Thread: How thick can you kiln dry wood?

Originally Posted by codeMunk3y

For the past few weeks, I've been looking into bush mills and milling my own timber (thanks to the many people on here who have posted their knowledge)

I'm wanting to build my house in the next 2-3 years and the plans include having 150 x 150mm (6"x6") uprights as part of the main structure. Somewhere I looked recommended a year per inch, meaning I'd be waiting 6 years if I want to mill my own posts and build with them.

My question is: is it possible to construct my own wood drying kiln (I'm thinking in a shipping container) and dry timber of that thickness without it cracking and warping? I've seen a few kilns on this site, but I can't' find anyone who has tried something that big.

Several members have their own kilns and can probably add more insight than mine but here is my 2c worth

A well constructed Kiln into which you pump energy should only take about 3-4 weeks to dry a 6" posts.

However, just putting timber into a sealed container will cook and destroy timber. It needs to be well ventilated. A naturally ventilated (ie air vents and whirly bird type fan will reduce drying time by about half. If you want to dry it faster energy has to be supplied firstly in the form of moving air and if you want it faster still, heat.

Something like a 20 ft container using about 1.5 kw of air movement both to turn over the air inside the container and exchange the air from inside to outside will drop the drying time significantly. my guess is you could maybe get it down to one summer for 6" posts.

If you want it faster you will have to supply heat. This could be in the form of electricity or a thermal heat store such as a pile of rocks. There are designs for heat store kilns on the Woodweb site.

Cheers Bob, I've been looking at solar kiln and was thinking of mounting something on top of a container with a system to move the dry air through the container.

I'll speak to my partners dad on the weekend as he has an engineering background and works with wood himself, together we should be able to come up with something.

My main concern was in regards to trying to dry it too quick and then ruining the wood.

Hi Code, Which species of timber are we looking at here for this project?. Old Pte

Originally Posted by codeMunk3y

For the past few weeks, I've been looking into bush mills and milling my own timber (thanks to the many people on here who have posted their knowledge)

I'm wanting to build my house in the next 2-3 years and the plans include having 150 x 150mm (6"x6") uprights as part of the main structure. Somewhere I looked recommended a year per inch, meaning I'd be waiting 6 years if I want to mill my own posts and build with them.

My question is: is it possible to construct my own wood drying kiln (I'm thinking in a shipping container) and dry timber of that thickness without it cracking and warping? I've seen a few kilns on this site, but I can't' find anyone who has tried something that big.

Looking at Eucalypt / Victorian Ash / Tasmanian Oak

I have a mate with a container kiln. The reason I don't have one myself is I saw what he had to go through. It was very much a trial and error exercise. It now works great and does an excellent job on thin material. Anything over 4 inches thick is not dried right through. Anything up to 4 inch is no problem and the load comes out with a very even reading throughout. The method I use for furniture grade timber is 6-8months air dry, then kiln dry, then recondition. This kiln is what youwould call a slow kiln, they seem to be more forgiving with hardwood thus why it wont dry over 4 inch in areasonable length of time. Have to watch fast kilns though, as they can case dry hardwoods.

I was thinking about air drying for a year and then kiln drying to get the best of both worlds (Faster dry time but without stressing the timber too much.)

The timber will be all the better for it.

Originally Posted by codeMunk3y

Cheers Bob, I've been looking at solar kiln and was thinking of mounting something on top of a container with a system to move the dry air through the container.

I'll speak to my partners dad on the weekend as he has an engineering background and works with wood himself, together we should be able to come up with something.

My main concern was in regards to trying to dry it too quick and then ruining the wood.

It's not just a matter of moving air in and out of the container, at least the same level of attention needs to be paid into moving the air around inside the container. In fact for something like a container 1.5 kW of air movement would be needed inside the container and only about 0.5kW to move air in and out of the container. The balance between air in-out-around is critical to reduce cracking. If all you do is drag heaps of air out of the container it will definitely dry too fast. You may need to reduce air exchange by only running air exchange in the afternoon / evenings or 20 minutes in the hour.

If you just relied on air drying that's 2 kw pr power or where I live thats 44c/hour, or ~$5 a day x ~30 days = $150/month x about 3 months over summer - see how it starts to add up! You wanna make sure you have premium timber in there to be drying at those prices.

I can see this will need more thought than I have done so far, thanks guys for the help.

We're planning on building on a bush block- maybe I'll get the solar panels earlier and use them to run this system, then put them on the house and run a smaller system if I'm not doing such large timbers.

Hi guys

Just be mindful of internal checking in such large square stock in the euc. species. Can be quite nasty and not really appear apart from hairline cracks until you dress the timber.

It may be worthwhile over cutting the timber to 175 x 175 as euc. tensions may give you bent sticks that need re-sizing.

Cheers
James

Cheers James,

I was thinking of going to 160x160 to give myself some leeway but keep the drying down, I don't think I'm going to dress them too much, I kind of like the old not-so-square look.

Solar drying of timber is a subject close to my heart, and I had intended to do it, but ceased my commercial milling activities before I was that far advanced.

I had visited a company that was operating and selling solar kilns based on a 20' insulated shipping container. Very simply it comprised a skillion roof above the container with UV stable plastic stretched over the sloping roof. This provided the heat source with temps >60C in the summer and >40C during the winter (Bellingen). A 1HP fan was situated above the container and in the solar collector area. A 1HP fan is ample and it directed the air into a distribution duct immediately under the container roof. There was also provision for drainage of moisture from the container at floor level.

This fan circulated air to the timber below. Carefull attention was given to block off either end of the container (with foam) to prevent the air "bypassing" the timber. They had rigged up railway line into the container so that a timber charge could moved easily in an out. Basic electronics shut the fan down when the kiln temp went below a certain value (at night and at other cooler times).

This resulted in a slowish but gentle drying regime. They air dried the timber for three months and then put the timber in the kiln for approx two months, but this was dependent on the time of year. This was for 150 x 25 flooded gum (E Grandis). The company is called Rosegum Timbers and they still produce solar kilns, but they now cater for a larger and more sophisticated market primarily in the commercial arena.

Solar Dryers Australia - world leading manufacturers of solar and gas kilns for timber drying

However, while this all sounds doable, the problem for you is the volume. A 20' container will take a charge of 7m3 to 10m3 depending on timber section. (I think they told me they had dried 100 x 100 successfully). Ideally this should all be the same section and for the kiln to work it has to be full or the airflows won't pass through the wood stack. You won't have those quantities for you house.

So really you need to go to a smaller structure using the same principles. A 10' container is too short for most of your structural timbers so you are faced with the issue of building an custom, insulated structure. The economy of all this is just starting to walk out the back door.

Slower drying is not an issue for hardwoods. Indeed it is the better way to go and you should be able to achieve dry material inside 6 months.

Timber framed construction traditionally used green timber. Is it essential to for you to use dry timber?

150 x 150 timber will be a problem to dry easily, but remember timber only shrinks in it's width not the length so green timber may be an option, particularly for posts.

Just another issue is that the species you have nominated in commercial drying require reconditioning to rectify "collapse." I am not sure whether collapse would occur in a slow, solar drying regime.

Regards
Paul

Cheers Paul,

I was looking to dry it, as I was under the idea that if it was too green, then it would move, and the house with it. I'm still new to this so I'm not sure how much drying is needed for framing, or whether I could use greener wood combined with sound building techniques to produce a stable house.

Originally Posted by codeMunk3y

Cheers Paul,

I was looking to dry it, as I was under the idea that if it was too green, then it would move, and the house with it. I'm still new to this so I'm not sure how much drying is needed for framing, or whether I could use greener wood combined with sound building techniques to produce a stable house.

Although I built my own house using traditional timber framing techniques, I would prefer that a professional chippy gives this information, but in principle the old ways allowed for shrinkage as time went on and the timbers dried.

Two old references that might assist are:

Australian Carpenter by C Lloyd
The Australian Carpenter and Joiner by FC Bloomfield (4 volumes)

I am sure there would be many more.

Where are you using the 150 x 150 posts? If they are not visible, don't worry at all. It may be an issue for aesthetic reasons if for example they are verandah posts.

Regards
Paul