Rain drains and waterproofing a flat (suspended concrete) roof
Last Post 03 Nov 2015 03:20 PM by jameserizer. 7 Replies.
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jameserizerUser is Offline
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29 Aug 2015 09:27 PM
Hello Green Builders!  I am building an ICF home in Western Oregon.  There is a three story elongated octagonal tower (about 16' X 24') as part of this home and the roof of the tower is a 350ft^2 patio, accessible via a spiral stairway and hatch in the roof.  All 12" ICF blocks around the top (Logix) and a Fortruss suspended concrete roof/floor for the top.   The roof will slop downwards from the center to the short sides >.25"/foot and there will be rain drains in four of the 8 corners, emptying through cast gargoyles.  I will try to attach a photo of this tower  so you guys can see what it is.

I am the owner/builder and until I started this project, I had almost no experience in pretty much any type of heavy construction.  I am both the architect and the G.C. and am building this with my own crew.  So, danger signs abound.  I am a mechanical engineer though (not civil) and have some ability and so far the major ICF constructions have gone pretty well with nothing worse than a minor blowout (broken web) and a bowed form in one other spot.  Now I am getting into new territory: waterproofing, and I have no experience or verified good ideas how to do it right.  I hope you all can keep me from totally screwing up this house

I have a couple of problems I hope someone can advise me on regarding this roof: 

1) I need rain drains in four corners.  I am thinking of using 2 ~ 3" PVC pipe penetrating though the ICF at the drain locations, set just a bit lower than the surface of the floor on the roof.  I have no idea how to properly flash something like this so the rain does not get into the walls. I'm a total newbie here and would really appreciate any suggestions.

2)  The roof is suspended concrete (Fortruss by Logix), is poured right up to a short ICF parapet which encircles the top of the tower.  I need to somehow cover the concrete and up the parapet walls with a lifetime waterproof membrane that I can then place some sort of tiles or slate or similar on top as the floor surface.  Again, I have no idea how best to do this and would really appreciate any suggestions.

The first picture shows what I hope to get to someday.  The second shows the general construction of the tower just before we poured mud.  The parapet walls will extend just past the rebar sticking up.

Thanks for your advice!  James



creynolds247User is Offline
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31 Aug 2015 06:11 PM
James,

I think I read a blog on your home build? Are you the guy behind the fish tank screensaver?

Anyway....

I am not an ICF builder, however we are planning on installing a flat, concrete roof on our project too - so I have been reading many different ways on doing this and I will be following this thread carefully when some of the pros chime in.

With that being said, I am concerned with water penetration on a flat concrete roof, especially one that will be walked upon.

For the roof drains, I would be tempted to use a standard roof drain and flash this prior to the poor and pour around it - probably not that clear, but then, I'm not a pro

As for the lifetime membrane, I had considered (as our home is planned to have 2 large flat roof areas, one of which will be walked on) essentially building to roof like an enormous shower pan. Whereby the slab is poured to required thickness and then a membrane is applied followed by the standard finishing that a shower area would use. This way would solve the issue of waterproofing and allow you to still tile without having to apply an external sealer such as a blackjack product.

With that said... I'm going to bow out here and wait for the experts
Chris
joasisUser is Offline
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07 Sep 2015 09:21 AM
I would not be looking for any membrane style roof cover, but more of a polyurethane type of sealer. Other opini9ons may vare, but I have had great results with underground homes and exposed concrete roofs using this, and an old school method of hat tar mopped on 30 day cure concrete, and graveled. You wil want the roof deck to reflect as much heat as possible.
Ladwig Construction<br>Hennessey, Oklahoma<br>405 853 1563<br>Residential and Commercial Contractor<br>ICF's and Steel
pacificstartUser is Offline
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08 Sep 2015 06:27 PM
I am an owner/builder wannabee having the same kind of questions and I am currently designing my own house and will most likely build it with a lot of DIY except the ICF+pouring.

I am planning for a roof top patio as well so here is what I found after a lot of of research, asking and reading:

- Any concrete surface will crack over time so there is no way to leave the concrete slab exposed - even if sealed with whatever method - especially if it will be walked upon.

- In general I would avoid any "lifetime warranty" kind of product. Lifetime warranty is a marketing gimmick and it always comes with a gotcha. This obviously is a more general statement and it doesn't apply only to roof membranes or sealers.

- Walking on the surface will dramatically increase the stress and the stress resistance requirements of the application.

- If left exposed to the elements, the life expectancy of the waterproofing surface will diminish significantly.

- Whatever method, a flat roof needs to be built in such way so it can be easily repaired so the walking surface should be separate and should not be considered permanent.

- In my opinion is best to have a tapered roof insulation (like Archfoam) with the minimum required slope and then apply a membrane on top of that and extend it up on to the walls with flashing, etc. From there you can install the drains, etc. Pretty much a giant shower pan as Chris said. Then install a decking system that can be walked upon and which also allows for good drainage. For that you can use pedestals (about $10/piece) like Buzon and FRP concrete pavers or similar to keep the weight low.

Steven
FBBPUser is Offline
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19 Sep 2015 10:23 AM
If you pour your concrete in contact with the wall concrete and have no insulation on top of the roof concrete, you bypass the wall insulation.
Concrete if very thermally conductive so the heat trapped in the ICF core will radiant out to the sky from the roof concrete.
jameserizerUser is Offline
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02 Nov 2015 06:36 PM
Thanks for the feedback, guys. I have learned more about this process since I started this thread and have pretty much decided to go with a 0.080" TPO membrane over the roof. I found some nice roof scuppers made of stainless steel with TPO clad bond flanges and I will use those. I gave up on the idea of placing the roof scuppers in the walls and pouring the concrete floor (roof?) around them...way too hard. Instead, I plan to put 4" ABS pipes in the walls where the scuppers will go, plug the ends with spray foam so the concrete does not flow into them, and trim them off flush with the ICF using an angle grinder w/sanding disks. Once the mud has cured, I will bust out the spray foam dams and fit the scupper drains in place, then secure them in the ABS with more spray foam. Then we will install the TPO membrane and roof access ladder hatch. After the membrane is in, we will put some sort of compressible pad on top of the TPO, then set some kind of floor tiles on top of that. Might even bond the whole mess together with an appropriate adhesive (BlackJack?). If anyone has any experience with this sort of stuff and can point out flaws in my plan, please bring it on! I don't want to screw it up and can only afford to DIY.  I considered using tapered pedestals like you suggest, Steven, but I did not make the parapet wall tall enough so I need to keep the roof (floor) covering fairly thin or I will need a railing and that would screw up the whole motif.  Castles don't have safety railings.   I talked to a guy who suggested what sounded like some kind of mat made from compressed polymer fibers.  It is about 3/8" thick, strong but compliant and I could bond the tiles to it with blackjack or silicon adhesive and not lose the flexibility.  This would protect the TPO and also let water drain through it. 

FBBP, thanks for the advice about the thermal bridging. There is no way around it, the joists in the concrete must penetrate the walls and tie in with the rebar. I have about 5" X 10" joist connections on 2' centers so total concrete conductive area is 7.6ft^2 for a 400ft^2 roof. Maybe I can put some sort of thin insulation under my floor tiles that will help but without doing some really fancy stuff with ledgers etc it is what it is. Fortunately, I think that that concrete is actually NOT very thermally conductive...at least compared to other materials. Aluminum, for example, is more than 1,200 times more thermally conductive but still I calculate I will transfer up to 200watts-hour thru this connection with a 50F temperature delta, or up to 680 BTU-hr. That's equivalent to about 1/20th ton of air conditioning or heating going during the hottest day or coldest night. I calculate the total manual J heat load of the entire house to be about 2 ~ 3 tons so the concrete connection at the tower roof is about 1/50th of my total heat loss on a cold night. You got me thinking, though...I'll call my structural engineer and see if I can use low density concrete for this roof. If so, I could maybe cut that heat loss by half.
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03 Nov 2015 01:07 PM
jameserizer - do some research into green roofs. It sounds like that is what you are doing only using tile (or patio stones) instead of dirt and grass.

I am going to assume your calc's are correct however my gut tells me you might be losing more heat then that. If you search back in this forum, you will find references to the temperature inside an ICF wall. It shows that there is no temperature gradient between the outside face of the concrete core and the inside face of the concrete core even when there is a large delta T between outside and inside temperatures. In other words the whole core remains the same temperature because the concrete is thermally conductive enough to move btu's quickly. Therefore it may be assumed that the heat will conduct quickly up the core and into the roof structure.

Maybe consider something like putting down 2" of eps on the TPO and then laying patio stones on that both for ballast and a walking surface. I believe eps and TPO are compatible but that would have to be ascertained. Also the weight of the patio stones would have to be taken into consideration.
jameserizerUser is Offline
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03 Nov 2015 03:20 PM
FBBP-

I like the idea of a true, green roof but I don't like the idea of having to mow and water it. Also, does not really support the kind of usage I envision for the top of this tower, which is more patio than a lawn. Also, I think the weight of the dirt & grass, especially when wet, might be higher than if I put down 1/2" tiles. I want to have some chairs & a table, and there will also be the outside part of a small, 1/2 ton split system up there as well.

Regarding the thermal issues, after I read your post I went back and reviewed my numbers. I checked my CAD models and saw that I had underestimated the actual cross-section area of the joist connections and left out the 4"^2 end connections where the longitudinal rebar mat tied into the wall. So I had altogether about 1.3M^2 of conductive area instead of the .8M^2 I had originally estimated. Therefore you are correct and my total heat loss due to the connection between the concrete roof and the concrete walls is about 360watts, or roughly 1,224 BTU's or less than 1/10th of a ton of air conditioning. But it is still something to improve if I can, although unless I can add gypsum or poly beads to my mud I don't know how I can decrease the bridging by much. It is what it is.  I would not call concrete an insulator, but actually heat does not flow through it very fast.  That's why it makes such a great 'thermal flywheel' inside the ICF walls to even out dinural temperatures.

Considering the temperature delta across the concrete inside an ICF wall, the main reason the concrete does not show much delta is because it is insulated on both sides by 3" of polystyrene, which is about 30 times better insulator than the concrete. Concrete has a K factor of about 1.5 for 3,500psi mix, and polystyrene is about 0.02. Also concrete has an area ~ thickness ratio of about 2 (1ft^2 X .5ft thick = 2 : 1). Here is a really good web link that I use quite a bit for working out heat transfer: http://www.engineeringtoolbox.com/thermal-conductivity-d_429.html

I am not an expert at heat transfer by any measure but I know a little bit about it and often figure out simple stuff like how to cool an electronics cabinet or thermally stabilize a photonic scintillator (I design x-ray machines for a living). Still, working out the heat calcs for this wacky castle I'm building is challenging and I appreciate all the advice I get from you guys. I see there is a thread on geothermal heat pumps on this forum and pretty soon I'll have to start lurking there. Some day when I can afford it, I plan to install a geothermal system using a small lake in front of my house as the ground loop.
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