Occupied Roof Live Loads IBC/ASCE

[Bisbee_Structural]

I find Occupied Roof Live loads to be very inconsistent in the ASCE/IBC and would appreciate your input!

What live loads do you use for a residential occupied roof (single-family)?

ASCE 7-16 Table 4.3-1 allows for a live load at the roof same as the occupancy served, which for a house would be a 40 psf live load. This was first introduced in ASCE 7-10.

IBC 2018 Table 1607.1 has no similar allowance for a residential occupancy. The only live loads given are for Roof Gardens (100 psf), Assembly Areas (100 psf), and "all other similar areas" which refers to footnote l: "Areas of occupiable roofs, other than roof gardens and assembly areas, shall be designed for appropriate loads as approved by the building official". This was first introduced in IBC 2012.

IBC 2009 Table 1607.1 gives live loads for Roofs used for promenade purposes (60 psf), Roofs used for roof gardens or assembly purposes (100 psf), and "Roofs used for other special purposes" which refers to footnote l: "Roofs used for other special purposes shall be designed for appropriate loads as approved by the building official"

I can't find specific recommendations for my project jurisdiction (NY State).

My thoughts:

1) In my judgement 40 psf is too little for a roof deck, and 100 psf is too conservative.

2) In the past I have used a live loads of 1.5 times the live load of the area served, based off the of the live load requirements for balconies and decks. 60 psf seems more appropriate to me, assuming people will crowd onto a roof similar to a balcony/deck. In the absence of more specific requirements from the authority having jurisdiction this seems like a reasonable and defensible position under footnote l of the 2018 IBC.

3) I am uncertain how this fits with the category 'roof garden' defined in the commentary in the 2018 IBC: "Roof gardens are landscaped areas where assembly use is not anticipated, such as a promenade deck located on the main roof of an apartment building". This seems to push towards a 100 psf live load even in roofs where assembly use is not anticipated.

4) My project features an occupied roof with a paved pedestal roof deck system and an extensive green roof system not explicitly designed for occupants but also not physically separated from the roof deck by railings/guards etc. Based on the above, I would assume 60 psf for the paved deck and 100 psf for the green roof. But this seems illogical- why would the green roof have a higher live load than the adjacent paved deck?

I would greatly appreciate any and all input!

 

[phamENG]

Because green roofs are typically saturated with water, so they can get real heavy real quick. Your paved deck is a dead load, so you their you're only worried about people in your live load. I think the 60psf for occupied areas of a residential roof and 100psf for a green roof/roof garden is a sensible way of doing it.

 

[Bisbee_Structural]

@phamENG:

Thanks for the quick reply.

I disagree with your reply, though, because I have always understood green roofs to be dead load. After all the weight of the green roof can vary greatly between different roof systems and can be much larger than 100 psf.

This is backed up by Section 1607.13.3.1 of the 2018 IBC: "The weight of all landscaping materials shall be considered as dead load and shall be computed on the basis of saturation of the soil as determined in accordance with Section 3.1.4 of ASCE 7."

 

[phamENG]

Fair enough. Thanks for the reference.

A quick search brought up a couple similar threads. Nothing pointing to anything definite that I saw, but most come back to the fact that the soil can move. So yes, design the roof for a dead load of the saturated soil. But what happens when there's a leak? Are they going to carefully remove the soil and place it little by little on the ground? No. That 10'x10' area is going to end up in a pile next to the area they need to patch. THAT is going to be a big live load. Note that as this doesn't exceed 100psf and isn't an assembly area, it's a reducible load.

 

[Bisbee_Structural]

@phamENG:

Yes, unfortunately I haven't found a lot of good/definitive info on the topic of live loads on occupied roofs, including on this board.

I am surprised since green/occupied roofs have become so trendy in the last 10+ years.

I totally agree with your point about leaking. I think leaking green roofs will be a big issue 20-30 years from now, and I wouldn't be surprised to see most green roofs disappear as owners realize the cost to repair/replace a green roof!

I suspect that a 10' x 10' opening will not be enough to find and repair the leak on a green roof membrane but have no experience with the issue