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I am planning a deck with dimensions of 14' x 20', where that is 14' out from the house, and 20' along the house wall. This will be an elevated deck with 2' of that outward span cantilevered. The plan calls for 16" OC joists and blocking at the halfway point of the span.

My question is about creating the stiffest feeling for the deck. Basically, is it better to do more blocking or 12" OC joists? I don't want that "springy" feeling at all. Nice and solid.

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    Use thicker deck planks.
    – Solar Mike
    Commented Aug 9, 2020 at 14:31
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    Deeper section (ie, turn a 2x8 into a 2x12) joists will be the least expensive (labor plus materials) upgrade. 12" spacing will improve things more, but at a considerable labor and modest material cost. You can also put a beam under the middle (high cost), or block more (thirds or quarters). There are online resources to determine deflection -- you should be able to see relatively easily where the bang/buck intersection is. Commented Aug 9, 2020 at 14:42
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    If you want to reduce deflection, the height of your joists (width as you buy them, height as you install them) has a cube relationship to beam stiffness - i.e. doubling up the joists side by side will be twice as stiff, but making the joists 2 times taller (wider as bought) will be eight times as stiff.
    – Ecnerwal
    Commented Aug 10, 2020 at 14:15
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    The drawback to 12" centers is more screws, which is both a visual and cost consideration. I agree with the others--upgrade the joists one nominal size.
    – isherwood
    Commented Sep 8, 2020 at 20:09

3 Answers 3

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Deeper Joists

  • Stiffness Gain: ★★★★★
  • Cost Adjustment: ★★★
  • Appearance Impact: ★★★★

I assume that you've spec'd at least 2x10s for a 14' span. Bump to 2x12s on the same 16" centers for a noticeable improvement in stiffness.

More Joists

  • Stiffness Gain: ★★★★
  • Cost Adjustment: ★★
  • Appearance Impact: ★★★

Taller joists offer a greater stiffness gain, but if height is a concern you could use more joists (say at 12" centers). The main drawbacks are cost and appearance, as you'll use more materials and have more screw stripes across your deck.

Thicker Decking

  • Stiffness Gain: ★★★★
  • Cost Adjustment: ★★
  • Appearance Impact: ★★★

By tying the joists together more robustly you split load between joists better. 2x6 lumber is much stiffer than 5/4. The main drawbacks (aside from cost) are that you'll need longer screws and the appearance won't be as polished (edges are typically sharper and not as uniform).

More Joist Blocking

  • Stiffness Gain: ★★★
  • Cost Adjustment: ★★
  • Appearance Impact: ★★★★

More blocking will have a similar effect to using thicker decking, spreading load among more joists, with none of the appearance compromise. Install two or three runs. It's more work, and you do need to fit them well. You'll use many more screws and quite a bit more lumber

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  • It is spec'd at 2x10 with blocking at mid-span. Does moving up to 2x12 eliminate the mid-span blocking? I am considering adding in a drainage system, which calls for sheets draped between the joists. Blocking could interfere with this, so eliminating that would be helpful. Commented Sep 9, 2020 at 10:43
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    That's somewhat discretionary and may depend on what your inspector wants. Here in Minnesota I've never blocked a deck (though I've cross-braced interior floor systems before engineered joists became standard) and have never seen joists move appreciably. You could use strapping on the bottom instead, but that won't add as much stiffness, or you could use cross-bracing, which would leave a trough for your membrane.
    – isherwood
    Commented Sep 9, 2020 at 12:50
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Give a short and simple answer because you have a relatively common deck size. 16" OC is fine. Put two rows of blocking with the same materials as the joist. The blocking should split your deck in thirds. If done well the blocking will appear to be some sort of magic beam that goes through your joists.

I mention that because I always add at least one row of blocking to all decks. You do not want your joists turning on you due to moisture/humidity issues. The best way to combat this is blocking. Therefore you need to do it anyway and with 16" OC you should have little to no deflection give you are using at least 2x8 on top.

Note: I want to add that your piers probably play a bigger part in the apparent "stiffness" of a deck. Any kind of sway in them and your deck will have a bouncy or flimsy feeling. This may mean that you want to sink your piers a little lower, get bigger piers/lumber and ensure that they are set correctly. If these things have any given your deck will always have give.

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If you want NO springy-ness ( is that a word?) I would consider using 2x12s (and ensuring supportive posts/columns are also beefed up to support the additional weight load). 12" o.c. would work as well--but the same caution about posts/foundation should also be taken.

If you ever plan to put a hot-tub or other heavy load on it, I would use 2x12s on 12" centers--or consider use of steel I-beams --@ 6ft and 12ft from home (with I-beam columns). Depending on your location, steel MAY be the cheaper option. (delivery costs are not insignificant).

I also would use 2x6 for decking (and use stainless steel screws for the all construction--costly? yes. But you won't regret it).

What are you using for support posts and foundations?

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  • Deck springiness is a function more of joist spacing than the joists themselves, joist sizing dictates maximum load for a given span. Replacing the joists with i-beams won't solve the problem if the decking cant rigidly bridge the span between joists. Commented Aug 9, 2020 at 16:50
  • 6x6 posts on footers/piers. Intermediates are 24", boundaries 16". Commented Aug 10, 2020 at 12:43
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    2 X decking is what makes it stiff. After 25 years my problem is most deck screws have rusted away. Stainless would be my only choice now. Commented Sep 8, 2020 at 21:01

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