I don't see how concrete would form this roof unless it is hand formed ferrocement. It may also be some kind of reinforced stucco.

To waterproof the plywood apply a layer of self-adhering modified-asphalt (Ice and Water Shield by Grace without substitute). To be super safe use their water based primer.

To waterproof the finished material there are many liquid applied roof coatings. Tremco is a major brand.

Google ferrocement roofing

Here is a link that might be useful: ferrocement

There are also deck systems which are composed of technical cements, LIFE PAINTS in Santa Fe Springs, is one of them.
I have a question; why hadn't you selected the product and have the roof framing engineered to carry the load?

Instead of ass-backwards, as you are attempting now.

RENOVATOR8: THANK YOU SO MUCH FOR THE POST. Our contractor had just been reading up on ferro, so your post was extremely timely! And also THANK YOU for the waterproofing tips -- VERY MUCH APPRECIATED! You have pretty much reinforced the direction we were headed in.

SNOONYB: Yes, well, the reason we didn't have it engineered is because we DID NOT THINK WE WOULD HAVE TO TAKE THE OLD ROOF OFF IN THE FIRST PLACE! This is a fire damaged property, and unfortunately, in such scenarios of restoration, you never know what you may encounter. Playing safe with the Department of Building and Safety and attempting to preserve the original historic architecture has been a delicate balancing act. In this instance, we thought we'd be able to get away with keeping the roof, but DBS basically said to us, "There's only so much you guys can do and you're already testing the limits." We decided at that point we'd better tear off the roof. We can, of course, put slate on the roof. But it won't match the Disneyland-esque walls of the house and so we're searching for alternatives that will mimic the original look of the house.

"in such scenarios of restoration, you never know what you may encounter. Playing safe with the Department of Building and Safety and attempting to preserve the original historic architecture"

This tells me that you did not have competent counsel from either an architect or building contractor.

Immediately upon making the decision, I'd have advised you to contact a civil or structural engineer, investigated the different processes and vendors available, which are LA city approved, and within 1 week of you making a decision on the product you would have had, in your hand, a wet stamped set of plans to submit to the city, with the vendors certificate for the process.

I've done business from Ventura too San Clemente for 35yrs. and am well familiar with both the City and County.

"This tells me that you did not have competent counsel from either an architect or building contractor."

This tells me you have not had much experience restoring unique, historic, fire-damaged properties.

"Immediately upon making the decision"...immediately upon making what decision? "Within 1 week of you making a decision on the product you would have had"...What product are you referring to?

The architect and structural engineer MANDATED BY THE INSURANCE COMPANY submitted plans more than two years from the time of the fire to the time of the financial settlement, and even those plans had a disclaimer stating there was no way to determine the extent of damage due to the extreme smoke damage in the house (the house baked for 10 hours). Everyone who works on fire damaged properties will tell you that you can't really determine the extent of damage until the fire damaged timbers come down. At the time, no one was focused on actually rebuilding the house as much as fighting for a decent settlement, and trying to cover worst-case scenarios in a construction bid so that my back as homeowner would be covered. The ASSUMPTION was that if he had to replace the roof we would do so with slate. The ASSUMPTION was that if we did not have to take the walls down we would sandblast them and start anew. No one could predict for sure what would happen once construction began, least of all me, the homeowner. Things are not so black and white in a fire damaged property. In the end, I selected the contractor whose sensibilities were most like mine: to attempt a complete historic restoration. But this was LOOOONG after the insurance companies (yes, there were THREE insurance companies involved) were pressuring me for the construction estimate.

And I'll say it again;"This tells me that you did not have competent counsel from either an architect or building contractor."

"This tells me you have not had much experience restoring unique, historic, fire-damaged properties."

You have no idea how many floors, attics and walls I've opened, crawled through, cut apart and assisted in the preparation of damage and loss assessment.

"Everyone who works on fire damaged properties will tell you that you can't really determine the extent of damage until the fire damaged timbers come down."

This is false, and again goes to competency, because those familiar with how fire affects wood species can rather easily determine how the wood member has been affected, as well as it's relationship with other members associated with it.

"immediately upon making what decision?"

As you said, to replace the "defined method" from, too cement.

"Within 1 week of you making a decision on the product you would have had"...

"What product are you referring to?"

Competency, again. A contractor or architect, would have had a working superintendent on the job who would be cognizant of the available alternate cement roofing products available and with little more than a couple of phone calls had the information in the contractor or architects hand and a job site review for a subsequent bid submission scheduled.

"The architect and structural engineer MANDATED BY THE INSURANCE COMPANY" submitted plans more than two years from the time of the fire to the time of the financial settlement,"

The loss assessment and report should have been completed in little more than 10 days.

Engineering is an exact science, based upon assumptions, and there are minimum standards established by municipalities, beyond which both product and process can affect the fimal product.

" and even those plans had a disclaimer stating there was no way to determine the extent of damage due to the extreme smoke damage in the house"

Competency, again.

"(the house baked for 10 hours)."

How long had it sat vacant?

Here in Los Angeles, NOBODY knows anything about concrete roofing because it simply isn't done. Not Building and Safety, not the code or plan checkers, not the engineers, not the contractors, not the roofers. If they did, I wouldn't have posted this question. But I am grateful to Renovator for giving me the constructive advice I sought through this wonderful forum.

The loss assessment and report was completed within three months. It is NOT EASY to assess the true and accurate loss on a historic, one-of-a-kind property, and there is no contractor who could completely do this accurately on the spot because there simply are no tradesmen who work in say, faux bois, which the house had all over the place. And who knew, at the time, how collectable and valuable this stuff is now?

To answer your question, the house sat vacant for more than 2.5 years after the time of the fire.

What we have decided to do, because no one knows about concrete roofing here, is to create a sample using ferrocement which will prove the weight of the intended application. We can document this and then provide to the engineer as well as dbs to prove the weight accurately and conclude that the shearing and concrete pads are adequately built. This is our game plan, as of now...

SNOONY...This is what we are up against...found in my email this morning from the engineer...

"I have never heard of this product. As mentioned previously, to have it approved would be opening a can of worms. Building and Safety will probably see this as a rigid diaphragm and will require a rigid diaphragm analysis. This will also cause a review of the existing shear walls at the ends of the gable roof, which do not meet current Code in terms of height to width ratio, but was able to get approved for you as is.
At a minimum, you could expect Building and Safety to have the end walls re-engineered with steel moment frames which means most of the end walls will again have to be reframed, on top of whatever other crazy comments they will come up with from using a non-standard product."

If any engineer said that to me, my shadow would never be seen near his office again.

"What we have decided to do, because no one knows about concrete roofing here, is to create a sample using ferrocement which will prove the weight of the intended application. We can document this and then provide to the engineer as well as dbs to prove the weight accurately and conclude that the shearing and concrete pads are adequately built. This is our game plan, as of now..."

Don't waste your time All specialty products must be certified by specific testing agencies and then those certifications submitted to the state board for approval, however, LA City has it's own building code, which is more stringent than those State standards.

KISS principal.

The simplest solution is to ask Ren. 8 for the contact information for the trade who applies the product and/or the certified supplier.

Your eng. is correct in his assumption regarding the Cities response.

RENOVATOR8 - Do you have contact info for tradesman (men?) who apply ferrocement, and/or a certified supplier? Snoonyb suggested I ask you, as an aid to help me present this to Los Angeles Building and Safety, which apparently knows nothing about ferrocement.

SNOONY: I'm just curious...what bothered you about what the engineer said? Also, I don't know how to reach Ren. 8 other than to post a follow-up here, which I just did. Do you? Anyway, thanks for following the saga, and for your suggestions.

ALSO, SNOONY - It is my understanding that ferrocement is simply 1 part Portland cement to 3 parts sand. It's a recipe, if you will, that anyone can mix up. So I'm not sure if there would even be a specific testing agency.

Ferrocement is a combination of cement and rebar and/or wire mesh reinforcement. Check out boat builders and swimming pool construction.

Here is a link that might be useful: ferrocement roof

zagy: You can email renovator8 through clicking on his name above. As long as he's got email enabled, he'll get it. Make sure you include your email address in the body of the note so he can reply to you. GW doesn't include your email info.

"I have never heard of this product."

Being cognizant of available building products and processes are an eng. firms life blood.
What was omitted from their response was, we will investigate the process and forward our recommendations,
Instead, you received a speculation, which, given the nature of the Cities continued evolution regarding the tectonics in play, may be an accurate assumption.

Curiosity is the mother of invention, as many sucesses have developed from it.

SNOONY - Agreed. I think part of what's going on here is that the engineer already got paid by my insurance company more than two years ago and, at at least at this point, he's not charging us for follow-up work and thus doesn't have a lot of incentive to investigate that which he doesn't understand or know about.

And the architects position?

The architect has been paid already as well, although he we still have a small amount of unused credit with him. It didn't even occur to us to go to the architect with the ferro because it seems more like a matter for engineering.

Ok, who pulled the permits?

Architect

"It didn't even occur to us to go to the architect"

You can't be serious!

You can't even move in until the architect authorizes it.

You need to, carefully, read your agreement with the architect.

Hey zags, you emailed me instead of Reno8. You need to click on his screen name above.

The architect is finished with us. The city inspectors will sign off, and then we're good to go.

Sorry, I'm in New England and haven't done anything with ferrocement since building a row boat out of it in school and launching into the Charles River.

Here is a link that might be useful: Fantastic Ferrocement

Reno8 - Thanks for the link!

off topic zz...but isn't there a link that tells more about your unique house? seems I had seen it in a post a couple of years ago...
tia!