By Paul Bianchina
When installing posts and large columns for anything from decks and patio covers to the load-bearing supports for a second floor, it's important that the post rest in the proper type of base. Post and column bases provide a positive connection between the post and the surface it's sitting on, and also protect the bottom of the post from moisture damage due to contact with concrete.
There are essentially two types of column bases available - surface-mount and imbedded. The choice of which one is right for your project depends on the type of construction, the framing members being used and the loads you'll be working with.
Surface-Mounted Bases
For lighter duty projects, surface-mounted column bases can typically be used. A surface-mounted base is easier to work with because it offers you more flexibility in placement; however, it does not have the strength or the uplift resistance that the imbedded bases do. Surface-mounted bases can be installed over both concrete and wood surfaces and are designed to accept 4x4, 4x6, 6x6 and 8x8 posts.
To install the surface-mounted base, a string-line is laid out along the line where the posts will be placed and then the center-to-center spacing of the posts is laid off. This will give you the positioning of each base.
Next, the base is fastened in place. Fastening methods vary according to the type of base and the load capacity and include nails and screws for wood surfaces and masonry nails, powder-activated shots, wedge anchors and epoxy-set bolts for installation over concrete. Your dealer can assist you with the selection of the proper fasteners.
Surface-mount bases are available in two-sided and four-sided designs, as well as an adjustable version. The two-sided type, which has a bottom, two raised sides and two open ends, allows you a little more freedom in the placement of your post. Once the post has been set into the base, it can be moved a little bit from side to side parallel with the two raised sides in order to simplify plumbing the post.
The four-sided version looks like a small box with an open top. Once it has been secured to the surface, the post is simply set into it from above and cannot be moved in any direction.
For additional flexibility in placement, the adjustable version of the post base is worth considering. Available as both two-sided and four-sided styles, adjustable bases have a slot in the bottom that allows for a small amount of side-to-side movement to make it easier to get the post plumb. Adjustable bases are designed for use over concrete and require that a bolt or threaded stud be set into the concrete surface first - wedge-anchor studs and epoxy-set bolts can both be used. Once the bolt is in place, the base is slipped over the top of it and then a washer and nut are installed over the bottom of the base. The base can slide from side to side, and once it's in position the nut is tightened to secure it in place.
With any of these three types of bases, once the post is in place it is secured using manufacturer-approved nails or screws.
Embedded Bases
The more commonly used type of column or post base is the embedded style, which is designed for use with poured concrete slabs or piers. The typical embedded base consists of a flat horizontal steel plate with two vertical straps above the plate and a vertical u-shaped steel strap below. Sizes are available that will accommodate up to 12x12 posts, and also glulams.
Embedded-style bases are installed in wet concrete. The u-shaped portion goes down and is embedded into the concrete so that the flat horizontal plate is flush with the finished surface of the concrete and the two upright straps remain above the surface. Once the concrete has set there is no way to move or reposition the base, so proper placement is very important.
Another type of embedded base is the elevated style, which raises the post further up above the concrete for applications where water may occasionally be present on top of the concrete. Elevated post bases are a two-sided style with a single steel rod that is embedded in the concrete - there are also ones with a threaded rod that allows for up and down adjustment of the post. Elevated post bases do not have quite the same strength characteristics of some of the larger embedded styles, so they are limited to use with 4x-and 6x-lumber.
Post and column bases are available in dozens of style and size combinations from any lumber yard or home center, either in stock or by special order.
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