I live in south west Florida on an island the humidity swing is one of the worst in the US. My lumber goes from 5.5-6 % in the winter to as high as 17% in the summer. My delimma is should I use high quality plywood or used lumber that has dried and stabilized for several years for the main frame. If I’m over complicating the subject let me know. ( One of my back ground is a machinist, tool & die maker)
It depends. Even wood that is dried will have seasonal movement.
Different woods move different amounts also. Some are more stable than others. You can look up tables that will tell you how much any given species will move.
IMO If your concerned then go with plywood or even metal.
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Thank you for your response , I was leaning towards ply but was curious about others input
Hey Paul,
if you want to suppress the volume change of wood due to humidity fluctuations, there is a method in which the wood is completely saturated with polymerizing oils (linseed oil, tung oil, walnut oil, poppy seed oil, etc.). It is called “Wood-Oil Zone” or “Oil Phase”. These oils are also known as “drying” oils, but they do not dry, they solidify on contact with oxygen (polymerization). I have already done this successfully and the result is amazing. The wood can get a surface that is hard and smooth. It can take 28 days for the oil to soak into the depths of the wood and then polymerize completely. I soaked a pine table top with pure linseed oil for two months, every day at first, then every few days, and since then it has been a kitchen table that forgives everything. If you treat a kitchen cutting board like this, you can put it in water overnight and it won’t swell or soak up the water. There have been windows here that were over 200 years old that were treated like this and they didn’t rot. However, regular re-oiling is advisable if the wood is used outdoors (re-oiling in the dry season)
As mere contact with air triggers the chemical reaction and generates heat, cloths soaked with a polymerizing oil can ignite spontaneously. It is advisable to enclose rags in a jam jar after use, where spontaneous combustion will not work due to lack of oxygen.
I enclose the translation of instructions from Livos Holzöle.
Professional tip: Wood-oil zone, mode of action and meaning
Term
The term “wood oil zone” refers to a wood surface saturated with natural oils, which prevents or greatly reduces the water absorption of the wood cellulose in the wood fibers. As a result, the water resistance and mechanical strength of the wood surface is increased. In addition, swelling processes and the associated dimensional changes in the wood body are prevented.
Formation
The applied natural oil is absorbed via the pore system of the wood, whereby the penetration of the oil is supported by the capillary effect of the narrow pores. At the same time, the molecular attraction between the wood substance and the oil causes the oil to adhere to the wood fibers. The flow process continues until the oil begins to dry out, whereby the penetration depth depends on the structure (pore diameter and structure of the vascular system) of the respective type of wood. The upper part of the wood-oil zone dries by absorbing oxygen from the outside air. Once the grip strength has developed, the deeper layers of oil dry by absorbing oxygen from the wood pores.
During the curing process, the oil molecules continue to cross-link with each other and with the wood fibers while absorbing oxygen, making them a solid part of the wood body. For this reason, natural oil coatings do not flake or peel off the surface. After about a month, the curing process is well advanced.
Formation of a wood oil layer with KOIMOS Wood Oil No. 299 on alder wood
The formation of the wood-oil zone is basically the same on all types of wood. The processes involved are illustrated using the example of an alder wood surface coated with KOIMOS Rolling Oil.
Technical data
Alder wood has an average pore diameter of 30 - 80 µm (1µm = 0.000.001 m). On the end grain there are 75 - 145 pore openings per mm².
KOIMOS Rolling Oil No. 299 penetrates about 200 µm deep into alder wood. With a density of 0.93 g/cm³, a consumption of approx. 15 g/m² can be expected, which corresponds to 16.13 cm³/m².
0 - 4 hours
After treatment, the rolling oil penetrates to a depth of 200 µm into the wood tissue. By displacing the oxygen from the surface layer, the expected color tone is achieved.
0 - 8 hours
Oxygen absorption leads to molecular cross-linking and the formation of a film matrix. After eight hours, the upper third of the wood-oil zone has reached grip strength.
8 - 24 hours
Deep-seated oil films through oxygen absorption from the air content of the alder wood pores.
1 - 28 days
Through further oxygen absorption, cross-linking structures between oil molecules and wood fibers are further expanded. At the same time, the wood-oil zone becomes harder and more water-resistant and this process is completed after around 28 days. The wood surface has reached its maximum load-bearing capacity.
– Source: Profitipp: Holz-Öl-Zone, Wirkungsweise und Bedeutung (PDF) (DE)
Note that such a treatment is not recommended on plywood or veneered wood, as the horizontal glue layers can be an obstacle for the oil.
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If you are worried go to your local lumber store (most likely not Home Depot or Lowes) and get an LVL (laminated veneer lumber). They come in all sizes and shapes. Basically it’s plywood but in the shape of 2x (whatever). They usually come in 5’ increments so often lumber yards will have cutoffs you might get for cheap. They are stable, like plywood and very strong. You can even use them for legs.
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Thank you Alex that is a great idea that I’m going to consider.
I thought LVL was pretty expensive? Looking it up here our local Home Depot has 2x4x96 three packs for $100. Maybe it’s cheaper elsewhere. If cost is a concern the you could laminate cheaper plywood strips in layers to make beams of any desired size.