on grade decks

Old 11-17-02, 10:02 AM
Bob Rich
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Question on grade decks

I am wanting to add a deck on one corner of my home. The ground in this area is reasonably level and construction of the deck will mean that the support runners would be below ground level. I am hesitant to bury lumber due to rotting, termites etc.

I was contemplating using treated rail road ties set into the ground and bolted to the support columns of the upper deck for support of the headers that the decking will nail to. Does this make sence? Is there a better way or a better product?

The support runners have to be below grade in order for the deck to be a the proper level for the two doors that open out onto the future deck.


Old 11-17-02, 01:59 PM
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On grade decks

Railroad ties rot, too. The railroad ties the previous owner placed in my landscape at my mountain cabin are nearly rotted beyond usefulness.

Deck Framing Details
By Tim Snyder
Illustrations by Bob La Pointe
Good framing is as essential for decks as it is for houses. And as you'll see, there are quite a few ways to handle the fine points of framing, from putting in ledgers and beams to pouring concrete piers and cutting stair stringers. Local building authorities can be a good source of free advice about deck details, and so can professional builders and experienced lumberyard salespeople. If you take the time to troubleshoot your framing plan, your deck-building project will go along more smoothly.

Installing a Ledger
The ledger is the board that connects your deck with the house. It's the first board to be installed when most decks get built, and it's one of the most important structural elements—the ledger's top edge is the reference line for all the framing to come. When your decking lumber arrives, pick out the straightest, most knot-free boards you can find and set them aside to be your ledger boards.

The ledger connection must be as strong as possible. How you'll install the ledger depends on the wall you're fastening into, but in all cases, strength matters. A considerable load of joists will attach to this framing member, and if the ledger pulls loose from the wall, the entire deck can collapse.

Water damage is the other major concern with the ledger because you attach it to the outside of your house. The installation details shown—especially the flashing—will direct water away from the house structure and forestall rot. Exterior caulk is also essential for sealing holes where bolts penetrate wall surfaces. Lastly, you can use metal sleeves or multiple steel washers to create a drainage space between the ledger and the house exterior.

Pier, Post and Beam Options
Together, piers, posts and beams create the foundation for your deck. Piers and posts are the vertical structural elements; beams run horizontally. Piers are typically concrete; posts and beams, wood.
The foundation types shown here are among the most common used in deck construction. Once again, it's important to consult with your building inspector or an experienced builder to make sure that your concrete piers and wood posts are sized and located properly. In general, you should always use carriage or machine bolts at all post-and-beam connections.

Beam on pier or pad. This is the type of support to use if you're building a grade-level deck. The beam—usually a double 2-by—can bear directly on a poured-concrete pier or pad that's close to grade level. You can also use precast bases, preformed footings available at building supply houses, but their size tends to make a grade-level deck look clunky. With the top of the pier or pad just a few inches below grade, you can backfill soil up against the perimeter joists and have the lawn grow right up to the edge of the deck. As always, any wood framing that comes in contact with the ground should be pressure-treated lumber or a rot- and insect-resistant variety such as redwood or cedar.

Post on precast base. Where building codes allow them, precast post bases can provide sound support for a low-level deck—one with posts no taller than 3 feet. Make a hole for each base, and put a layer of gravel in the bottom of it. Be sure to tamp the ground and the gravel well to ensure a solid foundation for the base. Notching the post to hold a 2-by-10 or 2-by-12 support beam on either side improves strength and appearance, and, again, be sure to use carriage or machine bolts as fasteners.
Post on pad. Elevated decks that include tall posts (8 feet or more) benefit from the stability of a post-on-pad foundation. This construction is suitable for all soils except sandy ones.

The pad must be at least 4 inches thick and sit below the frost line, so be sure the hole you dig for the cement is deep enough. This may not be an exact process, so it's usually easier to leave your posts long and cut them to final height after you get them in.

Let the pads harden overnight before you set the posts on them. Use a plumb bob to make sure each post is straight, brace it, and add fill, tamping it down hard to hold the post in solidly. You can pour additional concrete around the base of each post instead, but it's not necessary.

If you don't notch the posts for support beams, connect them on either side of each post with three machine or carriage bolts. Nail a 2-by top piece on top of the post and between the support beams to increase strength and rigidity.

Post on pier. This foundation works for all types of decks, and it's often required where the soil is sandy or site conditions are difficult. Depending on local code requirements, you can make your piers by pouring cement either directly into holes in the ground or into tubular cardboard forms that rest inside the holes. Either way, the pier should extend to below the frost line, and the diameter of each pier should be twice your post thickness. Attach either a steel post base or a base anchor bolt (see Deck Hardware for more on this) to the top of each pier. Take care to align the bases or bolts so that the mounted posts will be in a straight line. You can use solid wood beams or double 2-by beams for posts.

Span and Spacing
Span refers to the distance between support points beneath a joist or a beam. Spacing denotes the gap between parallel joists, studs, rafters and the like. Calculating the size and spacing of joists can be complicated, but here are a few rules of thumb.
Span ratings depend on the species and grade of lumber you select, as well as its size. Builders and lumberyards have span tables you can consult to find out what size your joists need to be to span a given distance. For example, you can use a treated pine 2-by-6 joist to span up to 10 feet, while a 2-by-10 joist of the same type can span up to 16 feet.

The thickness and composition of the decking material also affects joist spacing. Cedar, redwood and pressure-treated pine or fir are the solid woods most often used as decking. Decking sold as 2-by lumber—actually only 1 1/2 inch thick—can be installed over joists spaced 24 inches apart on center. Decking designated 5/4 inch, which is actually only 1 inch thick, requires joists spaced 16 inches apart on center. Most synthetic or composite decking materials also require joists spaced 16 inches on center, though some manufactured boards require 12-inch on-center spacing. It's best to check with your supplier about spacing and span ratings.

Stair Framing Details
Unlike an interior stairway, a deck stairway can stretch out, using broad platforms instead of traditional treads to move between levels. Framing plans for multilevel decks can help you with this type of construction.

For a conventional stairway design, use these guidelines:

Unit rise (the height of each step) should be between 4 and 7 inches.
Unit run (the width of each step) should be between 10 and 14 inches.
Unit rise plus unit run should equal 17 to 18 inches.
Nosing (or overhang) should be 1 to 1 1/2 inches.
For longer-lasting treads, use two or more boards to make a single step, instead of a single wide board. Before you install stair treads, finish them by coating their sides, ends and edges with decking stain and letting them dry completely.

Tim Snyder, writer, photographer and carpenter, was a senior editor at Fine Homebuilding magazine and executive editor of American Woodworker magazine. With TV personality Norm Abram, Snyder coauthored two books in the best-selling New Yankee Workshop series. He's also written books on deck design and furniture

Snyder, Tim. Deck Framing Details. Retrieved 17 November 2002. Retrieved 17 November 2002. http://www.cornerhardware.com/html/articles/art49.htm


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