Thread: Timber expansion per meter - across the grain?

Will it live entirely indoors? Exposed to direct sunlight?

My experience with indoor furniture has been that once it's in the house, you may see a small percentage expansion initially (in dry climates you tend to have higher humidity indoors), then it will be very stable.

I jsut had a floating bamboo floor installed, and they told me that wood expands 2mm per metre across the grain in the tropics, where you have high humidity

This link is from DPI Queensland. It lists properties for a variety of timbers including "Shrinkage to 12% MC". I believe that means the % shrinkage, or expansion, between 0% and 12% Moisture Content. While some of the percentages seems large, over 13% for Tassy Oak for instance, there's no way you will get that variation on timber indoors if it's been sealed with a finish, or even if it hasn't. So you need to estimate what the actuals will be instead of theoretical shrinkage as listed here. It is good though, for a comparison between species.

One of the contributors here wrote an article for AWR giving % expansion per 1% moisture change. I'll se if I can find it and post details.
It's "Estimating Wood Movement" by Jugo Ilic, AWR Issue 51.

I'll try to dig it out and post the table.

I always allow about 10mm - 12mm lip on the table buttons i've found that gives more than enough play for the expansion and contraction of any table top while not weakening the structure of the rails.

Cheers


Steve

Evan Dunston, one of the regular contributers to AWR, estimates about 10mm per metre for most eucalypts. I've use jarrah, karri, blue gum and spotted gum a fair bit and I know these timbers do stretch and shrink alarmingly (I've watched a 3" wide plank of blue gum open a split a whole millimeter wide in just two hours of sunshine). So basically allow 1% across the grain and you shouldn't go too far wrong.

Toolbags suggestion has my vote.

Like CT above said, about 1% on average I allow. I often have table tops 1m to 1.2m move by 10mm untill you get them sealed. They could still move 5mm or more sometimes even after sealing. Extreme dry or humid conditions over a few days will make them move. I had some people call me the other day who I made a table for about 9 months ago saying one end had shrunk back 5mm (over 1.1m) we had had about a week of dry westerly winds. I told them to watch it for a few weeks or untill the winds subsided and keep me posted. They live about 200m from the beach so normaly the table has a high humid environment but the westerlys changed that for a week and bingo, shrinkage. For anything with close tollerances I would check out the shrinkage rates and calculate the min/max into the job. Good luck. The joys and chalenge of working with real timber.
I also had some Aust Red Cedar doors 820 wide 40mm thick that would expand and contract 5mm over night each night untill they were sealed.

An extract from a site I just visited, has good info.
My only minor correction on this is the term should be hygroscopic not hydroscopic, as I said, minor.

Moisture content and expansion and contraction Top of Page

Timber is hydroscopic, i.e.: it attracts and retains moisture. As the humidity of the air changes so does the moisture content of the timber. There are really no timber finishes that will prevent 100% the transfer of water vapour in and out of timber. Finishes will only slow the process down (even two pack coatings).
Some timbers are more hydroscopic than others, mainly due to the extractives they contain. Different species also react differently due to the grain structure and boards from the same species can behave differently by the way the timber has been cut (back sawn or quarter sawn).
A big factor in using the Australian hardwoods is the strength of the swelling and shrinking that is directly related to the density. Timber of 950 kg/m3 has approximately 9 times the swelling strength as a timber of 550 kg/m3, the relationship between density and swelling strength is exponential.
All timber structures such as furniture and floors should allow for the expansion and contraction of timber however the Moisture Content (MC) of the timber should be as suitable for your application at the commencement. One method to do this is 'equalisation', see below.
A workshop on the east coast of Australia will generally have a year round average of 12% EMC. However your house which you have air conditioned, heated in winter, open the windows on a fine day and closed them on a rainy day will have an EMC ranging from 6-12%, a relatively harsh regime for furniture unless it is not designed properly. For issues with flooring see the relevant section.
How does timber expand and contract?
For practical purposes your timber will swell across and through the board not along the length.

Allowing for timber movement.

As timber seasons, a process that will continue for the rest of the timbers life, its movement due to changes in MC will become less pronounced but will never cease entirely. Anecdote: One night I heard a loud 'bang' from the living room. The side of a 150 year old cedar buffet split open (about 3-4 mm) from top to bottom. The buffet had been in the same living room for 5 years.
Example 1 Brush Box (Tristiana conferta) at 950 kg/m3 and a very high level of extractives has an average coefficient of movement of approx. 0.3. This means for a 300 mm board every 1% change in MC will result in a 0.9 mm change in dimension. For a 1200 mm wide table top taking it from a 12% EMC in the workshop into a 6% air-conditioned house may result in a 22 mm change in width.
Example 2. A timber floor laid straight out of the pack delivered at 10% is laid on timber studs above damp unventilated ground increases to 13% MC could see it try to expand 36 mm. There have been cases of timber flooring pushing brick walls outwards.


Furniture construction: Some do's and don'ts

• Do not screw drawer runners across the grain, screw the front of the runner but have a slot or oversize hole at the back so the side can expand and contract without problem. If using metal runners use the elongated holes.
• Panel and frame structures are good but when finishing make sure the finish does not glue the panel into the frame.
• Screw counter tops tight at one edge then the remaining screws need to allow for movement
• Do not fix a table top tight to the table frame. Your method of fixing (buttons or oversized screw holes) should let you be able to shift the top relative to the frame by a firm nudge with your hand.
• Waxing between frame members and table tops helps prevent sticking due to the finishes.
• Test the timber's MC. For furniture components where the timber is too moist (over 12%) dry further before final dimensioning.

Questions to ask yourself

• Is you timber at the right moisture content (MC)?
• Do you have a moisture meter to check what your supplier has provided?
• Does your supplier offer the results of their MC testing before the timber left their premises?
• Does your supplier (or their supplier) have a recognised and regular MC testing system?
• Does this system follow any National or International standards?

Here's the table I was talking about.

Awesome responses. Thanks to all.
Gave me something to think about when I am putting rails for draws across the grain too.

Hmmm actually I think I need to rethink alot of my ideas on when it is appropriate to "hard" screw or glue. Thanks

These are great responses and something to remember. Is there any good textbooks that describe incorporating timber movement into furniture design?

Im only just learning to work with 'real timber' and would be great to read up a bit more about it all. Obviously it will all come with experience, But a bit more understanding to prepare me for it