next, press and walk around the area. A ridge that shifts, flexes, or returns after repair may point to loose panels, moisture, or movement below the surface. Look for stains, swelling, soft spots, open seams, and fasteners that have backed out. A visible ridge can be a structural warning, not just a flooring defect.
Before sanding, grinding, or driving fasteners, check for pipes, wiring, and existing fasteners. If the hump is changing, cracked, or soft, do not cover it yet. Have a qualified flooring or building professional assess the cause.
Confirm Product, Perimeter, and Access Clearances
The product instructions control what can go beneath the planks and how much room the floor needs at its edges. Check the flooring and underlayment requirements before choosing a repair method. Also confirm door, transition, vent, and access-panel clearances so the correction does not create a new problem.
Some plank products have an attached pad; others require a separate underlayment, and some do not allow one. Use only a type and thickness approved for that flooring system. A second layer or a thicker pad is not a shim: it cannot safely correct a hard raised spot and may let the floor flex more.
Check the specified expansion gap at walls and fixed objects. This gap lets a floating floor move as the manufacturer requires. The correct size varies by product, so do not rely on a commonly used measurement as a universal rule. Keep the required clearance around columns, cabinets, pipes, and other fixed points.
Then test the finished floor height at doorways and transitions. A repair or approved underlayment can change the height enough to affect door swing or transition fit. Keep floor vents open and preserve access to any panel that must remain reachable. Do not bridge an access cover with permanent flooring if it needs to be opened.
| What to check | Why it matters | What to do |
|---|---|---|
| Plank instructions | Sets flatness and underlayment rules | Use the current requirements for your product |
| Perimeter clearance | Allows the floor to move as designed | Measure the exact gap stated by the maker |
| Door and transition height | Prevents binding or an awkward edge | Dry-fit the layers and check clearance |
| Vents and access panels | Maintains airflow and service access | Keep them clear and removable as needed |
Correct the Subfloor Before Installing Planks
Correcting the base means removing a sound surface high spot or repairing the condition that caused it. The method depends on whether the substrate is concrete, plywood, or another approved material. Do not use extra underlayment to bury the ridge, and do not alter framing or structural panels without qualified direction.
On concrete, first make sure the raised area is a surface defect, not a sign of slab movement or damage. A suitable grinding method may remove a small, stable high spot, but check for embedded services and follow tool and dust-control guidance. ASTM F710 covers preparation of concrete floors for resilient flooring; it does not set one flatness tolerance for every plank product.
On wood subfloors, find out whether the ridge comes from a raised panel edge, loose sheet, or framing below. A qualified installer may be able to secure a loose panel or carefully reduce a surface high spot. Do not sand through the plywood face veneer, which is the thin top layer, and do not plane or cut joists without qualified direction.
For low areas, use a patching compound compatible with the subfloor and flooring system. Apply it only as its instructions allow. A compound intended for concrete may not be suitable for wood, and the required primer, depth, and cure time vary by product. Let repairs cure before checking or installing flooring.
A ridge caused by a crowned, damaged, or moving joist is different from a surface bump. Sanding the top can weaken the subfloor without fixing the movement. Planks may then sit unsupported or hide a moisture or structural problem. Investigate and repair the cause first.
| Finding | Appropriate next step | Avoid |
|---|---|---|
| Stable, small surface high on concrete | Confirm it is safe to grind, then use a suitable method | Grinding without checking for services or damage |
| Raised or loose wood-panel seam | Identify the cause and secure or repair the panel as appropriate | Sanding through the face veneer |
| Low spot beside the ridge | Use a compatible patching product as directed | Using soft underlayment as filler |
| Ridge that moves, cracks, or returns | Stop and arrange a qualified assessment | Covering it or compressing it with planks |
Verify Flatness and Complete the Finish
Verification confirms the repair meets the chosen flooring instructions across the full installation area. Recheck with the required straightedge method after patching, sanding, or fastening. Then install only the approved underlayment and planks, keeping the stated edge gap and maintaining access to vents and service panels.
Clean the surface so dust and loose material do not interfere with the next layer. Check the repaired area in more than one direction, including the transition from the repair to the surrounding floor. A sharp edge at the repair can matter even when the center seems flat.
Compare your measurements with the product’s current flatness limit. If the result is outside that limit, correct it again before laying planks. Do not assume a thicker pad will make an out-of-spec surface acceptable. If the instructions are unclear, ask the flooring maker or a qualified installer before proceeding.
Before laying the first plank, dry-fit the layers at doorways and transitions. Confirm the required perimeter gap and make sure the floor does not block vents or panels. Follow the installation sequence, acclimation rules, and fastening or locking instructions that apply to the selected product.
Common installation situations
These examples show how the same visible ridge can call for different action. They are diagnostic scenarios, not a substitute for checking the floor on site.
- A stable concrete crown: The straightedge shows a small high area, with no crack or movement. The installer confirms the material and product requirements, checks for hazards, and removes the high spot with an appropriate method. The surface is measured again before flooring goes down.
- A plywood seam that lifts underfoot: The ridge shifts when stepped on. Rather than sanding the seam, the installer checks panel support and fasteners, then repairs the loose condition as appropriate. If movement continues, a qualified professional assesses the framing.
- A ridge that returns after patching: The floor looked flat after a repair, but the high line reappears. That change raises concern about moisture or movement below. Stop work and find the cause before reinstalling the planks.
Conclusion: A durable plank installation starts with a stable, flat-enough subfloor, not a thicker cushion. Measure the ridge, identify its cause, follow the flooring maker’s current limits, and correct the base with a method suited to its material. If the ridge moves or returns, investigate before covering it.
Frequently Asked Questions
Can I hide a subfloor hump by adding another underlayment layer?
No. Extra or layered underlayment is not a leveling method and may not be approved for the flooring system. Correct the high spot or its cause, then use only the underlayment specified by the manufacturer.
Can planks flatten a small hump when I install them?
Do not count on it. Planks may bridge the area, flex, or put stress on their joints. Meet the product’s flatness requirement before installation.
What flatness tolerance should I use for plank flooring?
Use the current installation instructions for your exact product. Some resilient-plank instructions state 3/16 inch over 10 feet or 1/8 inch over 6 feet, but those figures are not universal.
How do I measure a hump in the subfloor?
Place a straightedge across the area in several directions. Use a feeler gauge to measure gaps, mark the limits of the high area, and follow the straightedge length required by the flooring instructions.
Can I sand a hump in plywood?
Only if it is a suitable surface high spot and the work will not cut through the panel’s face veneer. First check for loose panels, moisture, movement, wiring, pipes, and fasteners.
Can I grind down a high spot on concrete?
Sometimes, if it is a stable surface defect and the method is suitable. Check for cracks, movement, and embedded services, and follow safe tool and dust-control practices.
What if the ridge moves when I walk on it?
Do not cover it. Movement can point to a loose panel, framing issue, or other problem that needs assessment and repair before flooring installation.
Does ASTM F710 give the flatness limit for my plank floor?
No. ASTM F710 addresses preparation of concrete floors for resilient flooring. The chosen plank product’s instructions set the applicable flatness requirement.
How much expansion space should I leave at the edges?
Use the exact gap required by the flooring manufacturer. Requirements vary by product, so do not treat one commonly used measurement as universal.
Should I fix a low spot at the same time as a hump?
Yes, if it is outside the product’s flatness requirements. Use a patching product compatible with the substrate and flooring system, and follow its application and cure instructions.
(This article was written by one of our staff writers, Michael Donovan. Visit our Meet the Team page.)
