I was in a DC attic recently and noticed that every one-in-five or so of the roof decking boards had a cement coating.
The boards looked just like our modern day forms boards after being dirtied from use. This roof framing is original to the house. At the time of construction of much of DC row homes, concrete (and therefore concrete forms as well) was not yet in use. So, I had to think and try to figure out where the boards could have been used.
After spending a few minutes thinking it over I realized the boards weren’t concrete forms at all, they were likely scaffold planks used by the mason who built the demising walls (probably not the front lumper).