Green roof live load

[UcfSE]

ASCE 7 specifies 100 psf live load for special purpose roofs. Would you consider a "green roof" to be a roof garden as noted near the end of chapter 4 such that the special 100 psf live load is required?

For this puropse let's assume that people are not meant to walk on this roof; it's only meant to be green without access for the public or assembly. The garden roof will be similar to these. We don't know which has potential for selection at this point.

 

[frv]

How deep is the soil?

A commonly used value for the weight of soil is 125 PCF.

Seems like 100 would be kind of low.

 

[cvg]

is soil considered live or dead load? what about water?

 

[Lion06]

frv-
that is DL not LL. We recently did something similar, and we only used the snow drift load. We used the masimum drift load everywhere on teh green roof (it was around 75 psf.
If people aren't meant to be up there, it seems hard to justify a 100 psf LL.

 

[cvg]

temporary loads much higher than 100 psf could be seen in small areas during placement of the soil. this could be a recurring thing for maintenance of the roof (repairing leaks). even light weight soil can weigh 80 - 100 pounds per cubic foot.

 

[frv]

I wouldn't say that it's so clear cut whether the soil is dead or live in this case. Especially considering it's on the roof. I once had a professor tell me that if it's not bolted to the building, you really should consider it live.

What happens 15 years down the road when someone decides do add a meager 4" of soil to revitalize the garden? That's over 40 PSF load you hadn't considered.

 

[UcfSE]

The type of green roof in question is "engineered" to be light and doesn't have typical soil with plants (see link above). The company's website lists weights from about 18 psf to a little over 45-50 psf or so. Sometimes you can find them specified with a dry weight and wet weight.

As stated above, it seems difficult to justify special roof loading of 100 psf live load if there is no access to the roof, but then I've been wrong before. That's why we have eng-tips! What would be great would be some literature convering this topic.

 

[UcfSE]

Section 1607.11.2.3 of the 2006 IBC is for landscaped roofs. I think this is more along the lines of what I'm thinking. This section provides for 20 psf live load and the weight of the landscaping is to be dead load and is to be based on the saturated soil weight. Makes sense to me.

I didn't find this earlier because we don't use the 2006 IBC in my state and the ASCE 7 does not have a corresponding section.

This came about because a coworker is saying we should use the 100 psf for special purpose roofs. I'm interpreting this requirement to be based on assembly and not applicable where there is no access for assembly.

What do you think?

 

[strguy11]

I have seen some "engineered" systems that weigh about 20-40 psf, however, they had integral drainage tubes. My worry is that these could become clogged and water could build up and be heavier.

 

[gsmith22]

just thought I would throw in my 2 cents:
engineered soil is expensive. dirt not so much. I have heard of (haven't been involved with to my knowledge) contractors replacing the engineered soil with regular dirt to pad their bottom line. Regular soil is as much as three times heavier than the engineered stuff. As someone else suggested, long term maintainence could force small amounts of soil to be added later without the guidance of a complete design team. I would be overly cautious with a green roof design for loading as there are many variables not neccesserily under your control.

 

[hokie66]

If it were my job, I would use 120pcf x the thickness of the landscaping material above the concrete as a dead load, and live load based on the intended or possible use of the roof. To say that no one will walk on it is ridiculous, but the actual use can be related to how easy the roof is to access.

 

[JLNJ]

Whatever you do the viariability of the loading possibilities dictate you be very conservative.

I'd be less worried about a possible load from occupants and more worried about the landscaping material. Just the water soaking into the material when it rains might exceed your rated "occupant" live load.

 

[archeng59]

I've used the weight of saturated soil in addition to roof live load and snow load. In my opinion, the soil and greenery are live loads because they can be changed. Pavers, water lines, and associated planters are dead loads.

 

[UcfSE]

All good advice. Thanks everyone.

I would be conservative if it were up to me in estimating the soil loads. The question I'm having relates more to a colleague getting too caught up in terminology, imo.

The project in question isn't mine but I am trying to offer some advice.

 

[Atomic25]

If people arent meant to walk on the roof then it's a landscaped roof. 20psf LL

If people can go up there, then its a garden. 100 psf LL

A garden is something people interact with, landscaping is not. Hope you straighten you college out.

 

[MDukh]

Atomic25 is correct! Obviously the load shall be taken LL=100 psf. Also, for green roof 1/4' in/ft slope does not provide sufficient drainage and in calculations shall be checked ponding instability from rain-on-snow by ASCE 7, paragraph 7.11 and rain loads by Chapter 8. Probably problem with drainage will increase with time even more. Because there are no Code provisions for green roofs, common sense shall be used heavily for this kind of roof.

 

[strguy11]

ASCE 7-05 indicates that "Yards and Terraces, pedestrian" should be designed for 100 psf. I would apply this along the paths of the roof, and where the actual planting beds are, I think a LL of 20 psf + DL is ok.