Expert: Bruce E. Johnson - 10/27/2009

Question
I basically need to know how to tell if people don't know what they are doing, and if they are even looking at the right kinds of materials or even looking at the problems correctly.

I have a commercial roof that is 26 x 123 feet. The roof slopes along the long side; the high edge and the low edge are 26 feet. This is a single slope roof. All of the roof framing spans the entire 26 feet with no center support, and the roof joists run at 90 degrees to the slope. The roof has a slope of 0.5/12 and currently has rubber as a surface. The deck under the rubber is varied width 1” thick planks running parallel with the slope. Starting at the high edge, at least half is framed with 2x14 (at least, it might be bigger than 14”) rough cut lumber at 12” O.C. The rear sections I can check appear to be 2x10 (9.25 actual) at 16” or 18” O.C., and there is some leakage in this area. The roof has a noticeable bounce in the rear when walking on it (in the front there is none). This roof currently is holding 3 (!) commercial AC/heat units which will be leaving. I won’t be having any AC/heat units on the roof. This roof is inside parapet walls on the high side and two long sides that don’t extend more than 14” above the roof surface.

This is an old building.

For at least half of the building, the structure is dry. There has been leaking in the rear section (it worsens as it progresses, which appears to have been made worse by the fact that insulation was shoved between the joists and then a ceiling was put on (without fixing the leaks), and it was retaining the water. The roof has a noticeable pond on the rear in rainy weather. The rubber is old and bubbled, and pulled away from the parapet walls.

Someone was talking about sistering dimensional lumber joists to the existing ones in the rear using bolts and replacing the planking with plywood in the rear (where the pond is). I was under the assumption that it would be best to use an engineered product like LVL joists, rather than dimensional, because of the length. I am not even sure if it would be better to sister them or install new, sturdy joists in their place. I also don’t know if it would be bad to try to lift the roof decking in the center (it appears to have been done before). I want to be as economical as possible, but not at the expense of an unstable roof. The roof will have coatings and/or rubber applied to it.

I also need to how plugging AC roof curb openings is normally done. I don’t want leaky rectangles in my ceiling.

Also, some people want to apply OSB or plywood over the whole roof, on top of the existing plank decking.

I realize this is long, so if you have thoughts on any part of it, lease share them (you don’t have to respond to everything I have mentioned if you don’t want to).

Answer
HI Joseph, where do I start?  It sounds like you have done some legwork already on this one.  First of all, the rough 2x14's or whatever they are, sound like they are hanging in there at this point.  The 2x 10 roof section is probably ruined because the 2x10s were not sufficient for the span, they sagged, water gathered at the sag, the sag got worse, etc..If you are going to go through the trouble of fixing this section of roof, fix it properly.  Rip out the bad joists and replace with engineered joists or engineered trusses.  As far as leveling out after the new section is sheathed and ready for roofing?  That can be as simple as hmving the new joists manufactured taller so that when sheathed they line up with the good section of roof.  As far as the A/C curbs go..you have to tear them out and have the roof patched.  If you are doing a new roof then the point is moot, the new roofer can rip out the curbs, infill with insulation board to bring the roof back up to elevation and then proceed with the new roof..At a 5/12 pitch you want a built up heavy, fiberglass reinforced roll roofing of 90 lbs or better.  I like double coverage (twice the normal lap) on my commercial 90# roofs.  As far as OSB goes?  We use it all the time.  But I like to use it with a polyisocyanurate insulation glued to it.  This way we can insulate the roof deck and free up the interior roof space for utilities and general esthetics..I hope this information helps, please feel free to write again regarding this or other matters, sincerely bruce e johnson..bejohnsonconsulting.com