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Old 10-03-2008, 04:42 PM   #1
Patrick Haskell
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Default Building a raised platform

Got sort of a unique problem. I not only have a terribly uneven garage floor (with frost heaved concrete in places), but the garage floods, so I need to raise my platform approximately 4 inches above the floor. My intention is to build a 6x8 platform with 2 alternating CDX layers, topped by particle board and rubber matting.

I'm anticipating framing this with 2x4s running along the 6-foot length of the platform. To raise the platform appropriately I was anticipating using 4x4 posts lag bolted to the 2x4s. My question is for anybody who has done more building than I and has a better idea of how much force is generated by 250# of bumpers (some day, I hope) being dropped from overhead. How would you space the 2x4 and posts? My initial thought is to do 16-inch spacing of 2x4s and to lag a post every two feet on each of those. This would mean 28 individually cut 4x4 posts, as the floor won't be the same height in any two places except by chance. Am I going to smash this thing right away? Am I drastically overdesigning? Does anybody want to wish me luck?
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Old 10-03-2008, 05:02 PM   #2
Dave Van Skike
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Quote:
Originally Posted by Patrick Haskell View Post
Got sort of a unique problem. I not only have a terribly uneven garage floor (with frost heaved concrete in places), but the garage floods, so I need to raise my platform approximately 4 inches above the floor. My intention is to build a 6x8 platform with 2 alternating CDX layers, topped by particle board and rubber matting.

I'm anticipating framing this with 2x4s running along the 6-foot length of the platform. To raise the platform appropriately I was anticipating using 4x4 posts lag bolted to the 2x4s. My question is for anybody who has done more building than I and has a better idea of how much force is generated by 250# of bumpers (some day, I hope) being dropped from overhead. How would you space the 2x4 and posts? My initial thought is to do 16-inch spacing of 2x4s and to lag a post every two feet on each of those. This would mean 28 individually cut 4x4 posts, as the floor won't be the same height in any two places except by chance. Am I going to smash this thing right away? Am I drastically overdesigning? Does anybody want to wish me luck?
Your basically building a small deck but your joists are undersized for the load.


frame construction on 16" centers is very burly. but you're relying on a relatively small cross section for the horizontal members (2x4). 2x8's might do it easily but you're likley going to see a little flex with anyhting less than that. If it's possible, an underframe of 2x4's on 16" centers that is shimmed to the floor level that a lot of the framing members are actaully bearing on the floor would be really strong.

Is it your garage floor or a rental? were it me, I'd bust out the concrete, level it out with 5/8" minus gravel, compact the shit out of it and lay pressure treated sleepers down to roughly level, then just make your 2x4 frame on the sleepers, if you want it ungodly strong, sheath both sides with plywood.
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Old 10-03-2008, 05:36 PM   #3
Patrick Donnelly
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Quote:
Originally Posted by Dave Van Skike View Post
Is it your garage floor or a rental? were it me, I'd bust out the concrete, level it out with 5/8" minus gravel, compact the shit out of it and lay pressure treated sleepers down to roughly level, then just make your 2x4 frame on the sleepers, if you want it ungodly strong, sheath both sides with plywood.
Why not just lay new concrete so that its higher than flood water?
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Old 10-03-2008, 06:03 PM   #4
Dave Van Skike
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might make re-use of the garage problematic to have an elevated 8x8 section 4" higher than the balance of the floor. most people suck included really suck at pouring big slabs one bag of concrete at a time...

plus, you can re-use the platform or even haul it outside for the summer.
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Old 10-03-2008, 06:30 PM   #5
Patrick Haskell
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Thanks for the quick replies. I could go readily go up to larger lumber. Having the floor flex on a hard pull would be really disappointing when all is said and done.

I did consider pouring an entirely new floor. Unfortunately, the flooding is a design feature, as there is a dry well in the rear corner that drains the entire front yard, so pouring a new floor would entail putting in some pretty impressive french drains and even then, I'd want my platform raised, as in the winter when the ground freezes the dry well can back up and freeze the 1 to 2-inch deep flood water solid for a month or two. At that time, I'll probably be at the Y most mornings, as sub-20 degree bars are more character building than I find to be ideal

If I move to 2x8's, I assume I can get away with fewer posts for this, especially if I put 2x8s on either end to frame it in fully. I'm thinking as few as 6 or 9 posts, provided my cross-piece 2x8s are tied into the end pieces with some kind of hardware. Am I off base here?
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Old 10-03-2008, 08:15 PM   #6
Frank Needham
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Wow, sounds like the real problem is drainage. Somebody did a horrible job designing that lot. Just curious, have you investigated running a swale or gulley to fix things? Probably a lot of work but something like what you describe would drive me batty.
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Old 10-04-2008, 02:25 AM   #7
Patrick Haskell
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I have definitely considered a drainage swale/french drain, but it would be no mean task. It would have to be sizeable, since it has to manage all the water removed from my basement by the sump pump, in addition to the runoff from the roof and lot. I'd have to bury it, since the only place to run it would be across my driveway. The frost line is 4.5 feet down, and the soil is all dense, bouldery till. I supposes my neighbor might let me borrow his mini-excavator/tractor, since he doesn't use it enough anymore, but man, that's a project. Even then, I'm faced with the task of building a raised platform. Now that we've committed to staying here for three more years (we were planning on moving), I've been considering the earthwork project, but I'm not ready to tackle it this fall, as it would absorb all my free time for a little while.
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Old 10-04-2008, 11:10 AM   #8
Garrett Smith
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If you do 2x8s, make sure to use a lot of them (more than 10), and use 2 or 3 screwed together specifically in the areas where the bumpers will be hitting the platform.

I did not do this on my outdoor platform and after dropping ~200 pounds from overhead enough times, the platform floor started to break in those specific areas.

Now my platform (what's left of it, anyway) is taken apart and in pieces in my garage. As I have no plans to put it together again anytime soon, I really should figure out what would be best to do with all that lumber...(sob).

At least I now have a floor-level platform in my garage gym that is perfect for cleaning and any other lifts that don't go overhead (darn garage door!). I'll figure it out...

Oh yeah...critters of all sorts will likely love the area under an elevated platform.
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Old 10-05-2008, 04:01 AM   #9
Patrick Haskell
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Thanks, Garrett. I had hoped for some direct experience like this, although I wasn't sure whether anyone else had tackled this same project. Did you elevate the platform on posts, or was it simply constructed of 2x8s laid on the ground and shimmed to be level? Also, did the plywood part break through or did the 2x8s fail?
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