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waterproofing concrete on metal decking
4

waterproofing concrete on metal decking

waterproofing concrete on metal decking

(OP)
I have an exterior mezzanine that will have an occupied space below. This mezzanine needs to be completely watertight. The problem is that metal deck below the concrete and waterproofing above the concrete traps all the moisture of the slab inside. Has anyone ever had to waterproof an elevated concrete slab on metal decking?

RE: waterproofing concrete on metal decking

With occupied space below, I'd go with a topping and membrane system. You can get deck with pre punched weep holes.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: waterproofing concrete on metal decking

sponcyv....in your area, your code requires that any topping on concrete over occupied space serve as a roof as well, so you have an added issue of having "traffic" on a roof membrane system. This can be handled in several ways. First you can put a proper roof system such as a modified bitumen roof on the concrete deck, and then cover the roof membrane with pavers for protection.

A second option is to provide a fluid applied, reinforced membrane on the concrete deck in multiple layers with the final layer having coarse sand broadcast onto the membrane before it dries. If you use this option, make sure the manufacturer approves its membrane for roofing application and that your local code jurisdiction will also approve it as a roofing membrane. Option 1 will not require this step as it is already an approved roofing system.

RE: waterproofing concrete on metal decking

(OP)
Kootk - thanks for the info about pre-punched holes. I didn't know that.

Ron - there isn't an architect involved on this one and the code issue that you brought up is a good one. I recommended a roofing membrane to the client (contractor) and they told me that the owner is ok with some leaking (going to be a bathroom below). I'm going to call out a waterproofing system on my drawings to cover my rear and they can do what they want.

RE: waterproofing concrete on metal decking

With that approach, I'd CYA even further and design the slab with some rebar so that the deck can be considered sacrificial if need be.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: waterproofing concrete on metal decking

Owner is OK with some leaking but how about his tenants? Did a flood test on a floor once and the slab "leaked" just like the tenants had been complaining about. The daycare had actually moved one of those kiddy sandboxes under one of the "leaks." It was overflowing in less than 15 minutes. Owner found a way to pay for an upgraded waterproofing system. What about health issues related to "leaking?" Maybe there's a code issue you can use. Having a leaky floor will look bad for everyone involved and, in the long run, will be bad for your shortsighted client.

RE: waterproofing concrete on metal decking

(OP)
Kootk - for sure - definitely don't want to count on the deck.

Teguci - I'm strongly urging for them to properly waterproof the slab. I don't think they realize how much water can get through a small crack.

This is a patio at a restaurant and they will have bathrooms below it. May not be end of world if water gets through, but would be stupid if they didn't try to protect their investment.

RE: waterproofing concrete on metal decking

Plaza system all the way or get rid of the patio.

RE: waterproofing concrete on metal decking

sponcyv....in this age of design/construction defect litigation, you have an obligation to design at least to the code minimums and YOUR standard of care, not the owner's or the contractor's. I just spent 3 days testifying in direct examination and 1 day in cross examination in a large construction defect trial.....so maybe I'm a little jaded on the subject, but please be careful for your liability sake.

If the code expects a roof, then design a roof (See Chapter 15, IBC). No more, no less. If they want to "value engineer" (btw...I hate that term!)your design away and you're not involved in construction/contract administration, then so be it...you won't know. If you are involved in C/CA, then you will have an obligation to write a letter recommending against their actions, but you still won't likely be able to do anything about it....the difference is that you will have met your standard of care in two respects...the design and the obligation to warn against code violations.

One thing to keep in mind....if they get a permit to construct based on your proper design and they fundamentally change it, there might be a violation of the code from that action alone, without regard to what they change it to. The administrative portion of the code (Chapter 1) requires resubmittal to the AHJ (Authority Having Jurisdiction) of design changes.

Good luck.

RE: waterproofing concrete on metal decking

I've never meet an owner who approved of leaking after said leaking has occurred (unexpectedly of course).

This seems like once you have seen it, the it cannot be unseen and must be addressed by all involved. If ignored you are potentially opening yourself up to a world of pain later on.

RE: waterproofing concrete on metal decking

(OP)
I honestly don't want to get involved in the waterproofing period. It is not my specialty. That would be a building envelope consultant and I am planning on putting the following note on my drawings:

"It is the understanding of the structural engineer that the elevated floor shown on this plan will cover an occupied space. Waterproofing is outside of the scope of the structural engineer, but it is strongly recommended to provide either a waterproofing membrane system rated for pedestrian use or a waterproofing coating such as sikalastic mt primer with sikalastic 710/715/735/ AL traffic system. If waterproofing is installed, metal decking should be vented. Manufacturer will pre-punch holes. It is highly recommended that a building envelope consultant be retained to fully detail the waterproofing system.

Furthermore, I intend to add a note similar to this: "Structural drawings provided by ......INC cover only items related to structural portion of project. ......LNC takes no responsibility for architectural, mechanical, civil, building envelope, or any other disciplines as these are outside of the services provided by ......LNC. ......LNC recommends that these disciplines be consulted to ensure the constructed deck meet all code requirements as this deck is considered a roof assembly."

I don't have any legal experience and am sure that you could shoot holes through this. If you have any advice regarding this, I'm all ears.

RE: waterproofing concrete on metal decking

I think that you want the vented deck when the waterproofing system is absent.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: waterproofing concrete on metal decking

(OP)
It was my understanding that the venting is to allow moisture to dissipate out of the bottom because the top is completely sealed. If the top is not sealed and water gets through cracks, then it would just go right through the deck, so it doesn't seem like venting would be good to me. I think it all boils down to they absolutely have to waterproof or else they will have issues down the road.

RE: waterproofing concrete on metal decking

No!No!No!.....if you are going to distance yourself from the waterproofing, don't then specify the waterproofing or even recommend it!!

Quote (sponcyv)

"It is the understanding of the structural engineer that the elevated floor shown on this plan will cover an occupied space. Waterproofing is outside of the scope of the structural engineer, but it is strongly recommended to provide either a waterproofing membrane system rated for pedestrian use or a waterproofing coating such as sikalastic mt primer with sikalastic 710/715/735/ AL traffic system. If waterproofing is installed, metal decking should be vented. Manufacturer will pre-punch holes. It is highly recommended that a building envelope consultant be retained to fully detail the waterproofing system.

I would edit to the following:
"It is the understanding of the structural engineer that the elevated floor shown on this plan will cover an occupied space. Waterproofing is outside of the scope of the structural engineer. It is highly recommended that a building envelope consultant be retained to fully detail the waterproofing system."

Leave it at that!


RE: waterproofing concrete on metal decking

I see it differently:

1) If the top is completely waterproof then you're inside the building envelope like any normal composite deck and there should be no water. No vents required.

2) If the top is not waterproof then water will seep down through the concrete and get trapped at the deck causing rampant corrosion. Vents required.

Building envelope is not my strong suit however.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: waterproofing concrete on metal decking

(OP)
Ron - I was kind of thinking that myself. Thank you and I think that is a great recommendation.

Kootk - I could see that if they didn't water proof and I called out for vented deck and they got cracking in the slab and water poured through the deck that I would get fingers pointed at me. If the deck is not vented, the deck will rust out over time, but I would make sure the reinforced concrete cans pan without the deck.

RE: waterproofing concrete on metal decking

Quote (sponcyv)

Kootk - I could see that if they didn't water proof and I called out for vented deck and they got cracking in the slab and water poured through the deck that I would get fingers pointed at me. If the deck is not vented, the deck will rust out over time, but I would make sure the reinforced concrete cans pan without the deck.

So you want the deck to serve as poor man's waterproofing and rust out? Keep in mind that, if water is getting to the deck and corroding it, the rebar may not be far behind. You know, unless you're planning on galvanized/stainless rebar.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: waterproofing concrete on metal decking

If there was an architect involved, this wouldn't even be a question, there would be waterproofing. Architects are even more risk adverse than engineers!

(I agree with Ron, just reference to others for waterproofing. If there are no others, the client should seek advice from others. You can guide them in the right direction but if you aren't comfortable with specifying something then don't be forced into it)

RE: waterproofing concrete on metal decking

(OP)
Kootk - guess I'm damned if I do, damned if I don't. I do see your point. My primary responsibility is to design the structure so that it remains intact. That means that if I anticipate water intrusion into the deck that I should probably vent the decking. This way if water gets through, it won't be trapped. If water gets down into the occupied space below, this will be a waterproofing problem, but the structure will not be effected as badly.

Agent666 - yes, I agree - architect should be involved.

RE: waterproofing concrete on metal decking

It's a tough spot to be in. If you treat the deck as sacrificial and/or spec vented deck, then you're creating the impression that you expect there to be inadequate waterproofing. Kinda flies in the face of plausible deniability.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: waterproofing concrete on metal decking


The reason for venting the deck is so that moisture from the concrete can escape thru the venting. The general principle is that concrete should be allowed to breathe and should not be totally encapsulated (as would be the case if you put a non-perforated metal deck on the soffit and a waterproofing membrane on top).

Where a metal deck is used on the soffit, it is required to be able to breathe when a waterproofing membrane is placed on the top surface -- see CSA Standard S413 (although this Standard is for parking garages, the general principle applies to other structures).

The issue of venting has nothing to do with whether the waterproofing membrane leaks or not. If it leaks, you are in trouble irrespective of whether the deck is vented or not. Acoustic deck has punched holes that may be sufficient for the venting, but the juice from the concrete may come thru the acoustic perforations, although that mat be acceptable. There are membrane people (such as Neoguard) who claim that their membrane is "breathable" but there are no standards for breathability, so I am not comfortable about it. A few years ago membrane manufacturers would not warrant the membrane if placed on slab on un-vented metal deck. Not sure what their current position is.

I have to run now but on the weekend I can send you the excerpt from CSA S413 that deals with this if you like.

If there are corrosive deicing chemical applied in winter, then the steel deck is even a worse idea.

The whole thing would be much better if it were a formed soffit and not a metal deck. Yes it is more expensive to form it, but you tend to get what you pay for.

Sounds like you have an unwise client if he/she says that leakage is acceptable. Don't become a prostitute by going along with something you know is wrong. The person telling you leakage is ok, will have disappeared when the leakage occurs and you will be left with the problem.

RE: waterproofing concrete on metal decking

I would like to check out the S413 stuff. I have the doc if you can point me to a section.

The attached position paper from SDI actually seems to be in line with sponcyv's thinking.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: waterproofing concrete on metal decking

(OP)
one other question that I had was what is the best practice for crack control for this mezzanine exposed to weather? I've read most of the threads related to this topic, but there doesn't seem to be a general consensus. This deck is 16'x24'. I'm planning on showing it without joints. I'm showing #4 bars @ 12" o.c. ea. way. Supports are at 4' o.c. The deck is 4" total thickness using 0.6 non-composite deck. I am also showing fibermesh in the mix 1.5#/cubic yard. #4's @ 12" provide a lot more than needed for flexure and temperature and shrinkage. I'm planning on specifying 0.45 max w/c ratio, 4,000 psi strength, 6" max slump, max 1" aggregate, 520 lbs minimum of cement per cubic yard, 6% +/- 1.5% air entrainment

RE: waterproofing concrete on metal decking

(OP)
ajk1 - I agree with you, but I also think the best thing for me to do is to clearly state that waterproofing is outside of my responsibility and that I recommend a waterproofing consultant be retained. I personally don't feel confident specifying waterproofing items and have not been asked to do so. I don't want to completely ignore the waterproofing issue. I just want to make it 100% clear that I'm not providing it, but I recommend they get consultation by a building envelope professional. I want to cover my rear and separate myself completely from the waterproofing. I shouldn't have to specify waterproofing. They need to hire an architect. That's my way of not being a prostitute.

RE: waterproofing concrete on metal decking

(OP)
Kootk - that's exactly what I read earlier today - the verbiage that I plan on using on my drawings is "if vapor impervious materials are applied to top of slab, metal decking must be vented".

RE: waterproofing concrete on metal decking

There are significant differences between Lightweight Insulating Concrete (LWIC) and structural concrete. It typically weighs less than 40 pcf and is specified for insulation and fire rating. Concrete on non-vented deck (form deck and composite) is commonly used on roofs and typical floors with impervious (and moisture sensitive) flooring. Some people believe the concrete moisture content goes down quicker with vented deck, so it is used in hopes that the flooring can be installed sooner. For lightweight structural concrete and hardrock, I leave the decision to use vented deck up to the contractor. It is means and methods. The specs allow it but don't require it. Vented deck is typically required for LWIC.

I am curious to see the CSA standard for parking garages. I wonder if the vented deck is required because the concrete can be flooded without any waterproofing. I did a little searching and here are a couple articles mentioning vented deck for use in parking garages. The second also recommends vented decks when the concrete is exposed to weather (I don't consider concrete covered by a roofing membrane to be exposed to weather)
http://msc.aisc.org/globalassets/modern-steel/arch...
http://eng.lacity.org/techdocs/technical_papers/Me...

RE: waterproofing concrete on metal decking

I see nothing in the comments about the deck serving as a vehicular traffic surface, therefore you do not need a traffic membrane, only a waterproofing system.

If a true roofing waterproofing system with vented deck is not in the cards, I would consider the following:
1. Use heavy mesh instead of bars for better crack control
2. Require G120 galvanizing on the metal deck - never use G90 for exterior
3. Protect the top surface of the concrete with a 100% solids silane sealar. It is vapor permeable and will not cause problems over metal deck, unlike like a urethane traffic membrane. Lasts at least 5 years before needing re-applied. Cost about $0.80 per square footing in my neck of the woods.
4. f'c = 5 ksi min and w/c 0.38 with integral corrosion inhibitor along with proper air

Hope that gives some ideas

"It is imperative Cunth doesn't get his hands on those codes."

RE: waterproofing concrete on metal decking

2
MacGruber22....it is a concrete deck over occupied space. A silane/siloxane sealer is insufficient for such purposes. There HAS to be a waterproofing system that meets the following qualifications:

1. The minimum deck slope shall be 1/4:12.
2. If an overburden or wearing surface is not to be installed, the waterproofing system must be approved by the manufacturer for use in vehicular and/or pedestrian traffic locations.
3. The waterproofing assembly must possess a Class A, Class B or Class C fire rating.
4. If any portion of the waterproofing membrane is to remain exposed, the waterproofing system shall be
ultra-violet resistant.
5. Flashings must be installed in accordance with the waterproofing manufacturer's published specifications.
6. The waterproofing system shall be flood tested in accordance with ASTM D 5957.
7. The flood test shall take place after installation of the waterproofing membrane and prior to the
installation of any above membrane components, wearing surface or overburden.
8. An approved testing lab shall provide written verification to the building official confirming that
the flood test was performed along with the results, prior to final inspection.

RE: waterproofing concrete on metal decking

For some reason I was envisioning an open mezzanine. Completely agree with Ron.

"It is imperative Cunth doesn't get his hands on those codes."

RE: waterproofing concrete on metal decking

(OP)
MacGruber22 - I was planning on specifying rebar because there is a better chance that it will not just be laid down on top of the deck and actually be placed in the desired position. For additional crack control, I am adding fibermesh which has performed well historically on jobs that I've been involved in. I'll specify the G120 decking as you suggested. I've never seen a mix with 0.38 w/c ratio. This is the bare minimum required for hydration of concrete from what I read.

Ron - I completely agree. I'm showing 1/4"/ft slope on my drawings and as you mentioned previously, I'm assuming no responsibility as it is out of my scope. I'm clearly stating that my drawings do not consider waterproofing or fire rating or anything relating to other disciplines. I'm clearly stating that my drawings only cover structural items. Honestly, I don't care if the occupied space below gets wet. It is not my responsibility to waterproof the deck and I'm clearly making the owner and contractor aware.

RE: waterproofing concrete on metal decking

Sponcyv - we use 0.38 regularly for high moisture exposure. Low w/c ratio's like this are used in combination with high range water reducers. It seems you need all the durability you can get.

"It is imperative Cunth doesn't get his hands on those codes."

RE: waterproofing concrete on metal decking

MacGruber22...agree completely with your last post.

RE: waterproofing concrete on metal decking

Thanks, Ron.

On sort of a side note, Sponcyv, I would also consider using this project as an opportunity to start learning how to specify drainage and waterproofing systems - specifically for parking and plaza structures. Competent structural engineers are perfectly capable of performing that task for common construction, and for small projects (such as the one you describe) it is a cost-saver for the client.

"It is imperative Cunth doesn't get his hands on those codes."

RE: waterproofing concrete on metal decking

MacGruber22...agree again. I routinely design waterproofing systems for structural protection. Sometimes a lot more difficult than the structural design!!lol

RE: waterproofing concrete on metal decking

Thanks, Ron. Yes, it often is more complicated!

Speaking of client complications, I am assuming that the contractor/owner has shot down a formed concrete slab for this mezzanine, sponcyv? If not I would say that you recommend that over stay-in-place centering, because of being exterior. They may rather form it up than order G120 deck, which is not in stock. On the plus side, in order for a membrane manufacturer to release their warranty, they need to approve the substrate. The manufacturer can run some vapor transport calculations and specify the percentage of venting required for a non-vapor permeable soffit. Always lean on the material manufacturer's for their vast specialized experience.

"It is imperative Cunth doesn't get his hands on those codes."

RE: waterproofing concrete on metal decking

(OP)
McGruber22 and Ron - I value your insight and appreciate you taking the time to answer these tough questions. McGruber22 - do you have an example of the concrete mix with 0.38 w/c ratio that you have been using? Perhaps a concrete mix submittal? Being that water intrusion issues are so litigious, I'd rather not learn the hard way. I would like to learn how to specify them, but don't want to bite off more than I can chew especially for this project. The contractor has not shot down formed concrete. I can run that by him.

RE: waterproofing concrete on metal decking

(OP)
McGruber22 - if going with cast in place slab on temporary forms, supported by steel beams, would you do one slab pour for a 24'x16' area with no control joints? I was not planing on doing composite beam action. How do you detail concrete slab supported by steel beams? I won't have headed studs. Is there a need for any mechanical connection between steel beam and concrete slab?

RE: waterproofing concrete on metal decking

sponcyv - I will have to take a look in the project folders, but LaFarge concrete is our go-to supplier for high-performance concrete. It may be quicker for you to just call your local LaFarge rep and ask for it. They are very responsive. I don't think you need 0.38 if you go with a formed slab, then only ACI minimums of 0.40 (chloride attack) 0.45 (no chloride) for exterior. Regardless, I would require an integral corrosion inhibitor. LaFarge tends to use this Grace product: https://gcpat.com/construction/en-ca/Documents/DCI...

My thoughts on the other items - No control joints - just be generous with your crack control steel and provide good details. Provide standard detail for construction joint in case their pour is interrupted for some unforeseen reason. Yup - no need for composite beam there. Non-composite beams are designed with the slab cast against the side of the compression flange all the time without studs, to prevent positive moment LTB. When the spans get long, you need to consider a cross-bracing for fascia beams to the interior beams for wind and generally stability. Obviously, when diaphragm transfer is an issue for a larger structure, some sort of mechanical connection is required. See the attached - it is a repair detail but you will get the idea. I show the repaired slab cast against the existing beam top cover plate (don't ask why that cover plate was there). Remember, if you take your moment diagram, scale it down by 98%, and then divide by the lever arm between the flanges, you have the required top flange brace force per foot. The value is always surprisingly smaller than you think you need.

"It is imperative Cunth doesn't get his hands on those codes."

RE: waterproofing concrete on metal decking

(OP)
Thank you MacGruber22! I appreciate the advice.

RE: waterproofing concrete on metal decking

Quote (ajk1)

Where a metal deck is used on the soffit, it is required to be able to breathe when a waterproofing membrane is placed on the top surface -- see CSA Standard S413 (although this Standard is for parking garages, the general principle applies to other structures).

Quote (wannabeSE)

I am curious to see the CSA standard for parking garages.

Just following up on this.




I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: waterproofing concrete on metal decking

I am going to agree with KootK

RE: waterproofing concrete on metal decking

Darn KootK and his ability to solicit agreement! bigsmile

I also agree.

"It is imperative Cunth doesn't get his hands on those codes."

RE: waterproofing concrete on metal decking

Pfft... I've got myself a pretty wicked non-concensus brewing in another thread right now. Win some, lose some...

What are you guys agreeing with anyhow? I just posted ajk1's references without even adding any sparkling commentary.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: waterproofing concrete on metal decking

Good point....DARN IT %$@%@#! wink

"It is imperative Cunth doesn't get his hands on those codes."

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