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Shrinkage of hardwood floor

Shrinkage of hardwood floor

Shrinkage of hardwood floor

In my new condo, the hardword floors are developing cracks between the boards. I want to demonstate that the wood was at an unacceptably high moisture content when installed.  How can I do this calculation with wood if I can measure the ratio of strinkage divided by wood width?

RE: Shrinkage of hardwood floor

Just like lining up against the Fridge, you are treading on somewhat dangerous ground here.

Most commercial wood products are considered dry at 15-19% moisture content.  If you keep your condo excessively dry, you may have contributed to the problem if it is indeed drying.  Most wood floors have an allowance for thermal expansion at the perimeter of rooms(under the baseboard).

If the wood was not properly fastened together at the joints, the boards may be shifting into this area that was provided for thermal expansion/contraction.

RE: Shrinkage of hardwood floor

Ok, bosy, this is one of those "fuzzy" logic answers that is necessary because you are dealing with wood - a material that is not only anisotropic (it does not have the same properties in all directions), and is quite frustratingly hygroscopic (it can readily absorb moisture), but also is so diverse from one piece to the next (even within the same species) that it is necessary to love it to really work with it.  Wood will continually expand and contract (in the direction perpendicular to the grain far more than parallel) for its entire life.  Indeed, the great problem with antique furniture that was brought over to North America from Britain in the old days was that these exquisite pieces invariably shrunk drastically when placed into our centrally-heated environments.  They had been made by cabinetmakers who knew the ambient moisture conditions that existed in British homes, and designed appropriately.

First of all, the cracks that are opening up in your floor are most likely occurring, as bengineer has stated, due to loss of excess moisture.  Although your flooring was surely kiln-dried before laying, it may have not been brought down to the required moisture content (MC) for your particular locale.  What is recommended as a tolerable MC range for construction lumber is not necessarily the same MC needed for finish lumber, or for furniture.  In cooling climates (where your air conditioner load exceeds your heating load), summer humidity may be the major issue, and higher design MCs would probably be mandated.  In heating climates, the correct MC for indoor wood, including furniture, hovers around 7 to 9%.  Where I work, wood installed at higher MCs will almost invariably shrink during the dry, hot winter heating mode, and swell again during the summer, when the relative humidity is higher.  Among other things, I have been designing and making furniture for over 30 years, and the trick is to always allow the wood to move – because it's going to whether you like it or not.

This is the reason that most flooring comes in narrow "strips" - usually 2.25 or so inches wide - so that the shrinkage in each strip will not be too noticable.  Wide "plank" flooring is not only subject to large lateral shrinkage, but will usually cup as well. Thus, all wood flooring develops small cracks between the strips over time. But certainly a crack of over 1/16" in 2.25" strip flooring would be excessive.  Testing the wood with a moisture meter at this stage will probably not tell you anything, for your floor is reaching ambient conditions.  However, it may be possible to get samples of flooring from the building supply company, and test them.  

As for the equation to tell how much the wood will shrink (or has shrunk), this will probably not be very useful to you at this stage.  Each wood species has different percentages of shrinkage, and in addition, the amount of shrinkage will depend on whether the surface grain is predominantly "radial" or "tangential" (we are not worried here about longitudinal shrinkage).  Better to ask qualified experts in your area to give their professional opinion as to why you are having problems – and then you may be in a good position to ask the contractor to fix the situation.  This will probably mean ripping up the flooring and re-laying and re-finishing it once it has stabilized – yes, it is very feasible to re-lay!  I have re-laid many beautiful old hardwood floors.

This post is long-winded as it is, and I do not wish to blather on.  So, if you would like the equation, and the explanation that must accompany it, please ask, and I'll be glad to post it another time.

One final note:  if the floor is exposed to excess humidity for a prolonged period of time, an added danger is that each board will swell to such an extent that it will put such pressure on the adjacent strip that the fibres of each will be irrevocably crushed – increasing the gap when normal temperature and humidity conditions are restored.  Although coatings such as polyurethane will retard the absorption of moisture by the wood, they will not stop it.  Only a drastic measure such as encasing each board (all surfaces) in an epoxy coating would virtually stop the intake of moisture – and this is hardly practical.

Sustainable, Solar, Environmental, and Structural Engineering: Appropriate technologies for a planet in stress.

RE: Shrinkage of hardwood floor

Another thought, bosy, to throw at you: The time of laying may also have had a significant effect on the subsequent behaviour of the floor. If the bundles of wood were left in the open (even in the warehouse) for a long period in the humid summer after kilning, it could have absorbed enough moisture to cause excessive shrinkage come heating season.  

Sustainable, Solar, Environmental, and Structural Engineering: Appropriate technologies for a planet in stress.

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