Where is PRODECK? I am considering building a 10×25 deck over an area that used to be dog run/kennels. It has concrete slabs about 3’x12’x4″ with solid cap block turned on edge between runs. It will be 8′ above ground to match an existing porch. I do not want to have to cut the slabs and pour footers. Will probably add a spa of medium size in one corner later. Figured 5000# for tub, 4444# people, 2500#dead load.
Planning books indicate 4 2×8 beams and 2×8 joists 12’oc. Calculations indicate about 1000# load per post (12) average. It seems that a post bracket attached to the slabs would be adequate. NC code only calls for 12″x12″x6″ to support 40 sqft tributary area. The soil seems very stable and almost no chance of washing out under slab. Even if it cracked I don’t see how it could go anywhere. Published frostline here is 12″ but think that is overkill. Thinking that I would not directly attach to porch but put a floating bracket that would prevent any horizontal movement without causing any vertical stress.
I considered using the dek-blocks but that limits to 4×4 posts and they say not to use them (blocks) for decks over 60″ high.
Suggestions?
Edited 8/31/2003 10:18:49 PM ET by RASCONC
Replies
Up north (here, anyway) to build a deck is to make an addition to the structure, which requires a building permit. With the permit comes various inspections, the foundation, including footings, being the first. (The last being the final.)
Have you called the Building inspector in your area to determine what they require for a deck foundation? You should also inquire about permits. Build a deck (an improvement to the structure which escalates its retail value, and hence its taxable value) and you change the assessed value of the house.
Had a friend who added a deck onto his house two years before he sold the house, but didn't pull a permit. When he sold the place, the town made him detach the deck and pull it away from the house before he could close. They get upset when you circumvent their tax base.
Thanks, here in hillbilly country things are a bit looser. There is no inspection for a sale. Yes they do like to get the tax base up but do a periodic on-site revaluation. If you do anything without a permit and they (inspection dept, not the tax guy) find it you have to get a permit, if you have built it to code there is no problem. If you have buried electrical or plumbing in a wall you might find yourself having to open it up for them. Which is a chance you take and fair. It is always better to get the permit and let them share or shoulder the resposibility.
A freestanding deck which is legal can be considered a moveable structure like a storage building and could be built without permit. I am probably stretching the gray areas a bit.
If your calculations satisfy you that the 4" slabs will be strong enough to take the posts (I would be hard to convince on that, BTW), you can deal with the frost-line issue by simply dumping at least 12" of fill on top of the slabs around your posts.
A better way: Drill a 2" deep hole for rebar into the slab for each post. Set a sonotube with a footing base on top of the slab, and wire in a 12" piece of rebar set into the hole. Pour in some Gel-Crete concrete bonding agent, then pour the tubes to come about 15-18" above the slab. When the 'crete has set, strip the cardboard, dump fill and grade to about 3" below the tops of the concrete posts, and build your deck.
You don't have to compact the fill; it's there to insulate the slabs, not to support anything.
Dinosaur
'Y-a-tu de la justice dans ce maudit monde?
Thanks for your input. If the deck is free standing I do not think frost heaving ( even if it were to happen here in the sunny south) would be a problem if not attached to the house. Code here allows a drystack of four 4x8x16 precast blocks for 70 sqft of tributary area.
I didn't figure frost heave would be your major concern, either. Not like here where we go to 5 feet down or the bedrock, whichever comes first....
My main focus would be on the point resistance of those 4" slabs you've got in place. I won't put a post supporting the main beam in a basement directly on the slab unless we've placed a minimum 12x12x12 ancillary footing form in the gravel before the floor is poured. We generally pour the floor 3" thick; I don't see an extra inch of unreinforced concrete as being enough stronger to take the kind of point load deck posts will impose on it, especially if the HO decides to throw a few dance parties out there each year. Since you can't (more like 'don't want to' and I don't blame you) put a footing for each post under the slab, I was just suggesting a way to put it on top of the slab. The load will be nicely spread out and I'd be surprised if you had trouble with it in that configuration.
Caveat: If you're not sure, ask an engineer to calculate it for you. I'm just going by rule of thumb and what my bellybutton tells me....
Dinosaur
'Y-a-tu de la justice dans ce maudit monde?
Thanks, it appears that our code here would be covered with the drystack of 4x8x16 precast cap blocks giving 8" thick and 16" square, go ahead and use 6x6 posts. The only problem I would see would be if by some strange quirk there were large voids under the slabs. That does not seem likely as the slab is up against a retaining wall on one side and another block wall on the other and flat yard on the other two. I agree on the basement post but I believe they are carrying a much greater load even with a one story house. We are still looking at around 1000# per post. On a 6x6 that translates to equivilent loading of 4700 per sqft under the post, agree that it is quite a bit much.
I will shoot a picture and take to a meeting with an architect Wed. and get his take on it. Better safe than sorry by golly.
Thanks again for your help!
4444# people,
This tub will hold 20 people?
You have 2 conditions to think about,
The deck could use standard framing, but the tub should be engineered as a 4 person tub could weigh 5000lbs by itself.
You might want to consider doubling that 20 people number just to be on the safe side as you may get real popular with an unforeseen special attraction. fireworks, wedding, etc.
SamT
Thanks, the 20 people were not for the tub, that is extra folks standing on the remaining 200 sq feet. Tub calculation was 350 gal @ 8.5#, 500# tub, 6 or so people for a worst case of around 5000. My Jacuzzi four person spa holds 220 gal, weighs 307# empty and calls for floor loading of 77#. This is 1870 of water, 307 tub, and 564 for people.
The initial plan is for beams on the hot tub end of 3 ea 2x8' with a span of less that 5'. Joists will be 2x8 12"oc with every other one doubled under the tub. Joists will ride on top of beams, not hung with joist hangers. I am confident that this far exceeds the requirements for the upper framing. A 4x4 syp post is rated for 10,000# vertical load at 8", these will be about 6 1/2' so that should be fine. The Dek-blocks claim specs that would let one block support the whole deck. They have a base of 11x11 and I cannot see them punching down through 4" of cured concrete. The perimeter will be braced with 2x6's.