on this one, I've got 2x4 joists into a flush-mount beam at the high side, dropping down to 2x6 joists flush-mounted on one end and cantilevering over the outer perimeter. I've got the beams strapped to concrete pier blocks again, our frost line here is 12" so I use the precast blocks for ground level decks .
But I would tell you that using 2x4s as joist is problematic. They are much weaker than 2x6s so you will have to have a much tighter joist spacing, certainly no more than 12" from center to center. Secondly you will have to have many, many deck blocks along the length of each 2x4.
IF you are allowed to use 2X4's for the joists, it'll cost you more money than using 2X6's would. The 2X4's won't span as far, so you would need more beams and more footings to support them. The 2X4's won't span as far, so you would need more beams and more footings to support them.
Low Profile Deck Using 2x4 Joists by: Editor - Rich Bergman 2x4 joists would be sufficient depending on close they are spaced no further than 12" on center, maybe even a bit closer and how long the 2x4s will span from one support beam to the next. 2x4s can not span a very long distance without starting to bend.
The patio is not actually going to use beams, but I am going to dig several footings along the entire area of the deck in order to keep the patio as low as possible. The joists are going to be 2x4x8's. I am debating whether to attempt to span 4' with the 2x4's thus needing to put 3 footings per 2x4 or if I need to go with 4 footings per at 2 2/3 foot spans.
The problem you'll have with trying to use 2x4's as floor joists is that most span tables don't include 2x4's. This means that it will be hard for you to convince your planning department to approve your plans without some sort of proof that the 2x4's are adequate.
The 2x4 also wouldn't make for a sufficient floor joist, so you'd have to use new structural lumber for that anyways. If it's old lumber, I'd consider saving it for another construction project, or donate it to your local re-use center or habitat for humanity.
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