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  1. #1

    Default This deck is gauranteed to COLLAPSE, correct?

    My parents are building a 20x20 deck attached to a ledger board on their house. They live near Rutland, Vermont, about
    ten miles from the killington ski area. The frostline is typically stated as 42".The contractor has spec'd a quote, and
    while I know little to nothing about constuction, I feel pretty certain that something ( lots of things ) are wrong and
    this deck is almost gauranteed to fail. So, if I misuse terminology, I'll happily clarify or take any pictures anyone
    needs to determine if this deck is obviously going to collapse.

    The entire support structure will be PT #1. The ledger board is 2x8 and extends 5 feet past the corner of the house. The
    ledger board is two boards, and is currently lag bolted to the house with one bolt every 20 inches or so, in the center.
    ( I believe it isn't water sealed propertly, but that is a separate issue, so that should give an idea of how badly
    designed I think this deck is )

    So, the ledger board is really a roughly six foot board, then a roughly 14 foot board extending 5 feet free floating past the
    corner. There is a post hole there, so cantilevered isn't the correct term since a support is at the end?

    All posts are going to be 6x6 PT resting on post anchors embedded in 8" sonotubes. ( The brand is handiform, but I'll
    still call them sonotubes ) The deck, at the ledger, is 6 foot above grade measured to the bottom of the ledger. The ground
    slopes away from the house, so the the level at the far end of the deck is 18-20 inches below the grade of where the ledger
    sits. I used a line level to quickly make a rough estimate.


    There are a total of 7 footing holes dug, space roughly ten feet away from any other post support or the ledger,
    except for the one hole under the end of the ledger that extends past the house corner, which is five feet.


    The first footing, the one under the ledger, hit a large rock. At this point, even though mud is caked on the rock, he just put the

    sonotube in the hole extending about 3 inches above grade, because he hit it will a large prybar and said based upon the sound, it

    can be used as the footing. ( I don't doubt you can use a rock to support the concrete, but it seems to me it would be better if

    you hit the rock near the frost line. ) I went out and excavated around the rock, making a hole that is at least two foot by two

    foot, opened up the bottom of the hole, to be wider and longer than the top, removing 6 inches all the way around the boulder,
    and going down around the boulder 6 to 12 inches. The rock is relatively flat, though sloped slightly towards the house, and
    in, away from the direction of the deck, which is just about exactly opposite the slope of the ground at the corner of the
    house)

    The ground is wet, and clay. I am not a geotechnic engineer, but I'm sure no one would refer to this as ideal soil.


    So, for the first hole, I thought I would put rebar in, build a small wooden form that is above the rock and extends
    above grade. Keep the form square or rectangular, and pour the concrete, allowing it to cover the rock, extend under
    the completely undisturbed soil and then rise up through the footing. Oh, and water is slowly seeping into the hole,
    and the deepest depth is now roughly 30". I dug the downside of the slope deeper, cause for some reason that just seems
    to make sense to me. I couldn't figure out what was supposed to hold the concrete on the rock the way the contractor
    was going to pour it. So, my idea was to use the rock as the key, to hold the concrete from sliding, and throw in
    rebar for good measure.

    The contractor used an auger to dig the holes, and hit rocks and a few wood like pieces, and none of the 7 holes actually
    reach the frost line. The deepest is about 35" at one of the far end holes. We could backfill the entire area after pouring
    to bring the grade level, making the sonotube sit near the frostline?


    Which brings me to my questions. A 6x6 will barely fit on an 8" round tube. Shouldn't a 6x6 be on a 12" nominal tube?
    Shouldn't there be a larger footing at the base of the sonotube, especially given less than ideal soil conditions AND
    the fact that this is New England, and this area averages about 66" of yearly snowfall?

    I found load calculations used vary somewhat. Ten psf seems to be a consensus for dead load. Live loads I've seen some
    say 40 psf, for a 50psf total. Some say minimum 60 total. Some say add 10psf to the total for extreme cold and or snow
    areas. I don't consider this area extreme but it does at least a few times a year reach below zero, and that's not
    in Celsius. It wouldn't be an unusual event to get 24+ inches of snow in one snowfall. Winds frequently reach 30 miles
    and hour, and occasionally 50+ miles and hour.

    Now, assuming the seven post, which is two rows of three, plus the one at the end of the ledger, the center post will
    be ten feet in every direction from and other support. The load on that post is 100 sq feet, correct? I take half
    the distance to the supports, which is five feet, it extends in all directions, making a 10x10 foot square being
    supported by the center post?

    So, the deck is elevated 6 to 8 foot above ground on a slope. It's a 400 square foot deck. One foot of snow on the deck
    is roughly 400 cubic feet of snow. ( some falls through, etc ) A cubic foot of water weighs around 62 pounds, 62.4 I
    think. So, at a 20% moisture content, which is somewhat light powdery snow, a cubic foot of that stuff weighs about
    12.5 lbs. ( 62.4 x 20% (.2) is roughly 12.5, and makes quick calcs easier ) So, 400 cubic feet of powdery snow weighs
    about 5000 lbs. A two foot snowfall is 10,000 lbs, and if the snow is wet, heavy snow, say double the moisture contect,
    would be 10 and 20,000 pounds respectively.



    I suggested at least (4) four holes per row, using at least a 24" footing ( I've seen preformed plastic ones made
    to accept sonotubes ) and 12" nominal sonotubes. But I'm not sure about it. I'm thinking 28" would be better. I'd
    be comfortable with the 12" diameter tube, though I don't really have any idea, I'm just guessing by what seems to
    make sense to me.

    I also though maybe free standing the deck would add strength and stability, but I'm not sure about digging large
    holes so close to the house foundation?

    As far as the wood goes, the local guys around the corner have 20 foot to 24 foot long boards in PT #1. The only
    20 foot plus boards they carry are 2x12, which seems fine. For the beams, should the 2x12's be adhered together
    and then nailed? Or is nailing OK. Keep in mind, my mother is in a wheelchair, and my father is 70, I'm hoping
    to find the most structurally sound method, not the fastest, nor the cheapest. Which reminds me, the decking will
    be 20 foot 5/4x6 Trex composite decking.

    If I understand correctly, all beams, are doubled up. The three outer sides, ( end joists are the two sides? the far
    side is properly called what, the end beam? ) and there will be a beam in the center.

    Is it better to notch a 6x6 and carriage bolt the beam through the post? Use a post cap to a double beam holder ( I
    see these type of connectors at mfgs like Ellis Mfg. ) Is it better, actually I should say, more structurally sound,
    to run the joists on top of the beam? Or hanger both the ledger and the beam. Then hanger the other side of the beam
    and again hanger the end beam? Do the answers change if you are using joists running as one joist from the ledger
    to the end joist? It's obvious it is easier to use one 20 foot member, and one joist should be stronger than two, but
    wouldn't the joists have to be much thicker, or because there is a supporting middle beam could the joist thickness
    be lower? While costs aren't the main issue, We're not trying to impoverish ourselves.

    Do the double end joists get connected to the ends of the ledger? Or, is one board cut 1.5" short to sit inside the ledger,
    and the other then overlap the end cut?

    The contractor is supposed to be arriving to start pouring cement and I'd like to know whether the deck is going
    to definatly fall before he pours.

    Oh, and this is going to sound completely naive, but when thinking about it, it made sense to me. The concrete doesn't
    really have anything to do with stopping your deck from failing, right? I mean, the purpose of the concrete is just
    to transfer the 'Downward' forces into the ground, over a large enough area to stop it from sinking, but doesn't
    really have anything to do with stopping it from falling? I imagine if the post was in the ground, in the cement, it
    would provide some structural support, (until it rots), but the reason that isn't recommended is because if you have
    to replace a post, it is much more difficult? All a post anchor really does is make sure that updrafts don't lift the
    post high enough to jump it off the column? That is why, even though telephone poles are replaced regularly, they
    aren't sitting on a footing with and anchor, they would just tip over, correct? ( Cost issues aside )

    Oh, the 6x6's are twelve foot. The quote calls for them to be left extended above the deck to be used to fasten
    the handrails. That kind of makes sense to me, but I rarely see that done. Is that because the surface area of a 6x6
    contains enough that in high winds, the post itself can become unstable if it is tall enough?

    Aside from aesthetics, is the reason many decks use diaganol decking to provide stability? The more I look at it, the
    more I think, most decks are built poorly. Everything is perpendicular. What is stopping lateral forces from swaying
    the deck? Blocking is going ( as in must be ) to be needed between the joists in order to stabilize that lateral force,
    correct? But, if I lay the decking boards diagonally, that would seem to be to alleviate the need to put blocking ( or
    would that be bracing ) between the joists?

  2. #2

    Default Re: This deck is gauranteed to COLLAPSE, correct?

    My post was too long to fit in one thread.

    I know this is a crazy number of questions, but it just seems to me, that this deck as currently spec'd isn't likely
    to fail, it is gauranteed to fail. But, since I play with computers for a living, my opinion doesn't really count since
    the 'contractor' said it's fine. No one has to answer all these questions at once, I don't expect someone to, I'm really
    just looking for enough replies with some explanations, in order to convince my parents not to have this contractor
    build the deck to those specifications. If anyone needs any additional info, or pictures, etc. just ask and I will post it ASAP.Thank you, sincerely.

  3. #3
    Join Date
    Aug 2007
    Posts
    7,234

    Default Re: This deck is gauranteed to COLLAPSE, correct?

    Quote Originally Posted by stevemcmaine View Post
    My post was too long to fit in one thread.
    As jkirk suggests, very long post that most won't read or respond to. I made it about half way through before giving up and will sum up with this statement. First and foremost, contact the building department governing the area your parents are located and make sure a permit has been pulled for this project. The building department will assess the plans and specs to determine if this structure meets the local codes. From the sounds of it, it does not. The items that smacked me were the spacing of the lag bolts on the ledger and the lack of adequate water proofing to the structure. Next was the footing under the end of the ledger and the over all depth of all footings.

    As to the size of the sono and the post, as long as the post sits on top of the sono-footing, it will be fine, if the post is embedded in the footing, then you need at least 2" off each corner of the post for adequate hole sizing. Again, local code will dictate.

    As I mentioned earlier, I only made it to this point before I quit reading. I recommend taking the plans and specs to the building department for assessment. It is here that you'll find what your local regulations are. Also, be prepared to enlist an engineer to stamp the plans (means he's done the calcs for snow loads, etc and approves ). I would also recommend stopping the work until your questions have been resolved, possibly check into the credentials of the contractor and the status of their license and insurance.
    I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!

  4. #4
    Join Date
    Aug 2007
    Location
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    Default Re: This deck is gauranteed to COLLAPSE, correct?

    While it's nice to have details .... like jkirk said ... long post.

    Just skimmed over it and a couple of things come to mind ...

    1 - you might be overstating the snow loads a bit
    ( as mentioned I just skimmed over .... when i get some time I'll review your figures)

    2 - I would have to guess if your local building codes are similar to here .... since that deck is over 2 feet above ground and it's an attached deck you probably require a permit.
    If so then the piers and footings would have to approved / inspected as well the deck design would have to be approved and inspected.
    If you have doubts contact your local building department to come on site to check things out...... they would have answers to all your concerns.
    "" an ounce of perception -- a pound of obscure "
    - Rush

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