Before you can use the Deck Post Size Table in a meaningful way you have to understand the loads that your deck must withstand. The "load" is made up of the "dead load" and the "live load".
Live Load consists of the weight of everything that will be on top of the deck. This would include you, your family, friends and all the chairs, tables, grill and anything else.
Dead Load consists of the weight of the deck structure that sits on top of the posts.
An average deck would be designed to handle a load of 50 psf (pounds/square foot) and is made up of 10 psf for the dead load and 40 psf for the live load. This is a minimum.
If you expect larger groups of people such as in a commercial building you might have to build for a load of 70 psf. And if you would like to place a hot tub on the deck you will have to consider building for at least 100 psf.
You have to know the load area for each post. Take the load area and look for its corresponding post size and height dimensions in the Deck Post Size Table.
But to determine the load area you will have to do some basic math calculations. Let's do one as an example...
Start with a simple rectangle deck, 12' wide by 12' deep. The 12' wide side is attached to the house. Assume you would like to use 4 posts (you could use 3 if you preferred but let's use 4 for now).
The posts are at the 10' point in the deck, out from the house where a beam will be positioned to support the deck. So there will be a 2' cantilever beyond the posts and beam.
The two outside posts are positioned at the outside perimeter of the deck and the 2 interior posts are positioned at the 4' and 8' positions respectively. So you have exactly 4' between each deck post.
You are going to have to break the deck into zones. The midway point between support points determines each zone. A support point is defined by either a post, the outer edge of the deck, or the part of the deck that attaches to the house - which makes the union of deck to house a support point. But don't forget to include the area of deck in the cantilevered portion in your load area calculation.
The distance between the house and the beam and post is 10', so the midpoint is 5'. Add to that 5' the 2' distance of the cantilever that extends past the beam and post. This makes 7'. The distance between the first post and the second post is 4'. The midpoint would be 2'.
Multiply 7'x2' and this zone occupies a load area (for this particular post) of 14 sf.
We already know the length of this zone is 7'. The distance between the first, second and third deck posts are each 4' so we know the midway points will be 2' on either side of the second post. Therefore the width of this second zone is 4'.
Multiply 7'x4' and this zone occupies a load area (for this second post) of 28sf which happens to be the largest load area of the deck. Use this load area when referring to the Deck Post Size Table.
The dimensions in the following table are based on a total load of 50 psf for the particular species of wood. So it is irrelevant if you are building a deck designed to withstand 70 psf.
In that case you would have to refer to the building code or other engineering manuals usually found at your public library to determine incremental differences.
From our example, take the largest load area of 28 sf and refer to the chart to determine the maximum dimensions of post that you should use given the load area.
In other words, look at the wood species, post size and load area and you will see the maximum height of post you can safely use. In this example, 28 sf falls under the 36. If you chose to use cedar post, its maximum height would be 10 ft.
Some Other Useful Reading:
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