Q&A / 

Deck Support Columns

DEAR TIM: It is time for me to build a fairly tall deck. I have seen wood support posts twist and crack. I do not want that problem. What are my alternatives? I am looking for something that is sleek, simple and easy to install. Mark O., Scottsdale, AZ

DEAR MARK: The hot and dry climate you live in can quickly cause wood to shrink. This shrinkage creates all sorts of internal stresses within the wood that lead to twisting and cracking. Some wood species are much more resistant to these defects, but I doubt a lumber retailer would ever give you a bulletproof guarantee. I think you should consider another building material - steel.

This 4 x 4 steel column is about 9 feet long. Welded to the top of it is a U-shaped saddle that is just the right width to handle two 2 x 12s. The steel fabricator punched the 5/8 inch diameter holes in the saddle so it was easy to drill the beams and insert the through bolts to secure the wood beam to the steel.

There are several reasons steel makes sense in your situation. Tall decks typically need larger diameter support columns than decks that are closer to the ground. If you were to build with wood, a structural engineer might specify six inch by six inch or larger wood support columns instead of traditional four by four columns. Switch to steel and I am quite confident a four by four steel column will perform as well or better than a six by six wood column of the same length. A structural engineer can quickly confirm this for you.

The smooth and uniform appearance of painted steel also makes it a good choice for you since you desire that sleek look. Steel keeps its shape and requires very little maintenance if you paint it correctly from the start.

I have installed steel support columns on more than one deck. The material is easy to work with and connecting the steel to the underside of the deck is a simple matter. Making the connections to the concrete piers buried in the ground is also an easy task. Perhaps the hardest thing is taking accurate measurements to determine the height of the steel columns. A local welder can build the columns for you and should be able to help you confirm the accuracy of your dimensions.

The steel columns almost always connect to a beam under the deck. You need to determine the thickness and height of this beam. These dimensions dictate the width and height of the metal channel that will be welded to the top of the steel post. This U shaped channel cradles the beam. The welder will punch or drill 9/16 or 5/8 inch diameter holes in the U-shaped cradle. Once the beam is in place in the cradle, you drill through these holes and insert one half inch bolt and nut to permanently attach the wood beam to the steel column.

Wood deck surfaces need to be cleaned and sealed to keep them in good condition. The cleaner I recommend is Oxygen bleach.

Believe it or not this is one 21 foot long steel column! It passes through the first deck on its way to support the second triangular deck off the master bedroom. It was very cool to assemble this deck I must tell you.

The steel column is connected to the concrete piers with one half inch diameter stud anchor bolt. The welder should install a 12 inch square 1/4 inch thick steel plate to the bottom of the steel column. A similar 9/16 inch diameter hole should be punched near each corner of this plate. Once the steel column is perfectly plumb and resting on the center of the concrete pier, you drill though the holes in the steel plate into the concrete below. Hammer in the stud anchors and tighten the nuts. Be sure the nuts and washers are installed on the stud anchors before you strike them with a hammer. The hammer blows can damage the uppermost threads of the anchor preventing you from installing the nuts.

The steel column assembly should be completely primed and painted before it is installed. Painting it this way minimizes any future rust problems. I prefer to use special rust inhibitive primers that are formulated to bond well to bare steel. There is a very good chance your welder will have some high quality primer for this purpose. Many welding shops deliver primed fabricated steel to job sites. It may make sense for you to have the welder prime the steel and you apply the finish coat.

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Be sure the welder produces good solid welds at the top and bottom of the column. Ask if he can punch a drain hole in the middle of the bottom steel plate. This hole allows any water that may one day find its way into the column a place to drain from the column. Water that gets trapped inside the column can cause the column to prematurely rust from the inside out.


3 Responses to Deck Support Columns

  1. Hi Tim:
    Can you please tell me, what is the 4x4 tickness that you used in the first example? (9' tall post)
    Thank you
    Queens, NY

  2. Hi Tim,

    This is Somashekar living in India. I have to extended a concrete pillar which is already built for another 10 feet height. There is existing roof that i would like to extend by about 10 Ft * 12 FT. The current roof rest on main concrete walls. We also will raise the main brick walls adjacent to the pillar so that the roof can extend and rest on the pillar and the new wall. What is your suggestion on extending the pillar do you suggest a steel or concrete structure. If need to extend the pillar should be drill holes are release some concrete to get access to the steel in the concrete pillar.

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