How to Take and Log Wood Moisture Readings Before Hardwood

Why this matters

Every hardwood cupping, gapping or crowning claim ends in the same question: what did the flooring and the subfloor read on the day it went down? A crew that can hand over a signed log with corrected readings by room, the meter setting, and the air conditions has an answer. A crew that "checked it, it was fine" has three numbers from one carton at the tailgate, and the inspector's first question has nothing behind it. The log is also how you catch the one wet room before the floor covers it, which is cheaper than any claim.

What the numbers mean (MC, equilibrium moisture content, the strip and plank match rule and the subfloor ceiling) is owned by 'Wood Moisture Content and the Match to the Subfloor'. This card is the procedure for producing the readings and the log.

Before you start

• Equipment. A pin or pinless wood moisture meter with a species setting or a species correction chart, the meter's check block or calibration check if it has one, a thermo-hygrometer for the room air, the flooring maker's installation instructions, and a log sheet.
• Conditions. The HVAC is running at the temperature and humidity the house will live at, and the flooring has been on site in the rooms it goes in. Acclimation itself is 'How to Acclimate Flooring and Confirm It Is Ready'; this procedure is how you confirm it.
• Shop defaults for how many readings. Flooring: 4 boards per 100 sq ft (40 per 1,000), rounded up. Subfloor: 2 spots per 100 sq ft (20 per 1,000), never fewer than 10, spread across every room in proportion to its area, plus every suspect spot. These follow the counts NWFA guidance has commonly used; where the maker or the current NWFA guideline asks for more, use theirs.

Opening cartons and pulling boards from the middle of a stack means lifting cartons that commonly weigh 40 to 60 lb; lift with the stack at waist height, not off the floor in a twist.

Step 1: Set the meter to the species and prove it

Set the species before the first reading. A pin meter reads electrical resistance, and resistance depends on species as well as water, so the same board reads differently on the wrong setting, often by a point or more, which is the whole width of the plank tolerance. A pinless meter reads through a field and needs the species or specific-gravity setting for the same reason. If the meter does not compensate for temperature and the wood is far from about 70 F, apply its temperature chart.

Then check the meter on its check block or calibration function and write the setting on the log. What breaks if you skip it: every reading is consistently off, the error never shows inside the log because all the numbers agree with each other, and an inspector re-reading on the correct setting gets a different floor.

Step 2: Record the room air first

Read and record temperature and relative humidity in each room with the thermo-hygrometer, plus how many days the HVAC has been running. Do this before you start taking wood readings so the conditions describe the same moment.

The air reading is what tells you where the wood is heading. The flooring should be close to the equilibrium the house air sets, and if it is not, the floor is still moving. What breaks if you skip it: a log of wood readings with no air conditions cannot show the floor was installed near in-service conditions, which is the point of the log.

Step 3: Read flooring boards from several bundles

Pull boards from at least five different cartons or bundles, from the middle of the stack as well as the top, and read each board in the middle of its length, on the back or the face as the meter maker directs. Avoid the last few inches of each end; end grain trades moisture faster and reads drier than the body of the board.

Number each reading and note which carton it came from. What breaks if you skip it: boards from the top of one carton acclimate faster than boards buried in a stack, so a single-carton sample reports the best-acclimated wood on the job and hides the rest.

Step 4: Read the subfloor on a spread, and on purpose

Lay out spots across each room in proportion to its area, then add every place moisture comes from: along exterior walls, beside a bath or kitchen wall, over a crawlspace or unheated garage, near a water heater or laundry, and anywhere with a stain. Stay off spots directly over a supply register or duct run, which dry locally and flatter the number.

On 3/4 inch panels a pin meter reads at the depth of its pins and a pinless meter reads through roughly the panel thickness; know which you have and do not read a pinless meter on a spot with a metal fastener head under the pad. What breaks if you skip it: a subfloor averaged from comfortable spots passes, and the wet corner by the dishwasher becomes the cupped corner six months later.

Step 5: Compute and compare, room by room

For each room, average the subfloor readings and write the difference from the flooring average. Apply the match rule from the owning card: in substance under NWFA guidance, within 4 percentage points for strip under 3 inches wide, within 2 points for plank 3 inches and wider, and a wood subfloor over 12 percent is not ready whatever the flooring reads. The maker's instructions win where they are tighter.

The unit of analysis matters. Compare by room average for the match, and check every individual spot against the 12 percent ceiling. Any single spot more than about 2 points above its own room's average is a location, not a rounding problem, and gets investigated before any average is trusted.

Step 6: Hold if it fails, and find the source

A failure is not fixed by waiting it out without knowing why. Find what is wetting the subfloor (a crawlspace with no ground cover, a plumbing leak, a slab-edge water path, a humidifier set too high), route the fix to whoever owns it, and re-read the same spots after the source is fixed and the area has dried with the HVAC running. Never wet the flooring to bring it up to a wet subfloor; the house sets the target, not the subfloor.

If the failing area is a separate room behind a threshold, installing the passing rooms first is a judgment for the lead and the customer. If the floor runs continuously into it, the whole run holds.

Step 7: Sign the log and file it

Complete the log, sign it as the lead, note what the customer was told and when, and attach it to the job in the shop's system before the crew leaves. Re-reads after a hold get their own dated log, not edits to the first one.

Worked example: the filled-in log

850 sq ft of 5 inch solid white oak plank over 3/4 inch plywood: a 350 sq ft living room, a 150 sq ft hall and a 350 sq ft bedroom over a crawlspace. Plank, so the match is 2 points. 4 boards per 100 sq ft is 34 boards; 2 per 100 sq ft is 17 subfloor spots, split 7, 3 and 7 by area.

WOOD MOISTURE LOG          Job 3108    Date 03/14
Flooring: 5 in solid white oak plank, 850 sq ft
Meter: pin, setting WHITE OAK, auto temp corr,
  check block read OK
Air: 71 F / 41 pct RH, HVAC on 6 days, all rooms
Rule: plank, within 2 pts; subfloor 12 pct max

FLOORING: 34 boards, 7 cartons, mid-board
F1-5    7.6 8.2 7.9 8.4 7.8
F6-10   8.1 8.0 7.7 8.3 8.0
F11-15  7.7 8.3 7.9 8.1 8.2
F16-20  7.8 8.0 8.0 7.6 8.4
F21-25  8.1 7.9 7.8 8.2 8.3
F26-30  7.7 8.0 8.0 7.9 8.1
F31-34  8.2 7.8 8.4 7.6
  avg 272.0 / 34 = 8.0

SUBFLOOR: 3/4 plywood, 17 spots
Living 350  9.2 9.6 9.4 9.1 9.5 9.3 9.7
            avg 65.8 / 7 = 9.4   diff 1.4  PASS
Hall   150  9.4 9.0 9.5
            avg 27.9 / 3 = 9.3   diff 1.3  PASS
Bed 2  350  10.9 11.3 11.8 12.4 11.6 11.1 12.1
            avg 81.2 / 7 = 11.6  diff 3.6  FAIL
            12.4 and 12.1 over 12.0       FAIL
DECISION: HOLD ALL. Run continuous hall -> Bed 2
Signed (lead): ________  Customer told: 03/14

Reading the log. The flooring is tight, 7.6 to 8.4, and an average of 8.0 sits close to what 71 F and 41 percent RH holds wood at (a little under 8 percent on the USDA EMC table, as the owning card lays out), so the flooring has acclimated. The living room passes by 9.4 minus 8.0, which leaves 1.4 points, and the hall by 9.3 minus 8.0, 1.3 points. The bedroom fails twice over: 11.6 minus 8.0 leaves 3.6 points against a 2-point rule, and two spots are over the 12 percent ceiling on their own.

Why the whole job holds. The plank runs continuously from the hall through the bedroom doorway with no threshold, so laying the living room and hall now would mean either a tie-in later across a doorway that was designed to run unbroken, or installing over the wet room. The lead holds everything and tells the customer that day.

The source. The bedroom readings are high everywhere in the room rather than at one spot, which points at what is under the whole room, not at a plumbing wall. The crawlspace under it has a torn ground cover and wet soil. That fix goes to whoever the shop sends for crawlspace work, and whoever enters the crawlspace treats it as its own job, with its own hazards (wiring at floor joists, standing water near electrical, pests). After the ground cover is repaired and the bedroom subfloor has dried with the HVAC on, the crew re-reads the same 7 spots on a new dated log.

What would change the call. If this had been 2-1/4 inch strip, the 4-point rule would apply, and 3.6 points would pass the match, but the two spots over 12 percent would still hold the bedroom. The ceiling is what catches it when the match does not.

How to verify the log is complete

• The species setting and the meter check are written on the sheet.
• Air temperature, RH and HVAC run time are recorded for the day of reading.
• The board count meets the default (34 here) and came from at least 5 cartons; the subfloor count meets the default (17 here) and every room has spots in proportion to its area.
• Every room shows its average, its difference from the flooring average, and a PASS or FAIL against the rule written at the top, and every spot over 12 percent is flagged.
• The decision, the lead's signature and the customer notice are on the sheet, and the sheet is in the job file.

References

• See related: Wood Moisture Content and the Match to the Subfloor
• See related: How to Acclimate Flooring and Confirm It Is Ready
• NWFA Installation Guidelines, current edition, for the moisture match and subfloor limits; flooring maker's installation instructions, which govern where tighter.
• Moisture meter maker's instructions for species settings, temperature correction and reading depth.