Thread: Moisture content and outdoor furniture

I am very new to woodwork, and keen to build an outdoor table. I’ve done a bit of readong, and there is surely much I don’t know, but right now I’m interested in the issue of changes in ambient moisture content and the effect it may have on what I build.

At this stage, my plan is to build a large “farmhouse-style” table, 2.5m x 1.2m x 0.7m high, something like this. The top will have 5 180x40x2500mm lengths, with a 10mm gap between each one. I live in Brisbane, Queensland, and will use Queensland Blackbutt, a hardwood that we’ve used in our garden retaining. I will probably use pocket joinery. It will be VERY heavy. The plan is certainly not finalised and all comments are welcome!

The table will be entirely exposed to the elements (on a concrete patio with no roof, and full sun approx 8 hrs/day in summer). Brisbane is a sub-tropical climate, usually a very wet, humid summer, and gets massive downpours from time to time. My concern is the change in relative humidity. According to the Aust Weather Bureau yesterday (Feb 11 2012) it ranged from 93% (6am) to 52% (11.30am). On Jan 11 it was 13%.

Our blackbutt retaining walls (which are dressed but untreated) have been in place for 6 months and many have cupped and warped, and some have split at the end. That timber is 180mm x 40mm in lengths up to 4m.

I want to avoid cupping/warping in my table. I’ve read this and many other such pages. I plan to seal the wood with Cutek CD-50 to mitigate some of the effect of changes in ambient MC. I plan to orient the grain appropriately. I plan to attach the top lengths using floating joints (screws in slots) to allow the boards to move across the grain. (Apologies for any confusing terminology).

So assuming I’m on the right track so far... how do I order the wood? My understanding is that the wood should be “acclimated” to the final environment it’s going to be in. My environment is pretty extreme from what I understand. I’m worried that if the delivered wood has a very low MC (e.g. kiln-dried and stored inside) it will warp during the acclimation period (under my house), making it unusable for the job. Can I order it “for outdoor use” with an appropriately high MC level? What is an appropriately high MC level anyway - it changes so much every day?

Do I need to worry about it at all? I suspect I do given the way my retaining walls are moving.

Thanks,
Andy.