It is established in the pilot episode that the year is 1876 and Heath is 24. So if Heath spent the last 7 months of the Civil War (which ended in 1865) as a prisoner of war, he would have been 13 at the time.
When the beams of the timbers are raised and temporarily secured in place for the logging sluice, the ends of the square beams are shown as freshly hewn wood colored beams. Moments later, as they fall, the same ends come crashing down in the same shot set-up: they have black paint marks on those ends.
After the timber is felled and made into square beams for the logging sluice, the supposedly solid beams are seen on the ground being finished. The ends of several beams are shown to be pieces of, probably, one inch thick boards nailed together to form a box shaped beam look - the edges are obvious. This is seen again when the assembled sluice falls down.