A common request for structural engineers it design the steel beam to support the structure over opening for sliding or folding doors, as you can see in the accompanying photo. Since I started my consultancy over 30 years ago these openings and the availability of doors for bigger openings has grown. The opening here looks a like anything up to 6m long. When an engineer designs a beam he checks that the beam is strong enough but they also check that the deflection is such that it will not cause any unsightly cracking. This is normally done by ensuring that the deflection due to imposed loads (people, furniture, snow on roof) is less than the span divided by 360. But folding doors as here would probably get jammed if the deflection for a 6m opening was up to L/360 (17 mm). I have looked at some details for the support track for these doors and there is normally a 5 to 10 mm gap between the underside of the structural support and the track itself. I have not had any instances of homeowners finding that they can't open their doors with a heavy snow load on roof above. But in recent years I now ensure that the beam above is designed so that the live load deflection is less than 10 mm.
