Roof Thrust Overestimated To the Editor: Gordon Tully's short piece (Building with Style, 2/91) on building second stories with half-walls, or knee walls, was a good, concise primer on outward roof thrust and how to deal with it. A few things puzzled me, though. First, a second-floor ledger lag screwed into the studs seems like a poor way to resist outward thrust. If the thrust exists, it would be pulling on the threads of the lag screws — not a very positive connection. Second, I wasn't sure why the article mentioned, but then dismissed, the alternative of anchoring each joist sideways into a wall stud. It has some inconveniences, as noted (you have to line up each joist with a stud,