Building an elevated storage deck in my garage on concrete slab
I need extra storage space and my garage has 13 foot ceilings so my idea is to build an elevated deck in a side area of the garage. To build the deck, I need two 6 x 6 posts. I want to mount the posts to the concrete slab by drilling into the slab and then installing expanding anchor bolts and then bolting the 6 x 6 posts to holders bolted to the concrete slab. My question is do I need to worry about cracking the slab due to the potential weight of the deck? I know if I was building a deck I would be pouring footers 2 feet deep before pouring the slab (by code) and then mounting the post on top of the slab. Obviously, I can’t do this since the garage slab is already poured. I’ve read where some people drill or cut out the concrete slab to provide access to the dirt underneath where they then excavate to then pour a footer and then patch the concrete slab. Does anyone have experience with building a deck on an existing slab that is at least 4 inches thick with a compacted rock and gravel base? By they way the deck is going to be 14 feet by 16 feet with one side bolted to a ledger board attached to the garage wall.
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big question seems to be how big and what are you storing on it? The bigger the structure is, there's more dead load that you have to factor is loading at the bearing points on the slab. 14x16, but what size joists? spacing? etc. If you're putting Christmas ornaments and kiddie pools, I doubt the live load presents issues. If you're storing lumber or other heavy items, you could have quite a load at a few points in the slab. If there's a void underneath, you could crack, shift the weight, etc. I would think cutting out a square and excavating to put a simple footing would be time well spent.
Thx Mikeeljon - yes all good points reference to weight. Not putting engine blocks or storing lumber most of the items will be household goods (yes Christmas ornaments included), garden hoses for winter storage, etc. The joists will be 2 x 8's on 12 inch center with max span of 13' - 6". I've checked the specs for southern pine 2 x 8's and Visually Graded No. 2 pine will span that exact distance, allowing for 360 deflection, and 40 PSF live load, with 10 PSF dead load. This is the code in my county for building a deck on a house - following the exact same building codes/guidelines with my only issue having the existing slab.
It may be the slab is enough to get away with this; if it were me, I'd put in proper footings to bear the weight.
I think I would dig 12x12x12 pad, but that’s just for peace of mind. I doubt you would ever have a problem. On my commercial projects I’ve seen some of my clients bolt really heavy duty (to heavy to lift by hand) shop equipment to concrete slabs and never have an issue. One of the main reasons footing depth is important is for frost heave, but inside a garage where protected that won’t be a problem. In tropical climates where there is never frost some areas allow for 4” min footing depth.
Thx again Mikeljon and thx jlyda. Based on your comments, I ran the numbers for my SUV in the garage - curb weight for my AWD SUV is 4,250 lbs. That weight distributed over 4 wheels with a cross section of each tire at 8 inches (tire width) by 1 inch (SWAG on how much each tire tread contacts the slab) equals 0.22222 total square feet ((8 sq in * 4 tires)) = 32 sq in divided by 144 sq in per sq foot. Then taking the total weight divided by 0.2222 total sq feet equals 19,125 lbs. 19K lbs equals the total load represented by the SUV for one sq foot. If I then transfer this to a 6 x 6 post, I get 27.5625 sq inches (5.25 times 5.25 inches). Divided by 144 sq inches per foot equals .1914 total sq ft. Times 19K (rounding down for a slight factor of safety) equals 3,626 lbs. per post times 2 posts gets me a total "representative" load of 7,200 lbs for the deck. If I do the same exercise for 8 x 8 posts, I get 52.5625/144 = 0.3650 sq ft. Times 19K, I get 6,935 lbs per post times 2 posts gets me to almost 14K lbs for total deck load... almost double. I think I go with the 8 x 8 posts as a CYA and call it even... anyone see any flaws in my calculations/reasoning? By the way, really appreciate the dialogue and feedback. Wouldn't have thought about using the SUV loads as representative safe load values without your comments. Oh by the way, if I put a heavy work truck in the garage, say an F350, I could get the base weight load to 6K lbs plus so I feel I have plenty of margin for safety using my SUV Curb Weight of 4,250 lbs. Another thought I'm having is to get two 1 foot by 1 foot steel plates (1/4 thick) to put on the slab for each post to rest on to distribute the weight across an even greater area. Appreciate your perspectives.
Commercial buildings commonly use prefab steel storage mezzanies that just sit on the slab. I've recommended them to clients because they usually don't require permits. I'm not suggesting this for you, but just to let you know that your slab should handle the weight without a problem.
All this is inside so below the frost line is not an issue. What I did was to put the 'footing' on top of the slab. What you are trying to do is spread the load onto the correct sq ft of support. So if you calculate the load at each post and size a 'box' accordingly you can put the post in a standoff at the top of the box and fill the box with concrete. Ok, you now have a couple of big concrete cubes in your garage but you can downsize them some because you have 4" of the concrete slab to add to your depth calc.
Storage 'decks' where there is less than 7' of headspace can be designed for 20# live load.
If you are designing this with a post on the corner and one in the middle, (far end on a wall) the one in the middle gets twice the load. So its box will be bigger. If equal loads (20ll+12dl) * 14 * 16 / 4 = 1232# or less than 1 SF. Double that for a middle post or if you want to stick with 40# LL. At 1 SF you could Titan bolt (galv. 3"x5/8") a Simpson Post base in place and the metal plate of the holder and the added depth of the slab spread will exceed the 1 SF minimum. For the doubled loading you would need an 11" square box 13" tall (or 13" tall pyramidal block to 11" square base to spread the load over the 4" concrete slab floor. 17x17 > 288 (2 SF).
IRC also requires that any storage space over 200 sf be accessible by a staircase. Be sure to include that or downsize it a bit.
Mike - thx for the input... I've seen these racking systems for warehouses (think Lowe's, COSTCO, Home Depot, etc.) but was never really sure how thick the slab might be in these massive warehouses... any experience with this? Thx!
Thx kitcad - good info but not fully tracking with your calculation "(20ll+12dl) * 14 * 16 / 4 = 1232# or less than 1 SF". I get 1792#... am I missing something? And yes you are correct ref mounting one side of the deck on a ledger board to the garage wall but no middle post - all posts are external edge. Stairs are included - 9 steps on a 7 inch riser with a 10 inch run - then each tread width will be 11 inches so a 1 inch over hang. This matches the stairs I already have going from the garage to the main door to the house - 3 steps up and I'm going to integrate the deck stairs into the landing going into the house. This way I eliminate the need for 3 stairs and save myself some space.
I really like your idea of using concrete cubes to distribute the load but I don't understand the need for "depth" to the concrete block - isn't this about transference of the weight load over more square inches/feet of the slab to minimize point specific load (e.g. the 6 x 6 post pushing all of the load into a small area of the slab)? If I build a wooden box 4 or 5 inches deep and fill it with concrete (will add a bolt for Simpson post base) isn't that good enough? Or, is this more about a "pyramid" shape - 5 1/4" square at the top flaring out to 12" square (1 sq ft) at the bottom? My little pea brain is having a hard time conceptualizing how the load distribution is any different between a box and a pyramid - not saying it isn't - just saying my brain doesn't see it that way... please clarify if it is. If it is, what should the base of the pyramid be and what should the height be to properly transfer load across the base?
Thx again for the great info - really appreciate everyone chiming in on this!!