Structural requirement for candilever deck

[wdjordan]

An existing redwood deck, 24 lf in legnth extending 14 lf out over 45 degree slope. Existing joists are 2 x 8 attached with hangers onto 2x ledger 16" OC. 11' out, the joists sit atop a 4" x8" beam on 4" x 4" posts anchored to 12" diameter concrete piers @ 7'6" on center. I would like to raise the existing deck to be flush with my pool decking and extend it beyond the existing 3' past the 4 x 8 beam. My idea was to sister onto the existing 2 x 8's with 2 x 12 x 20'. This would put me at 9' past the 4 x 8 beam. Is this doable?

[Jerry Peck - Codeman]

Not as you describe it.

Cantilevered joists are only allowed to be cantilevered a maximum of 1/3 of their rated span length, and that rated span length is with the joist over one span, two supports.

For example: Let's say you have a joist which has a span rating of 12 feet (between bearing points) over two supports. That means the joist would only be allowed to be cantilevered out 4 feet (1/3 of 12 feet), and only if the remaining length were over two supports, i.e., the joist would have to stick into the structure a minimum of 8 feet, and be supported at the wall where the cantilever starts and at the inward end of the joist 8 feet in.

Now back to your project: If you wanted to cantilever out 9 feet, the joists would need to have a span rating of 27 feet, and a minimum of 18 feet of the joists would need to be inward of the wall/beam where the cantilever starts. Finding lumber large enough for a 27 foot span will be hard enough, finding lumber in lengths long enough to do what is needed will be even more difficult. That would leave you with using engineering joists, such as glulams or LVLs (and the glulams or LVLs would need to be treated for use outdoors) in the size and lengths necessary to accomplish what you want.

Now, an engineer could run the numbers and do the calculations which may permit using smaller lumber and shorter spans, engineers are allowed to do that. Constructing a structure from the code book as though it was a cook book does not allow for those choices.

[wdjordan]

Thanks. Okay then, since it is not really a Candilever deck, with the support I have is it structurally safe to go out ant further? Again, the deck is 24' x 14'. I would use hangers for my new 2 x 12's along the 24" legnth ledger. The 2 x 12's would also be sistered into the existing 14' legnth 2 x 8 joists, but would extend past the 2 x 8's whatever distance is allowable (if at all). There are (4)ea 12" concrete pilons/piers, 7' 6" oc that support a 4 x 8 beam that runs the 24' legnth at 11' out from the ledger. The joists currently extend 3' past the beam. What I want to do is install a 3/4" moisture resistant plywood over the joist, then torch apply (I'm a roofing contractor) (2) layers of smooth APP membrane-Install two deck drains at necessary locations (The deck will have a 1/8 in 12 fall but there will still be two small bellys that need drains) then have a high-quality simulated-turf over drain grid, installed flush with my stone deck. I could do the area as it stands but would love (If Safe) to expand my yard a tad. Thanks in advance for putting up with my redundancy!