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  1. #1
    Join Date
    Jul 2004
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    Default Concrete over wood deck? Opinions, please.

    I've built new sloped concrete decks over membranes before, with many years of trouble free use, but this is a renovation of a flat wood deck.

    This time, I'm thinking about putting a concrete surface on top of the existing cedar decking. The cedar is in pretty good shape, with just a few boards showing a bit of rot. The P.T. framing is in great condition.

    The concrete would be poured in squares, 3 - 4 feet across, with 1/4" gaps between them on all sides. My thought is to put down #30 felt paper to protect the cedar, and wrap the edges down into 1/2" gaps cut into the decking in line with each edge of the concrete panels. In other words, this 1300 sq ft deck would have a concrete surface that has a grid of drainage channels at about 3 or 4 feet on center.

    Is this any crazier than putting concrete tile roofing over #30 felt?

    Pre-emptive comments: Yes, I know there are structural considerations for the weight, and details for water protection at the walls of the house. The height works because the deck is down 2 risers from the floor of the house, and the steps would also have new surfaces and reworked geometry for equal rise heights. I'm really just wondering about the long term prospects for the roofing paper and the cedar.

    Gentle persons, please discuss.
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  2. #2
    Join Date
    Mar 2006
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    Seattle, WA
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    11,339

    Default Re: Concrete over wood deck? Opinions, please.

    I'm thinking about movement. The concrete will be heavy, but what's to stop it from moving and wearing holes in the felt?

    Just pushing the felt down into the gap between decking may not allow it to drain well enough- are you somewhere that gets enough cold to wind up with a lot of ice? PNW is pretty big, with quite a variety of climates. Gaps on all sides means that there are going to be 2 sides where you will have boards beneath the gap, preventing drainage. Or will those get wrapped somehow (Ice & water? Metal caps? Combination?

    It sounds like the concrete is going to be poured, then placed later (like tiles)?

    Seems like it should work if you can figure out how to keep everything from moving much. Metal flashing with some tabs to hold everything from moving nailed over the felt in strategic locations? With a little silicone or similar adhesive?

    If you can get the felt & flashing details, I'd expect the existing decking to do fairly well. And if it's built so the tiles can be pulled to check what's below...
    http://www.lavrans.com

    "He uses statistics as a drunken man uses lamp posts; for support rather than illumination." -Andrew Lang

  3. #3
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    Default Re: Concrete over wood deck? Opinions, please.

    Good comments - thanks.

    I'm in Portland. We get a lot of rain but we hardly get into freezing temps much. I'm thinking I'll pour the concrete in place, and maybe taper the deck boards a little toward the drain slot. That taper will give something like an ear, or tab to hold the tiles in place.

    It occured to me after the o.p., that I only know of #30 coming in rolls up to 36" wide. That means I'd only make the tiles 32" or less, cause I don't want to lap the paper.

    Edit - I just saw that you're in Portland, too. This project is in Ridgefield, Near the Clarl County fairgrounds.
    Last edited by brujenn; 08-30-2010 at 11:31 PM.

  4. #4
    Join Date
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    Portland, OR
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    Default Re: Concrete over wood deck? Opinions, please.

    #30 felt is NOT, I repeat NOT, a sufficient protection for a wood deck.

  5. #5
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    Default Re: Concrete over wood deck? Opinions, please.

    Quote Originally Posted by NW Architect View Post
    #30 felt is NOT, I repeat NOT, a sufficient protection for a wood deck.
    Okay, thanks, but this reminds me of a line from Flight of the Conchords: "Try to be more constructive with you criticism."

    What would you suggest using, if not #30 felt?

  6. #6
    Join Date
    Apr 2009
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    midwestish
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    7,229

    Default Re: Concrete over wood deck? Opinions, please.

    Quote Originally Posted by brujenn View Post
    Is this any crazier than putting concrete tile roofing over #30 felt?

    I'm really just wondering about the long term prospects for the roofing paper and the cedar.

    Gentle persons, please discuss.

    Okay, thanks, but this reminds me of a line from Flight of the Conchords: "Try to be more constructive with you criticism."
    Notions of "gentle" vary widely. One could make the same observation about the question/s/er in all honesty. What's "long term" on an existing [___ year old] deck? Why try to float the boat if the deck structure can't carry the load? You're talking about adding a 22 -28 lb/sqft / 28,600 - 36,400 lb total additional load guessing off your sketch. You that sure about that structure? I'd want to know that before going any further.
    “I find the curiosity of our men with respect to this animal is pretty much satisfied.”
    ~ Meriwether Lewis

  7. #7
    Join Date
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    Portland, OR
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    Default Re: Concrete over wood deck? Opinions, please.

    How about actual waterproofing or roofing? PVC, EPDM, TPO, built up roofing, elastomeric waterproofing. If you have been using felt as waterproofing, start saving your money for when you get called back to do replacements of rotted wood underneath your work. Or, call all those clients and ask nicely if you can come back and redo your work at no cost to them, before the rot happens.

    Working a case right now involving the collapse of a wood deck with concrete topping poured over #30 felt.

    So, based on my experience, the long term prospects for the roofing paper and cedar is that it will fail and somebody will get hurt, at least economically, possibly physically.

  8. #8
    Join Date
    Jul 2004
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    Pacific NorthWet
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    Default Re: Concrete over wood deck? Opinions, please.

    I have never used felt for waterproofing. I've used a membrane that I think was a Duradek product. I wouldn't consider the #30 if I wanted it dry below, or if I didn't plan to have drain channels (slots, really) every 3 feet in both directions. This deck averages 2' above the ground, and the rain can fall harmlessly through the gaps in the concrete, just as it has for years through the 2" x 4" decking.

    I'll be putting an additional beam and row of poured concrete footings between each of the existing beams, so the existing 2" x 8" p.t. joist @ 24" o.c. will span just 3 and 1/2 feet. The existing beams will also get new poured concrete footings in addition to the existing footings.

    I apologize if my reply to NW Architect seemed glib. I was trying to ask advice on alternatives in a humorous way. Although I did say "gentle persons", I was using it to be more inclusive than "gentlemen". I try to be repectful and avoid sarcasm in forums, and would not ask anyone to take it easy on me. I understand the language of on-line forums, and am prepared to let most anything slide off my back.

    Thanks,
    Brujenn

  9. #9
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    Mar 2006
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    Seattle, WA
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    Default Re: Concrete over wood deck? Opinions, please.

    Backing up to the beginning, it seems to me that saving the decking might not really be saving much in the way of money.

    The labor needed to make the existing deck work and be long-lasting may be reduced by pulling the cedar and adding a pan or metal grid made to suspend the deck? The time needed to dig and pour all those footings under the deck is going to be increased, or at least more painful, if you don't pull the decking. Just a thought.
    http://www.lavrans.com

    "He uses statistics as a drunken man uses lamp posts; for support rather than illumination." -Andrew Lang

  10. #10
    Join Date
    Jul 2004
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    Pacific NorthWet
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    Default Re: Concrete over wood deck? Opinions, please.

    I'm not sure what you mean when you suggest adding a pan or metal grid. It's a flat, 1,300 square foot deck. I've looked into Mutual Materials 2' X 2' slabs, with their pedestal system, but my clients don't like the look, and it's difficult to do any kind of pattern with them other than a full on grid. Do you know of a metal grid system?

    The embarrassingle low quality photo attached is the similar to the look we're after. The mottled look is way exagerated because the concrete had just been hosed off, and the photo was taken at night with a cell phone.

    Saving the deck isn't just about money, although some would be saved. I'll have to remove and replace the decking up in a couple of 2' wide strips for adding the beams. The size of squares that I want can only be poured in place, so I need something to pour them on. Tearing the decking up involves labor, hauling, and dump fees, and buying something to replace it, and the labor to install it. Whether with the existing decking, or something that replaces it, I'll have to prep for the concrete by cutting my drain slots into the surface anyway.

    I'm very into renewal and renovation rather than bulldoze and start afresh.
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    Last edited by brujenn; 09-01-2010 at 12:56 PM. Reason: add comment

  11. #11
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    Portland, OR
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    Default Re: Concrete over wood deck? Opinions, please.

    Two feet above the ground? Knock it out, figure out a good set of details against the stem or pony walls of the house, perimeter stemwalls, backfill, and pour a sloped concrete slab.

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