How to Fasten a 4X4 Post to a Concrete Slab

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Whether you're building a cover for a concrete patio, mounting posts on concrete steps or installing a railing around a concrete porch, you have to attach the 4x4 posts so they remain upright and rigid. Depending on the project and design, you can choose from one of three methods to attach the posts.


  • surface mount
  • core and set
  • or side mount

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Surface Mount Posts

You can surface mount a post that will have more than one point of support, such as a post that attaches to both a concrete slab and a covered roof, or a post that supports a handrail that's also attached to a house. Post bases, typically made from metal, attach directly to the surface of the concrete. You drill into the concrete and insert concrete screws or anchors. Once the base is in place, the bottom of the post slips right in. A variety of post bracket styles are available, and most require additional bolting through the side of the bracket into the post.


Core and Set Posts

You cannot surface mount a post that has only one point of contact -- at the base -- because a post bracket does not offer adequate support against lateral force. If you're installing a fence on concrete or a freestanding porch railing, coring through solid concrete, or concrete block, to a depth one-third the height of the intended post will allow you to set and concrete the post securely. Portable core drilling units are available from some construction rental stores, but unless you're familiar with concrete coring, it's probably better to call a coring company.



Before drilling or coring, ensure that no mechanical elements, such as in-slab heating coils, plumbing lines or wiring lie beneath. If you’re unsure, have a concrete imaging company X-ray the slab.

Side-Mounted Posts

A third option for attaching a post to solid concrete or concrete block is side mounting, which works well on an elevated masonry base, such as steps or a raised concrete porch. A good rule of thumb when side mounting is to attach at least one-third of the post below the top level of the concrete. A minimum of two sleeve anchors inserted to a depth at least the width of the post will hold it in place.



Glenda Taylor

Glenda Taylor is a contractor and a full-time writer specializing in construction writing. She also enjoys writing business and finance, food and drink and pet-related articles. Her education includes marketing and a bachelor's degree in journalism from the University of Kansas.