Dealing with Wood Movement
Solid wood expands and contracts (grows and shrinks) as its moisture content adjusts to match the surrounding environment. This guide will help navigate this tricky issue!
What types of wood move?
In the context of furniture building, we are primarily concerned with solid woods, meaning planks or slabs cut directly from a tree trunk. Engineered boards, such as plywood, MDF, and chipboard are manufactured in a way that minimises movement, making them less of a concern.
Why does wood move?
Solid wood is composed of growth rings and a microscopic, straw-like structure designed to transport moisture and nutrients from the roots. This structure remains functional even after the wood has been cut and dried, allowing moisture to pass through, resulting in expansion/contraction across the grain.
The below image shows how planks can be cut from a trunk:
Plain sawn is a cheaper cut of wood, with less waste. The uneven grain pattern means it is more likely to warp and move in an undesirable way.
Quarter sawn boards have a consistent grain pattern, making them far more stable. They also produce a nice visual grain pattern which is desirable for furniture manufacture. They are more expensive to the increased labour and waste.
How much does wood move?
The exact movement depends on the type of wood, the exact cut of the wood (which section of the trunk it was taken from) and the relative moisture change. The vast majority of woods move 0.1% - 0.3% with every 1% change of moisture.
Oak: 0.15% to 0.2% movement per 1% change in moisture content.
Pine: 0.2% to 0.3% movement per 1% change in moisture content.
Wood is very stable in the direction of the grain (along the trunk) so movement generally only needs to be considered across the grain.
Practical examples of wood movement
In the following examples we assume:
The raw material starts at 8% moisture content (kiln dried)
The table is used indoors in a home ranging from 40% - 60% relative humidity
The wood can therefore range 6% - 10% moisture content.
This equates to a +2% / -2% relative change.
We are building a table top with a final size of 2m x 1m
Solid oak
This could be made of a single slab from a very large tree or made of pressure jointed/glued boards. Both will produce a similar result.
Assuming a 0.2% movement over 1% moisture content change
0.4% move over 2% moisture content change.
0.4% of 1m = 4mm movement.
This means the table top needs to be fitted to any supporting frames, legs etc. in a way that can allow both a 4mm increase and decrease in initial size.
Pine planks (4no. 25cm wide)
Planks that are butted up tight against one another will act as one piece. If gaps are allowed, each plank can be calculated on its own.
Assuming a 0.3% movement over 1% moisture content change
0.6% move over 2% moisture content change.
Plank Movement: 0.6% of 25cm board = 1.5mm movement.
Full Table Movement: 1.5mm x 4 = 6mm movement.
If planks are fitted with gaps, then only plank movement of 1.5mm needs to be considered. If planks are joined or butted tight against one another they should be considered a single unit with 6mm total movement.
The above examples all assume indoor use. If kiln dried wood is used outdoors with a moisture content reaching 20%, the expansion result would be need to be multiplied by 5x.
Considerations for furniture building
Quality of wood
Cut type
As shown in the image at the top of this guide, a quarter sawn board has much more consistent grain patter. This will make the movement of the wood much more uniform & predictable and is overall more stable. A plain sawn board is much more likely to warp, cup or even split as it moves.
Drying method
A kiln dried lumbar is generally considered superior to air dried.
It has a lower and more consistent final moisture content. Approx 6 - 8% vs. 12 - 20% air dried.
Less chance of pests and fungal risks
More stable, especially for indoor use.
Knots & Defects
Generally speaking a higher quality lumbar would be expected to have less defects. Part of this is due to the species and cut selection used on these boards, but also precautions taken during the quality control process.
Allowing Dimensional Changes
As demonstrated above, a solid wood top can shrink and grow by several mm's (best case). Care should be taken to ensure that the table top is fitted to any legs or frames in a way that allows this movement to happen unconstrained.
Bracing
When adding bracing to the underside of a table, slotted holes and washes are a simple way to ensure the screws are not over-constrained when movement occurs.
Metal C-channels are a great choice for bracing a table top, since they already have elongated holes to account for wood movement.
Apron / Skirt
A traditional table apron is used to add stiffness and structure to the table top. There are several methods for fitting these, including:
Figure-8 Fittings
Z-clips
Slotted Screw Holes
Buttons (wooden blocks)
Sliding Dove Tails
Legs
Legs should be fitted in a way that they do not over-constrain the table top when it changes in size. This is only really an issue when your legs span a large portion of the table and not so much of an issue if the leg fitting plate is small.
All of our legs are made with slotted holes to allow movement across the grain.
Preventing Warping, Cupping & Splitting
Warping refers to any kind of twisting of the wood that causes it to loose its initial flat state. Cupping is a specific type of warping that is common in plain sawn timber, where the wood inconsistent grain structure causes uneven warping. Boards that go through aggressive warping, can be prone to splitting.
Wood Cut
Quarter sawn boards are much less likely to warp, due to the contestant wood grain.
Plain sawn boards are extremely prone to cupping with boards cut from the extremes of the log, while boards cut nearer the center of the log are more likely to split and crack.
Bracing
Strong bracing that helps to keep the boards flat, while allowing for size changes can be very effective at controlling warp. Metal c-channels are an especially good option due to their strength and slotted holes.
Acclimiatising Material
When dealing with a large change in moisture content, it will be very difficult to control warping, even with the strongest of supports. The power of wood movement is quite incredible and is very hard to hold back.
Drying boards to a suitable moisture content prior to building your table (cutting, planing, joining) is by far the most effective way to ensure the table does not warp, cup or split over time.
Acclimatising your material prior to building is one of the best way to prevent issues appearing after everything has been built. Achieving a moisture content of 8% is great for indoor use, while 15 - 20% is better for outdoor use.
Specific Material Examples
Kiln Dried, Quarter Sawn Solid Wood (Furniture Grade)
In terms of movement, this is your best case. Starting with a stable, dried wood prior to building your table means that the final shape/size you end up with will be maintained over the years of use, with relatively low levels of expected moisture content change. (approx +/- 2-3%) when used indoors.
Tips
Consider how the boards will be joined and ensure suitable fixing methods to allow the wood to expand and contract across the grain.
Construction Lumbar
Construction lumbar can often be kiln dried (to around 12%), but is usually plain sawn pine wood. There are different levels of treatment that can go into construction lumbar and it is generally needed to be strong, but not visually perfect.
Tips
Try to ensure moisture content is close to 8 - 10% when used indoors, by acclimatising the material first.
Consider making a plank table, with suitable gaps between planks for wood expansion.
Employ techniques such as using c-channels to help control warping and cupping.
Scaffold Boards
High quality scaffold boards are made from and has strict strength requirements for health and safety. They are generally used outdoors and air dried during production, giving them a moisture content of around 20%.
Tips
Try to ensure moisture content is close to 8 - 10% when used indoors, by acclimatising the material first. If you are yet to buy your scaffold boards, it is possible to buy them kiln dried from various stockists.
Consider making a plank table, with suitable gaps between planks for wood expansion.
Employ techniques such as using c-channels to help control warping and cupping.
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