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How much will dry oak swell when wet?

Started by gspren, December 09, 2015, 08:32:03 AM

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gspren

  I am building a small trailer for behind my Kubota RTV. The floor will be oak 1x6 & 1x8 that have been stacked in a corn crib for 20 yrs. I used a wire cup wheel on a disc grinder to clean & de-fuzz the boards and am thinking of mixing some used oil and diesel fuel to help preserve, any better stuff? Do I need to keep a gap between boards for times it gets rained on?
Stihl 041, 044 & 261, Kubota 400 RTV, Kubota BX 2670, Ferris Zero turn

starmac

I do not have a scientific answer, and don't know about just getting wet, but in 79 when the two big floods hit houston, I saw houses with old real oak flooring push house walls completely off the foundation. This was soaked, not just wet.
Hardwood tounge and groove flooring on semi trailers doesn't have gaps in them.
Old LT40HD, old log truck, old MM forklift, and several huskies.

beenthere

Just getting your trailer decking wet will not be a big issue, but soaking it like in a flood will likely cause swelling problems. Wood will not take up much moisture within from just a rain shower.
south central Wisconsin
It may be that my sole purpose in life is simply to serve as a warning to others

GeneWengert-WoodDoc

As  rule of thumb, you will see 1% increase in width for each 4% MC increase up to 30% MC.  (30% MC is 100% RH.)  So, if you have kiln dried oak that was kiln dried to 7.0% MC, that will be a 23% MC increase or about 5.7% increase in width  This means that a 6" wide piece will go from 6.00 to about 6.33" or 6-5/16".

Now to reach 30% MC or higher will require liquid water for a day or so.  The shell will reach 30% and will try to swell, but the core will not reach that high MC value in 24 hours...it takes longer for all the water to get inside.  Because of this, especially with thick pieces, you will not see quite this much expansion, but with a week of rainy, wet weather (or a flood), you can indeed see this much.

Now, the rain stops and the sun comes out and the process is reversed and the piece start to shrink.  However, if you dry the shell really fast while the core is still wet, you will create the same conditions that result in checking.  In this case, any checks will grow (deeper and wider).

The the 1:4 rule given is for flatsawn; you can cut this in half for quaretersawn.  in fact, quality oak trailers use a lot of quartersawn.  You might be tempted to add wax to the top surface to prevent entry of rain--that will work, but the surface will be very slippery and that can be serious for someone walking on the surface.  So, avoid water repellent coatings probably.

Finally, the average outside humidity in most of the USA is 65% RH, which creates 12% MC in the lumber.  So, as a compromise, the wood trailer flooring for an exposed floor should be around 12%.

Note that you can actually calculate a precise value for swelling and shrinkage--the details are in the WOOD HANDBOOK.  As an example, for commercial red oak, the change in width of a 6.0" wide piece of flooring (flatsawn grain) is 0.0161 x (MC change).

Questions?
Gene - Author of articles in Sawmill & Woodlot and books: Drying Hardwood Lumber; VA Tech Solar Kiln; Sawing Edging & Trimming Hardwood Lumber. And more

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