Green Roof Design

[kxa]

What are the requirements for designing green roofs? Are there any references that define all the elements of evaluating, designing and installing a planting area on an existing rolled roof supported by wood joists. The bldg is located in northeast and is five stories high with a 42” parapet. Any help is greatly appreciated.

 

[Ron]

Are you designing the roof membrane system, the structural system or both?

The roof membrane system is easy...lots of details, but easy. Mostly try to accommodate energy efficiency through insulation, reflection, and "green" products. Difficult comes in having planters, sod, flowing water, and traffic on roof. All have significant detailing/waterproofing issues.

As for the structure, that's another story....could encompass anything...planters, sod on roof, water loads, etc. All increase the structural system and thus, the cost.

 

[JAE]

We've done a few of these lately - seem to hover around 30 psf additional load on your roof.

This is in addition to the code required live load so sometimes the roof structure doesn't have the ability to support the green roof without strenthening.

 

[msquared48]

I am currently designing one with 15 psf dead and 75 live. I figure the normal 25 psf snow plus 50 psf for the planters and grass. No pavers. Placement of the roof drains at the low point of deflection of the roof is important.

Mike McCann
MMC Engineering

 

[BSE05]

From a roofing standpoint this has to be the worst idea of the green century. What do you do if you have a roof leak? Dig up your garden?

Stay away from condos and keep the liability insurance payments current.

IMHO

 

[Den32]

Structurally, it really depends on what the Green Roof System entails (how much soil, etc.). We have designed 'Green' Roofs from anywhere between 30 to 200 PSF.

Be careful suppporting on Wood Joists - Even if joists work when checked under this additional load - possible issues with rot, long-term creep (is there a gypsum ceiling below?), etc.

 

[JAE]

SteelyLee,

The green roofs I've done are individual panels of grass or something that can be easily moved. They are not typically integral with the roofing system.

 

[kxa]

I believe they are looking for 5”- 9” of planting bed and there may be some brick pavers. In addition to the weight, I am also concerned about damage to the roofing membrane that after placing the soil and gravel on it will be hard to find and fix. How about using light weight metal sheathing that would not corrode but yet, provides for some protection.

Wind uplift may also be an issue here. After all, we don’t want everything to get blown off the roof.