Dried timber cupping?

[Midnight Man]

Sorry to ask a newbish question :(

Some weeks back, I purchased some rough sawn, dried timber (it's actually Blackbutt) - 50mm x 300mm, with the idea in mind of ripping this down to 100mm x 50mm and then running this through a planer and thicknesser to arrive at roughly 45mm x 90mm dressed pieces for making a workbench.

Time has slipped away, to be sure, and I stacked these boards on the concrete floor in the shed to await the work to be done on them.

I noticed today that the top board in each stack has cupped up at the edges a wee bit - it's enough to cause concern if they were to be used as 300mm wide pieces, but given I'm going to rip them down, the curvature will be so small as to not pose a problem.

From the point of view of learning and understanding though, I'd like to know if this sort of behaviour from timber is normal? To be sure, the top boards would be exposed to a few hours sun through a window each day, and in my newness to woodworking, I was thinking this may have caused it.

I've now flipped each of the top boards over, thinking that if that was the cause, they should now cup in the opposite direction, cancelling out the unwanted cupping.

But I'd like to learn how to properly store dried, rough sawn timber, as I will likely buy in reasonable lots and use it over a period of time for many projects... thank you in advance to anyone who can share their wisdom on this!

[elanjacobs]

100% normal. The cupping on the top board is due to it being exposed to changes in temperature and humidity.

If you want to stack them properly, place sticks between each board and under the bottom one as well so that there is even airflow over each piece. Direct sunlight should also be avoided if possible

https://cromartietimber.co.uk/images/site/air_drying_timber.jpg

[DomAU]

I'm sure someone with far more experience than I will chime in, but in the meantime I'll put my 2c in :).

If you are storing long or large boards, ideally you want to lay them down on stickers (pieces of timber or any spacing material) that have been placed to create a level surface (top of each sticker is at the same height). Most concrete slabs aren't very flat so the more care you take leveling the better. Then you want to stack the rest of the timber on top with stickers between each layer. If it is hardwood you the stickers don't need to be too thick, like maybe 2cm or so. The idea is just to have the board(s) laying on a flat reference surface to begin with and to provide airflow all the way around the piece so that each surface can lose or absorb moisture equally. The end-grain will always wick or lose moisture much quicker than face grain, so it's good to coat the end grain in wax or paint; particularly if the timber is still wet so that it doesn't loose moisture too quickly from the end-grain and check / split.

I'd say that in your case the cupping is a combination of the sun and airflow drying the top causing it to shrink, as well as possible the back wicking moisture from the concrete floor.

Also, in my experience, even "kiln dried" timber can still be reasonably wet. I bought a heap of blackbutt decking boards at 120mm x 35mm that were kiln dried, but they were probably still at 17%+ moisture content given how much they have shrunk since.

Also, be prepared for the timber to potentially bow on you when you rip it into narrower lengths. After cutting it to your 100mm widths give it a day before you plane and thickness it to allow any tensions in the timber to release. At least then when you plane it down it shouldn't move on you much more again.

Timber will constantly adapt to the atmosphere / environment it is kept in so will continuously expand and contract with the seasons, unless the environment is humidity controlled. You always have to allow for wood movement in any pieces you make. Remember that wood will expand across the grain as each wood fibre (imagine a straw) swells/deflates. It will barely change in length along the grain.

Probably telling you how to suck eggs, but hopefully some of this is helpful.

[DomAU]

Ha! There you go. Beat me to it.

[DomAU]

100% normal. The cupping on the top board is due to it being exposed to changes in temperature and humidity.

If you want to stack them properly, place sticks between each board and under the bottom one as well so that there is even airflow over each piece. Direct sunlight should also be avoided if possible

https://cromartietimber.co.uk/images/site/air_drying_timber.jpg

I want to go to there... Nice stacks of timber!

[Midnight Man]

Many thanks for the advices folks, and I'm glad to hear my thinking wasn't too far out :)

I plan for now on leaving the top pieces turned over, and will monitor them to see when they flatten out - and stop the sunlight hitting them when they do.

Thank you Dom for your advice on ripping, and leaving for a day or so - truly, I'd never have thought to do that, but it makes perfect sense thinking it through :)

Would I be right in thinking that once I rip them, allow them to release tension, then plane them down, that I can apply some sort of finish to the timber that will help to stabilise it a bit? I hadn't yet gotten to thinking through what finish I might put on the workbench - these timbers will be used only for the structural beams of the bench, the working surface will be a sheet of MDF (or possibly nice looking Birch ply) that will have dog holes made with the UJK Parf Guide system. I'm thinking I would like to leave that unfinished to prevent any finish or oil rubbing off on work pieces, but the structural parts I might finish with something?

[DomAU]

Provided it's stored correctly after milling I wouldn't worry about quickly finishing to prevent movement. It shouldn't move.

I would finish the top with something like a few coats of water based polyurethane. Dries quick and will provide a tough finish that stops the mdf/ply abosrbing every spill and stain. It won't rub off onto timber and is easy to clean. Just my opinion.

[elanjacobs]

Make sure you finish the bottom with the same number of coats as the top or you will be in a world of pain

[tonzeyd]

Go to a lumber yard and take a look, most store their lumber outside with a tarp or something very basic if any. Rough sawn timber is just that its rough and will be subject to movement, the amount of movement depends on the cut of timber. Even kiln dried lumber will warp due to changes in humidity hence its a good idea to let them sit for a while once you've brought them home before you cut them down to final dimensions.

Along with the above advice i'd also lay a sheet of tarp down on the floor and place stickers on the floor before laying the wood down, heaps of moisture comes from the floor and will also allow air to flow under the bottom slabs. Its also important to have your stickers in the same spot as per photo above, this will allow even weight distribution of the slack.

Also heard that sealing the ends is also useful to reduce cracking, but I'm impatient and just cut the ends off for firewood.

[Chris Parks]

Putting timber directly on or close to concrete will cause you problems because the concrete itself holds a certain moisture level. Any hard surface will cause some issues if the timber is directly on it with no air space but concrete is worse.