End-Grain Acclimation

end grain

End-grain wood flooring is cut from the tree so that the face of the board surface exposes the ends of the growth rings. This is also known as the transverse cut. End-grain flooring will shrink and swell according to the tangential value in the direction across the circumference of the growth rings, and according to the radial value in the direction perpendicular to the growth rings, with essentially no movement in thickness.

In laymen’s terms, what this means is that once installed, it will shrink and swell on all four of its sides, unlike plainsawn flooring that shrinks/swells primarily in its width.

Because of this cut of flooring’s ability to move in all directions, it is very important to ensure the facility receiving end-grain flooring is capable of maintaining a consistent temperature and humidity throughout the year. The process of aligning the wood flooring and the environment it is to be installed in, is defined as acclimation, or more accurately, conditioning. This process involves understanding that the wood flooring and the facility are compatible.

The general guidelines for acclimation/conditioning of end-grain are as follows:

Prior to delivery, and again when the wood is delivered, check wood flooring moisture content and the product specifications on the packaging to ensure it is compatible with the jobsite conditions. Record, date, and document all results. Check other wood materials on the jobsite for comparison. If the solid end-grain wood flooring is delivered at a moisture content that coincides with the manufacturer requirements and expected in-use (e.g., normal living) conditions of the facility, and these conditions are maintained, no on-site conditioning may be required.

The subfloor should also be within an acceptable moisture content range relative to the wood flooring. On- and below-grade concrete subflooring should be completely sealed with a class I Vapor retarder capable of blocking any potential moisture from below. For wood subfloors, a general rule of thumb is that the moisture content of the wood subfloor should be no more than 2 percent greater than the MC of the end-grain wood flooring. If the moisture condition of the subfloor is outside of these tolerances, the flooring should not be delivered until the facility can accommodate the flooring product going in.

When it is necessary to introduce or remove moisture from the wood blocks before installation, in order to align the flooring with a subpar facility, you must take into account that the increase/decrease in moisture will distort the wood blocks, which without manipulation, will adversely affect the installation. (NOTE: Imported/tropical species often require more time to equilibrate to their surroundings. Some tropical species lose moisture or gain moisture at faster or slower rates than domestic species due to higher overall density, oil and resin content, and their interlocking cell structure.)

Due to the orientation of the grain, end-grain flooring tends to equilibrate at a relatively fast rate (depending on thickness and species) as compared to other solid sawn boards. Conditioning end-grain flooring can be facilitated by separating the individual blocks into small lots and then restacking them, on-edge, in the expected in-use (e.g., normal living) conditions of the facility. The lower and more spread out the blocks are laid out; the faster the conditioning process will take.

Because of this cut of flooring’s ability to move in all directions, it is very important to ensure the facility receiving end-grain flooring is capable of maintaining a consistent temperature and humidity throughout the year.

Off-site conditioning of the flooring material in a controlled environment is possible when the off-site conditions reflect the expected in-use (normal living) conditions.

In facilities that do not have the ability to maintain a consistent climate control with their HVAC system, shrink/ swell should also be accounted for within the installation. End-grain flooring often requires expansion space (3/4”) to not only be left around the perimeter of the room and at all vertical obstructions, but should also be added within the flooring system. The amount of internal spacing is based on the season of installation and the tangential shrink/swell value of the species being installed.

It is not uncommon for many end-grain installations taking place during the heating seasons to be installed with a slight gap between each block to allow for seasonal swelling. Depending on the size and species of the end-grain blocks, it is not uncommon to have 1/8”-1/4” expansion built into the system throughout the installation.

When acclimating end-grain flooring, the customer should be made aware of any built-in seasonal gaps prior to installation of the floor. The easiest way to explain to them why you are doing what you are doing, or why their floor is showing gaps after being installed over the summer, is to share the data provided to us by the USDA, Forest Products Laboratory, which suggests the following moisture content average and ranges for interior wood products, knowing that the low number represents the heating season, and the high number represents the cooling season:

• Most areas of the United States; average = 8 percent MC, ranging from 6-10 percent MC

• Dry southwestern regions; average = 6 percent MC, ranging from 4-9 percent MC

• Damp, warm coastal areas; average= 11 percent MC, ranging from 8-13 percent MC

Actual interior moisture content conditions in any location may differ significantly from these numbers.

Brett Miller is VP of Education & Certification at the National Wood Flooring Association in St. Louis. He can be reached at brett.miller@nwfa.org.

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