Taking the sag out of a flat wood framed roof.

[Scratch3r]

I'm looking at an old flat roof framed with glulam and rough sawn lumber. The client wants to add an HVAC unit to it.

It's sagging and ponding intermittently between glulams. The roof doesn't have drains or scuppers.

Has anyone tried to reverse the sag to eliminate the ponding or should drains be added to the roof?

 

[Agent666]

Usually to solve the issue you'd need to introduce some meaningful falls. This would generally entail ripping off the ply or whatever the roof sheeting is, building up the joists a little to achieve some cross fall and then resheeting/waterproofing. Easy to achieve more or less if there are parapets and you have some room to work to raise up the roof a little, not so easy if there are none as you start messing with the whole envelope. If the sheeting is sagging then its probably due for replacement, you should really address the root cause in any repair otherwise you are really only resetting the clock on the same thing occurring again down the track.

As far as the HVAC unit goes, you could simply put it somewhere that has the least/no effect on worsening any water pooling. Provided of course that the client is ok with it. Provided the roof isn't leaking then this might be an option. Located it over an internal wall or put some framing above the roof to span between wall locations, etc.

 

[msquared48]

Add roof drains in the sagged areas. The roof structure has told you where place the drains, since the original designer did not.

Roof drains should be place on all buildings, and all roofs should be sloped. In case no one noticed, it pretty much rains everywhere...

There are ways to take the sag out, but this is much simpler...

Mike McCann, PE, SE (WA)

 

[Ron]

If the roof has no parapet around it, remove the existing membrane, provide tapered insulation to achieve proper slope (1/4" per foot), and provide a competent roof membrane system over the insulation. This is all assuming the sag is from ponding deflection.

If the sag is recoverable, only the roof membrane and slope need to be addressed.

If the sag is not recoverable, jack the joists up to take out the sag and provide flitch plates using metal or plywood, then reroof the building as described above.

If the building has a parapet, provide internal drains and overflow scuppers through the parapet and/or overflow interior drains. Reroof as above but slope to drains instead of to perimeter.