How do I calc load psf?


I’m currently designing a small office/storage building that will have 2 small green roofs. One on each end. To my surprise, my structural engineer is asking me to provide the anticipated load psf. The more research I do, the more conflicting information I get. These roofs are proposed to have 6” of medium with mostly sedum and similar plants. There will be irrigation but very minimal as required. How can I provide accurate dead load calcs for this?


Jan 9, 24 11:38 am

change engineer

Jan 9, 24 11:50 am  · 
3  ·  1

Find an experienced system provider and work with them. The hard part about that until you have more direct experience is that it is likely the most expensive route. But you don't want to be wrong on the system details (not just design weight). Get a good partner to assist.


Do some deep research & own this with hard data. Give your engineer a number you personally have confidence in.

Jan 9, 24 11:53 am  · 
3  · 

Who is supplying the green roof membrane assembly?  Good suppliers like Soprema have estimated weights for their systems.  I have done a few green roofs and the weigh more than you think.  Once the growing medium is saturated with water is gets very heavy and your structure will need to be beefed up to support it.

Jan 9, 24 1:22 pm  · 
3  · 
Wood Guy

My engineer just upped my estimate of 20 psf to 35 psf for a residential living roof, in addition to other live and dead loads. We are now looking at this range of products, which includes PSF loads:

Jan 9, 24 3:55 pm  · 
1  · 
Non Sequitur

DAMN, inflation hitting hard everywhere.

Jan 9, 24 4:05 pm  · 
1  · 

Well we are all getting fatter . . .

Jan 9, 24 4:18 pm  · 

First thing, look at Chapter 16 of the IBC or your state amended & adopted version, however they call it in the state. If you anticipate you are dealing with person that weighs 300# then you can assume that is 150# per foot and upto 300# per foot concentrated on ONE foot if they were standing on one foot. Usually we calculated by spreading the weight over a roughly 2.5 to 7.5 sq.ft. area per person's weight when we talk about uniform load distribution. If your code requirement is for l/360 then you best design beams and joist at sizes up one or two from minimum or otherwise the size for l/480 to l/600 to assure in most cases that floors won't deflect with the human cows exceeding l/360. You should be able to calculate that because the math isn't that hard. 

The Structural engineer SHOULD know this if they have been doing structural engineering for buildings. What the engineer is probably requesting that information from you to look up the code requirements and what load you are basing on and go from there because he or she doesn't want to spend 5-15 minutes of his or her time to look at the codes. 

Where I am, I do those calcs myself in most cases and even size the beams or joists. I do that enough in preliminary design and when engineer is involved they can adjust from there in the design process as we go. I know enough to be in the ball park for the sizes in most cases. No person designing ANY building should design buildings of any kind or size, if they don't know how to do basic beam and joist structural calculation. This is basic stuff that every person that has any business designing buildings (even houses) for other people, should be competent on this end. This doesn't mean you don't consult an engineer and involve them. Engineers do tend to like design professionals that can already be in the ball park on sizing beams and joists, rafters, etc. 

If the structural engineer doesn't know the stuff for your project type, then they it is a question you may need to inquire if they been doing structural engineering for buildings or is it for stuff that are not related to buildings like bridges. 

Feb 2, 24 11:13 pm  · 

It should be clear that not all structural engineers does structural engineering for buildings. Usually, you should have engineers as consultants with experience in the project types you do design for. However, you may need to involve engineers with background on things that they may mostly do on that might not be buildings because their experience may suit a particular exceptional case that is not typical for your projects in your project type.

Feb 2, 24 11:19 pm  · 

I'd be so tempted to jump on a tannoy shouting about engineers jobs if I was an architect, I'm not though, can't afford the training.

Feb 3, 24 3:20 am  · 

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