Loads for Reroofing

[pihbjdc]

I recently took a position with a roofing consulting company. I wanted to get some general information on allowable loads to existing for reroofing commercial projects. My company has taken the claim "anything being added to the structure that is less than 3psf is fine." I have asked where that number and methodology comes from and haven't been given a direct answer.

I tried to my research with Chapter 34 and 15 of the IBC and IEBC Chapters 3,4,5,6.

It is my understanding that reroofing is considered an Alteration Level 1 Under IEBC Section 606.2.

606.2 Addition or replacement of roofing or replacement of equipment. Where addition or replacement of roofing or replacement of equipment results in additional dead loads, structural components supporting such reroofing or equipment shall comply with the gravity load requirements of the International Building Code.
Exceptions:
1. Structural elements where the additional dead load from the roofing or equipment does not increase the force in the element by more than 5 percent.
2. Buildings constructed in accordance with the International Residential Code or the conventional light-frame construction methods of the International Building Code and where the dead load from the roofing or equipment is not increased by more than 5 percent.
3. Addition of a second layer of roof covering weighing 3 pounds per square foot (0.1437 kN/m2) or less over an existing, single layer of roof covering.

My interpretation of the code is that the only case where 3psf load is acceptable is if a second layer of roofing system is being applied over the existing.

My companies typical practice is to tear off to deck, add poly iso, coverboard and whatever roof system and try to keep that near the existing. there are cases where the projects are being peeled down to existing insulation, having poly iso, coverboard and the roof material added to meet energy codes. This can add up to 3psf to the existing structure.

The only thing I can't find in the code is allowance for dead loads for meeting updated building code energy requirements with insulation which usually require a coverboard for mopping the roof membrane to. The coverboard itself can add weight of 1.5-2psf.

My professional interpretation permitted by code to replace the dead load and add up to 5% increase in member internal forces without penalty.

Can anyone in the industry or with knowledge of this share some feedback or insight.

 

[Triangled]

curious as to the geographical area you're considering.
may be I'm wrong but upon a reread of the OP, sounds like the thrust of the project is not so much a re-roof for waterproofing, but a reroof for insulation; and the thrust of your question is can we add lots of insulation so long as the net increase in roof dead load from the existing is 3psf or less.
One thing I'd consider if in snow territory is that if the insulation value is significantly improved such that significantly less heat escapes vertically through the roof system, the snow melting value assumed by the use of Ct may be reduced.

 

[pihbjdc]

To be more specific - this is in the South - Oklahoma, Louisianna, and Texas where snow loads are minimal for 95% of the projects. The reroofing is both for waterproofing and to bring it up to current energy codes. To shorten it up - my company operates with a "less than 3psf add" rule. The only location i found for a reference to 3 psf is IEBC 606.2 exception 3 for layover roofs. Basically I don't see a justification for where my companies rule came from and am looking for external insight.

 

[msquared48]

If a homeowner does a simple reroof in himself, he may not tear off the first layer, only applying the second layer of roofing. The dead load then goes from 12 to 15 psf. I normally use 15 psf DL for the roof.

My parents did exactly this, applying comp over shakes in the 60's.

Mike McCann, PE, SE (WA)

 

[pihbjdc]

msquared48 - as I previously mentioned this is for low slope commercial projects - no residential.

 

[DaveAtkins]

I think adding 3 psf of dead load to an existing roof won't cause structural engineers any "heartburn."

If the Code required an Owner to hire a structural engineer for a simple re-roof project, then the Code would be too burdensome, in my opinion.

DaveAtkins

 

[BUGGAR]

A lot of jurisdictions in CA require us to design for a second layer of roofing, although that doesn't help you there. Do you ever get building documents for your reroofs that would give any idea of the design loads?
I really appreciate your company looking out for engineering.

Mike, how did the comp over shakes work? This was in the Northwest?

 

[Ron]

Unless you are adding insulation for a cold storage application, your weight estimate is high. Consider the following for roof membrane weights:

TPO/PVC/Hypalon.....1 - 1.5 PSF
Torch applied Modified Bitumen.....2 - 2.5 PSF
Hot mopped Modified Bitumen.....3 - 3.5 PSF
Hot Mopped Built-Up Roof membrane, smooth surfaced.....3 - 3.5 PSF
Hot mopped BUR with gravel surfacing.....6 - 7 PSF
Ballasted single ply.....12 PSF

Insulation other than lightweight insulating concrete or cellular concrete...<1 PSF
No net gain for re-roofing is a good goal, but if must add weight, check deflections at least as that can affect ponding which can lead to progressive overload.

 

[msquared48]

Buggar:

The shakes were not that deteriorated and older wood, so it did work for many years - at least 20 I believe. Next roofing though it was all stripped.

When we re-did our roof in 05, all the shakes were removed, plywood laid over the skip sheathing, and the comp roofing installed.

And, yes, this was in the Seattle area.

Mike McCann, PE, SE (WA)