Can I install 6' 6x6 fence posts onto a cement driveway?

Question

We would like to install a 6' privacy fence with 6x6 cedar posts, between our house and driveway, that would involve 3 posts on top of our cement driveway and 1 in the ground. My intention was to hire someone with a core drill to dig 3' holes for footings, and pour cement into 10" tubes, but I am now wondering if it is "ok" to just install the footings directly on top of the driveway.

I have not checked the depth of the cement yet, so is there a recommended driveway depth for this sort of thing, if I can install directly into the existing cement? Or should I go ahead and get 12" holes taken out with a core drill and create 3" deep footings?

Also, is this a proper post anchor to use? It is the only type they had at our store.

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Answer 1

A fence post has very little vertical force...mainly just the weight of the post and panels it holds. Your driveway slab would work fine as a footing for that. HOWEVER, the primary force on a fence is typically lateral--especially a 6' fence. The reason posts are put into the ground is to counter lateral forces on the fence (typically the wind).

I imagine there are types of brackets that could be used on the concrete to accomodate the lateral forces, but the type you have in your post is not that type...that's typically used to attach structural posts to concrete (it handles the vertical force, but not lateral).

The type of brackets you'd need would be something akin to a shelf bracket where there's a diagonal brace between the two plates.

So, one option is to cut holes in your slab and dig proper holes there. Alternatively, you could find plates designed for this (or have someone weld some up for you).

A third option would be to not build a straight fence, but rather stagger the posts in a zig-zag pattern.

Answer 2

I think your first inclination was correct. You should cut holes in the driveway and install the posts at least 36" deep in a concrete footing. Trying to mount to the surface of the driveway will not give the support you need for a 6ft fence. Wind, snow etc are powerful lateral forces against a large section of fence and could easily topple it.

The post support you pictured is designed for posts under a deck or structure. The weight of the structure bearing down on the post provides stability. These supports hold the posts from slipping horizontally where there is no chance of the post tipping over because it is secured to other framing members. I think your fence would fall over in the first stiff breeze if you used these instead of sinking the posts into the ground.

Answer 3

I did exactly this same thing last year. I rented a machine from Home Depot and "drilled" my own holes. The cores I cut were only slightly larger that the 4x4 posts that I used (6 inches I think). The hardest part was digging down the additional distance once the cores were cut. I have to admit that I only went down 24 inches, since I figure that the posts are laterally immovable where they exit the slab.