Here's what Brian Renk has to say about Van Pelt on this:
Van Pelt said:
"The genocidal program in Auschwitz had been adopted in August, the roof was being finished in December, so there was no reason to hack through the roof. They could immediately have made the holes in the roof as they were constructing it".
The 1943 photo doesn't show any evidence of holes or Zyklon- B insertion "chimneys", so there is a problem for van Pelt. At first, he testified that the chimneys can't be seen because "after the dirt was brought on top of the roof of the gas chamber or morgue number one, the [protrusion] would have been less. If we then add snow on top of that, it is very unlikely we would have seen much of these little chimneys".
Two days later, van Pelt recognized his big mistake (probably pointed out to him at the Stockholm conference that weekend). I'll quote from my essay:
"Realizing that the photo shows that there were only a few inches of snow on the roof, he stated that the holes would have been covered with boards, implying that the "introduction chimneys" had not been built in late January". (fn 23: Transcript, January 28, pp. 109-113)
Van Pelt completely contradicted what he had said about immediately making holes (and, by implication, the insertion 'chimneys' also) as they were constructing it, and completely contradicted what he had said about the chimneys being covered by soil and snow in the photo- once he had realized that there were only 3-4 inches of snow (no soil) on the roof in the late January 43 photo. It is the contradiction that is important. They're all over the place.