Yep, I am more then a little cautious about that. Put the oven on the concrete driveway about 8-feet away from the garage.
I did a bit of research (not an exact quote, I spell checked to remove British spelling and added the conversion from degrees C to degrees F.)
"Fire properties of wood
When the temperature of wood rises to 100 C, (212-F) chemically unbound water begins to evaporate from it. The thermal softening of dry wood begins at a temperature of about 180 C (356-F) and reaches its maximum between 320 C and 380 C (608-716 F). Then the lignin, cellulose and hemicellulose in the wood begin to disintegrate. The softening of moist wood begins earlier, at about 100 C (212-F).
The ignition temperature of wood is affected by how long it is exposed to heat. Wood usually ignites at 250 – 300 C (482-572 F) . After ignition, the wood begins to carbonize at a rate of 0.8 mm per minute. Fire progresses slowly in a solid wood product, as the layer of carbon created protects the wood, and slows down the increase in temperature of the wood's inner parts and thus the progress of the fire. For example, at a distance of 15 mm from the carbonization limit, the temperature of the wood is under 100 C (212-F). This property is utilized in dimensioning load-bearing structures, among other things.
In glued laminated wood, the speed of carbonization is less at 0.7 mm/min. The ignition sensitivity of wood increases as its density and moisture content decrease, and as the thickness of an individual piece of wood decreases. The sharp corners, rough surface, flaws and cracks of timber also increase the impact of fire."
What can be taken from this, I believe is: "The ignition temperature of wood is affected by how long it is exposed to heat. Wood usually ignites at 250 – 300 C (482-572 F)" Limit the time and keep the temperature of the wood below the ignition point.
Of course, there is a risk involved so it is up to each individual to evaluate such risk and take on only that which you can afford.
