Timber!

Besides the decking, we had a load of 4x12's sitting in the house. This is because we saw some thick stair treads at a local old mill. We thought that would be a unique way to do our stairs... but I think these were twice as thick... 

(Interesting to note that these giant boards varied wildly between under and over 11"... I knew that lumber wasn't really what it is labeled, but I didn't think it could be an inch and a quarter short in places....)