What is the larger building in the background? I can‘t figure it out.

[removed]

It's the old Patent Office Building (now the Donald W. Reynolds Center, home of the Smithsonian American Art Museum and the National Portrait Gallery). ETA: that's the G Street side, which I believe was the first wing completed.

And now I instantly recognize it. Thanks!

Buildings in the foreground are now the Pepco building at 9th and G St NW

This must be one of the oldest photos of the District.

What's the deal with the roof of the building on the right middle? Usually there aren't giant gaps.

I don't believe those are gaps. I believe that is a lead standing seam roof with either wet spots or tar sealing strips painted on it. That said, check the ladders on the roof next door. I think it was in the middle of a chimney sweep when the photo was taken.

Those looks like ladders resting on the roof.

That building in the back has a very important role in the history of fire and life safety design in the US. One of the most interesting stories in DC construction history imo.