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
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?