Pouring concrete for fence posts was tiring enough, thank you very much!

April 5, 2011 7:04 AM

ConstructionFilter: Mounting posts to concrete, but not in?

My fiance and I bought a house last May, and when we did so the contractor who owned the home asked us if we would like the concrete slab that was the floor of the (demolished) garage torn up. I said no, because it was in great shape itself and I liked the idea of the parking pad. Great!

But now I'm wanting for something over our cars so I can work on them in the rain and they can be somewhat protected from the elements. I don't, however, want to have to destroy the slab and pour new concrete just to mount posts into.

What resources are available online or otherwise for finding out about mounting corner posts into concrete. Is this, like, possible? I mean, I know I could get brackets and drill holes into the concrete slab and just screw those suckers down, but that can't be strong enough, can it? Has anyone done anything like this? I'm not really looking for a garage, though if I eventually wanted to close in the walls and was able to, that'd be sweet.

posted by InsanePenguin to Home & Garden (17 answers total)

Best answer: You could put a 1/4" steel plate, maybe 8 inches square, under the post, both for strength, and so you could weld or bolt brackets to that, without comprising the concrete directly under the post.
posted by StickyCarpet at 7:12 AM on April 5, 2011

Best answer: Wedge anchor mounting bolts are really, really strong if they are installed well. Buy good ones, big ones, and be careful putting them in and you have nothing to worry about.
posted by dirtdirt at 7:12 AM on April 5, 2011

Best answer: Use mounting brackets. Use tap-cons to secure the bracket to the concrete pad. Then, build your walls with pressure-treated 4x4s. Place each 4x4 into a mounting bracket, and secure it in place with 3in decking screws.
posted by Flood at 7:19 AM on April 5, 2011 [1 favorite]

Best answer: One common way to attach things to concrete floors is with powder actuated nail guns, like this one. They are availble for rent in a lot of place; if you go this route though, be careful, they can be dangerous if not used properly.
posted by TedW at 7:20 AM on April 5, 2011 [1 favorite]

Best answer: For an application like this, you might look at mounting some bolts in the concrete with epoxy, then bolting your columns to the epoxied-in bolts. You'd need to drill holes in the slab with a hammer drill in the desired pattern, then fill partially with epoxy, But your bolts (a big enough piece of allthread would probably work) and let it set. Then you've got your mounting solution in place, and you can attach whatever you need to that. Like StickyCarpet said, a steel plate with holes drilled to receive the bolts would probably work pretty well. You can weld your column to the steel plate and you're good to go. You'll need a specialty epoxy made for this application.

Also, I'm assuming that your slab will support a structure, since there was a garage on it originally.
posted by Shohn at 7:24 AM on April 5, 2011

Response by poster: Wait, so, you guys mean I'm not completely nuts? There really are materials for doing exactly this? I'm so happy.

Tedw, the diagram on the OSHA website is very slightly terrifying.
posted by InsanePenguin at 7:24 AM on April 5, 2011

Just a note about the low-velocity powder actuated guns---this is likely very, very old concrete. If that's the case, you'll need the very most powerful propellant (I forget which color, I want to say orange) and you still likely won't be able to sink 3-inch nails.

If you've never used one (they're really pretty safe, just loud), I'd recommend you use flood's method with a hammer drill drill. You could possibly skip the tap-cons (soooo expensive) for normal masonry nails if you've got the appropriate thunder to drive them. (Will still need to re-drill.) Or dirtdirt's recommendation of masonry bolts would work too.
posted by TomMelee at 7:38 AM on April 5, 2011

Is there bare ground either side of the slab itself? When i was a kid my father built a 'carport' over a small slab in the driveway.

ie a basic wood frame with a polyurethane roof. the posts were simply buried in teh ground 500-1000mm beside the slab. Since it is just a roof you don't need to worry about subsidence etc...
posted by mary8nne at 7:44 AM on April 5, 2011

I think it's important to note that the above methods are really only useful for preventing the bottom of the post from being knocked out from under the structure; they are not strong enough to keep the post vertical against lateral forces. If you installed four posts using such brackets, then placed a heavy structure (like a roof) on top of those posts, the whole thing could parallelogram over rather easily. It will be important to integrate the posts into the structure in such a way as to prevent them from flopping over sideways.
posted by jon1270 at 7:46 AM on April 5, 2011 [1 favorite]

Response by poster: It will be important to integrate the posts into the structure in such a way as to prevent them from flopping over sideways.

How would I go about doing that?
posted by InsanePenguin at 7:49 AM on April 5, 2011

I think you would definately need to use diagonal braces on two sides to provide enough structural integrity to avoid the 'parralellogramming' effect.
posted by mary8nne at 7:53 AM on April 5, 2011

Response by poster: I think you would definately need to use diagonal braces on two sides to provide enough structural integrity to avoid the 'parralellogramming' effect.

Okay, I figured something like that but I wasn't sure what jon meant.
posted by InsanePenguin at 8:18 AM on April 5, 2011

I don't know anything about this company in particular, but you might consider something like this.
posted by JohnnyGunn at 8:25 AM on April 5, 2011 [1 favorite]

The proper way of doing this where posts are exposed to weather is to use a galvanized steel standoff post base. The end of a post directly on exposed concrete will soak up water like a soda straw. As shown in the drawing above, you drill a hole in the concrete and secure the post base with an anchor bolt. Then you mount the steel standoff that supports the end of the post one inch above the concrete. Installation instructions are shown in this PDF.
posted by JackFlash at 8:42 AM on April 5, 2011

Adding to what JohnnyGunn wrote: a standalone roof structure has a lot of advantages if all you want is some protection from the weather. It is not the beginning of a garage, so if that's your ultimate goal than is this is probably the wrong way to go. However, these roof structures are excellent in their own right and will certainly keep your vehicles out of the weather and give you a place to work on them when it's wet out side. And as you can see the models range from "lean to" to "pretty much a garage".
posted by mosk at 8:46 AM on April 5, 2011

If all you want is lightweight rain protection a simple shed like the ones at Home Depot or Lowes would work fine, you can use the power nailers others have mentioned to mount them to the pad. I have one of the "portable" tent-like units JohnnyGunn mentioned and really like it.

But...

If you are planning a full sized structure like a garage, you really don't want to build on the pad without checking to see if proper footings were built into the pad. The building department might have information on what used to be there. Also check with them and zoning for the proper permits, the last thing you need is hassles from them after you start building.

My neighbor decided to put up a lightweight carport next to his house. Poured a slab, put up some posts, tied the rafters in to his roof, topped it with some corrugated fiberglass. It was a nice two day project, but when he tried to sell the house the building department wouldn't issue a certificate of occupancy because it was an "illegal structure." They wanted him to remove everything and they wouldn't issue retroactive permits, the buyer wanted a price reduction because the house wouldn't have a carport, zoning said he needed to request a variance because he built too close to the property line. In the end, he ended up tearing off the carport, giving the buyer a couple grand back, and got around the zoning by claiming the slab was a driveway extension.

Which is why I went with a tube and fabric shed. It's considered "portable," so it doesn't add to my taxes, if the building inspector has a problem with it I can always take it down. But it's been standing for six years without a problem.
posted by Marky at 11:41 AM on April 5, 2011

StickyCarpet writes "You could put a 1/4' steel plate, maybe 8 inches square, under the post, both for strength, and so you could weld or bolt brackets to that, without comprising the concrete directly under the post."

While this will support quite a bit of load it won't provide uplift protection.

InsanePenguin writes "How would I go about doing that?"

If you plan to attach the roof to your house then an easy is to actually construct 4-8' of wall in each direction at the outside back corner. This provides a lot of rigidity to the structure and gives you a bit of space for a bench somewhat protected from wind. Where the space is long enough constructing a shed under the roof most of the way across the back can be useful. Other wise some manner of cross bracing is required.

JackFlash writes "The proper way of doing this where posts are exposed to weather is to use a galvanized steel standoff post base. The end of a post directly on exposed concrete will soak up water like a soda straw. As shown in the drawing above, you drill a hole in the concrete and secure the post base with an anchor bolt. Then you mount the steel standoff that supports the end of the post one inch above the concrete. Installation instructions are shown in this PDF."

This is pretty well the way I would proceed. Because the pad used to be a garage there is probably plenty of strength at the edge to support the mass of a roof. Use pressure treated 4X4s if they are going to be exposed.

Marky writes "Which is why I went with a tube and fabric shed. It's considered 'portable,' so it doesn't add to my taxes, if the building inspector has a problem with it I can always take it down. But it's been standing for six years without a problem."

In my jurisdiction tarp structures like you describe are considered temporary and while you don't need any kind of permit to erect them you are only allowed to use them for 4 weeks out of every 52. Lots of people get away with them year round though it only takes a single squeeky wheel neighbour to cause trouble.
posted by Mitheral at 6:06 PM on April 5, 2011