Why has my table cupped?

[Parbynat]

Can someone help on this?
I bought a 4.8m length of 200mm x 45mm C24 softwood from my local timber merchants cut it into 3 and glued it together.
24 hours later I removed from clamps and give it a good sanding.
Then I fixed 2 2x1 inch support pieces 6 inch from the ends to the underside. I then applied 1 coat of danish oil to to the underside followed by 2 coats to the top surface.
5 days later I noticed that it had cupped I would say around 10mm over the 600mm span, lied flat surface facing up the ends are 10mm higher than the middle.
For 5 days it was leant vertical against the wall. The table top is only for a guy I work with for his garage so really no big deal but why did it cup?
Can different coats affect things?
Thanks

 

[Jacob]

Not dry. Needed 6 months at least in a warm room. Finishes slow down drying but all of them are "microporous" etc so it still happens but slower.
It's drier now - just plane off the 10mm?

 

[robgul]

Did you glue the boards together with the end-grain in alternate directions? i.e. curve up . . . curve down . . . curve up for the 3 boards, looking at the end grain rings . The cross pieces need to be "against the curve" of the end grain.

It's happened to me a couple of times with some tables made from scaffold boards even gluing/bracing them as described.

In theory uneven numbers of coats of finish on each can have an effect but that's unlikely with oils, and even less likely in the time. Timber having too high a moisture content is probably the issue.

 

[Parbynat]

Thanks for replying guys.

The timber was in my back bedroom for 4 days before I used it along with 2 other lengths I had bought. Something I have done to them has caused them to cup as the other unused lengths look fine.

Robgul- After watching a video on youtube some time ago this guy who seemed to know his stuff said that in his experience it made no difference regarding alternating grain curve directions. I'd like to hear more from you regarding this if you don't mind.

 

[Jorny]

Construction timber usually have moisture content of 16% when it comes out of the kiln. Timber for furniture should be around 6-9%, unless you are using techniques suitable for green wood. The tabletop has dried and therefore cupped. The geometry of the boards make the cupping more obvious than on the unused lengths.

 

[paulrbarnard]

If your timber is already dried in the location it will live and the humidity in that location doesn’t change then the growth rings can all go the same way. However if there is any likelihood of the humidity changing or your timber is still at a higher misfire content then alternating the growth rings will minimise the effect of cupping across joined boards.

 

[Bristol_Rob]

Definitely the MC is to blame.
It'll move again when it reaches its final destination. So worth factoring that in if you can.

 

[robgul]

Thanks for replying guys.

The timber was in my back bedroom for 4 days before I used it along with 2 other lengths I had bought. Something I have done to them has caused them to cup as the other unused lengths look fine.

Robgul- After watching a video on youtube some time ago this guy who seemed to know his stuff said that in his experience it made no difference regarding alternating grain curve directions. I'd like to hear more from you regarding this if you don't mind.

I can't really answer with any real evidence - the things I've had cup have been made from (new) 2.4m scaffold boards - obviously not the finest quality timber. My instinct was based on logic with wider boards and the fairly open/wide (fast growth) rings. I've seen at least one YT video on the topic - can't remember who it was - that did "believe" the alternating grain theory.

 

[Doug71]

The theory is something like this.

When the wood dries the growth rings shorten causing it to cup.

If you join boards and put them all the same way up you get one continuous curve, if you alternate the rings the whole piece should stay flatter but with a kind of ripple effect.

I tend to go with whichever face looks best and try to make sure the boards are at the right moisture content.

I think your situation wont be helped by the 2x1 screwed on the back. The 2x1s probably aren't thick enough to hold the top flat but are stopping the back of the boards moving as much.

 

[profchris]

You might find this thread helpful:

https://www.ukworkshop.co.uk/threads/humidity-and-wood-movement-the-basics.135336/
The cupping isn't caused by something you did, just by a large humidity change. Knowing why and how wood moves means you can design to cope with it.

 

[Sgian Dubh]

Construction timber usually have moisture content of 16% when it comes out of the kiln. Timber for furniture should be around 6-9%, unless you are using techniques suitable for green wood. The tabletop has dried and therefore cupped. The geometry of the boards make the cupping more obvious than on the unused lengths.

That's not quite so. The target for kilned construction grade timber in Europe is 20% MC ±2%. European furniture grade timber has a kilning target of 11% MC ±2%. In North America the kilning target for furniture grade wood is 7% MC ±2%.

As others have already said, Parbynat's problem was caused by construction grade material continuing to dry after making up the panel, with the panel presumably made up in lower relative humidity conditions than those in which the wood was stored at the time of purchase. Slainte.

 

[Parbynat]

If I was to rewind the clock and start again which timber should I have chosen to make the table.
The options were-
c16
c24
spruce rough sawn
redwood rough sawn (more expensive)
PAR softwood (Most expensive)

 

[paulrbarnard]

Any wood is just fine. You simply have to let it acclimatise to the required moisture content before using it.

 

[Doug71]

I would choose Redwood over Spruce/Whitewood every time.

As you have found wood can move as it acclimatises, if you are able to plane it up you might as well get rough sawn and plane it flat and straight once it's acclimatised. If you buy the PAR stuff it will still move as it's drying out so you will end up having to plane it again so that it's flat and straight which kind of defeats the object of buying PAR.

 

[Ozi]

That's not quite so. The target for kilned construction grade timber in Europe is 20% MC ±2%. European furniture grade timber has a kilning target of 11% MC ±2%. In North America the kilning target for furniture grade wood is 7% MC ±2%.

As others have already said, Parbynat's problem was caused by construction grade material continuing to dry after making up the panel, with the panel presumably made up in lower relative humidity conditions than those in which the wood was stored at the time of purchase. Slainte.

And my last (ever) Travis Perkins delivery 26%

 

[HOJ]

And my last (ever) Travis Perkins delivery

C24 softwood

Why buy from a builders merchants? as @Sgian Dubh outlined they sell construction timber.

I would choose Redwood over Spruce/Whitewood every time.

 

[Parbynat]

I have just bought a moisture content reader, they are not a lot of money but seem to be a good tool to have.

So in days or weeks how long is it recommended with these types of softwoods to leave for them to acclimatise?

 

[paulrbarnard]

I have just bought a moisture content reader, they are not a lot of money but seem to be a good tool to have.

So in days or weeks how long is it recommended with these types of softwoods to leave for them to acclimatise?

A rule of thumb is 1 year per inch when drying wood. Anything you buy is already much dryer than when it was cut so for the wettest construction lumber 3 months per inch should be safe. For your 45mm (bit under 2 inches) 6 months would be safe. For furniture grade a few weeks would be enough.

Also sticker it when you store it to let air circulate and don’t put it on a concrete floor.

 

[Sgian Dubh]

And my last (ever) Travis Perkins delivery 26%

What was it? Recently tanalised, wet rough sawn and not kilned intended for exterior use, greater than 100 mm thick so wouldn't have been kiln dried anyway. 26% MC is two percentage points greater than is allowable for dry grading of kiln dried construction timber.

How did you check the moisture content - moisture meter or oven drying methodology?

26% MC is a bit high even for stuff that's been kilned to a target of 20% MC and subsequently stored in an open sided shed, whether winter or summer and all seasons in between. I suspect there's a detail of some sort missing in your post that could put that reported 26% MC into context. Slainte.

 

[Doug71]

I have just bought a moisture content reader, they are not a lot of money but seem to be a good tool to have.

So in days or weeks how long is it recommended with these types of softwoods to leave for them to acclimatise?

First of all you need to walk around your house, new moisture meter in hand jabbing it into random wooden objects like your dining table, kitchen units and internal doors etc, it might end up looking like your house has woodworm but at least you will have established what MC you are aiming for.

Next bring the wood into the house and store it somewhere (apparently under the bed is a popular place) on laths so air can get around it. Check the MC weekly until it's the same as your dining table etc then it's ready to use

I try to store wood that's going to be used internally in my house for at least a couple of weeks before I use it but the longer the better. It's also important to try and allow for wood movement in the design of what you are making.

I do joinery for a living and encourage the use of MDF whenever possible, it's much less hassle