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Decks—foundations options

QuestionI am interested in building a wooden deck outside my back door. Right now I have a concrete patio level to the ground that measures 20 x 6. It's about six to eight inches below my back door and one whole side of the slab is butted up to the foundation. My question is do we need to knock all the concrete out? The concrete is in pretty rough shape with cracks every few feet. Any suggestions would be greatly appreciated.

AnswerAs for your question, the answer is no; however, please consider the following: a well-structured deck always begins with the foundation. This is not the place to cut corners. The foundation must meet the building codes for the area in which you are building, so you may need to obtain a permit. Your building department may not require a permit for a deck that is 30" or less above the ground but they can still provide required information. If it is 30" or less above grade, a guardrail is not required. The UBC (Uniform Building Code) in our area does not require permits for driveways and platforms 30" or less above grade. I suggest you consult with your building department anyway to determine if you need a permit and the requirements for setting posts, specifically:

  • How deep should the hole be below the frost line?
  • How wide does the hole need to be?
  • How far out of the ground does the concrete need to be?
  • What are the restrictions for using precast piers?
  • What are the restrictions regarding embedding posts?

Construction tubes are convenient. Home centers carry them and may cut them to the length you need, saving you time. If you run into a situation where a patio, driveway, or concrete slab like yours might be in the way, call a concrete cutting firm. They can cut into the concrete just the diameter you need for the tube, making the job much easier than tearing up the concrete. This subcontractor is well worth the cost. Detailed diagrams of the three types of post installations are in my book, Builder's Guide to Decks (be sure to check out diagram 6-8). This book will provide other ideas to consider during the planning stages and helpful construction hints on details as well.

In your case, where you are six to eight inches below the threshold of the door, you have some other concerns:

1. In order to use 2x6 joists you will need at least 7 1/2 inches below the threshold. This includes the space needed for the 1 1/2 inch decking material.

2. If your concrete slab is level both out from and with the house, you could use 2x4 treated joists on top of the slab if the clearance is 6 inches below the threshold. Depending how far out you go with the deck once you reach 6 feet (the end of your concrete slab), you will need to install a header in order to carry the extended joists, not 2x4s at this point but most likely 2x6 or 2x8 joists. Both the header and the new joists will need a minimum of 6 inches of clearance from the underside of the joist to the ground.

3. In your case, a wooden deck may require you to step up to the deck to be able to provide the clearance required under the deck joists. You could step out to a wooden landing that extends out from the house 36" at floor level, then up one or more steps to the deck. The deck could surround the landing and extend out from the house.

It may be simpler to remove the concrete slab in order to use one size of joist.

To order Builder's Guide to Decks, click here or click the purchase button below!


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Copyright © 1999 LAF/C.R.S., Inc. All rights reserved.
Question answered by Leon A. Frechette.



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