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diagonal bracing under deck joists

fossil's picture

I have a second story deck 14' by 50' with 2 by 10's. Wondering if running a 45 angle 2 by 4 from the center of ledger outward in both directions is acceptable Screwing them in with 3.5 stainless griprites to the underside of the joist on the flat. Thanks

(post #101314, reply #1 of 15)

It will help stiffen up the deck but also give somewhere for the birds to nest.

If you have yet to install the decking, run this at an angle for the same effect.(looks great too)

(post #101314, reply #2 of 15)

Not a common technique, but why not?  I normally put a temp brace in that configuration to hold the deck square prior to/during decking installation anyway.

Really though, lateral stability will come from the attachment to the house and diagonal bracing from the support posts to the girder/beam.


(post #101314, reply #3 of 15)

The previous deck had 2 by 10's mitered on both sides upright in every bay. Seems like a lot cutting.

The old deck had no angled supports on the beam, but if that's enough to stiffen it up I will do that.

You folks would say then the 2 by 4's on the flat is uneccesary/waste of time?

(post #101314, reply #4 of 15)

If you're installing new porch flooring, I concur with running it diagonally. It'll stiffen the deck up much more than diagonal bracing will. Barring that, I'd add full-depth blocking on an angle between the joists. It's pretty quick work with a circular saw. Just snap a chalk line under the joists as a guide.

Without seeing the overall design of the deck, I'm inclined to think diagonal bracing in some form is advisable. The flat 2x4s would be my absolute minimum choice, and I'd use something heavier to attach them, maybe 1/4" x 4" lag screws.

(post #101314, reply #11 of 15)

i would be using a 2x8  spiked into the bottom of the joists run on the diagonal right into the wall. You can always screen the bottom of the deck to keep out unwanted  birds or what have you.

(post #101314, reply #12 of 15)

Diagonal bracing on the post beam connection will somewhat help the wiggle then?

I'd like to diagonal brace the bays, but I doubt HO will tolerate the time and expense.

Any guidelines on how long the diagonal brace on the post beam connection needs to be?

There is a slider and cement slab under the deck.

(post #101314, reply #13 of 15)


I once used 6x6 diagonal bracing on a deck that was about 12x12 with the finished floor about 9' off the cement patio below.

I seem to recall the braces were about 4' or 4.5' long which means they were fastened 34-38" from the beam. I used an 1/2x8" lag screw to connect to the post and 2 1/2" carriage bolts to connect to the beam (the beam was a dbl. 2x12 with 2-1/2" in between so the 6x6 brace was cut with a tenon). This did help make the deck more rigid.


Jon Blakemore

Fredericksburg, VA


Jon Blakemore

Fredericksburg, VA

(post #101314, reply #14 of 15)

A simple and elegant solution here is Simpson flat strap galvanized steel, set with 10d hot-dipped galvanized commons in an x-brace pattern, tagged at each joist crossing, running from rim joist to ledger, at 45 degrees. The strap works in tension, and with both directions covered, you will be amazed at the rigidity that the deck platform gains. Multiple crossings can be done for long decks, such as yours,if your engineer feels the need. Available in rolls of 50', this stuff can work wonders in many applications.

Crossing Simpson strapping is also a very simple way to take the "whip" out of straight runs of stair, such as a free-running stair into your back yard. Best to you, Glen

(post #101314, reply #15 of 15)

I'm just not sure that stuff will last with the ACQ joists everyone is now using, even though Simpson probably does make 'Z' strap.

(post #101314, reply #5 of 15)

We did a deck about 20 feet off the ground and about 30' wide (length of ledger) and 15' deep. The girder (12' from house) was tube steel and was supported by two steel columns. The engineer spec'd an X-brace detail using 1" steel rods... first rod anchored to pier A and running up to the top of column B, second rod anchored to pier B and running up to the top of column A. Each rod had a large turnbuckle threaded into it.

You could do the same thing under the deck joists using steel cable, turnbuckles, and eye bolts installed into proper blocking at the corners.

I would not bolt a 2x4 to the bottom of your framing. It gives water a place to sit and rot out both the 2x4 and the joists. I suppose if you were to very carefully glue it in place (epoxy? poly?) under each joist you could mitigate most of that problem. Tightly fastened decking will probably give a lot of the shear you need anyway.

(post #101314, reply #6 of 15)

The post's and beam is 12' out from the ledger and are not currently braced. Would bracing this add lateral stiffness to the deck.

When someone is walking near the railing a wiggle is felt.

The old deck had no braces there, but did have vertical diagonal blocking in every bay.

(post #101314, reply #8 of 15)

I agree with Davidmeiland that cross bracing on the vertical plane is important on a tall (off the ground) deck. The idea of cable (and turnbuckles for tightening) running diagonally from corner to corner horizontally sounds good too. Not much place for birds and critters and will shed water.

(post #101314, reply #9 of 15)


The old deck had vertical diagonal blocking, which IS BRACING. Known as "cut-in diagonal blocking," it acts mainly in compression, but going both ways in a vee or x-pattern it is very effective. Unlike the 2x on the bottom, it provides no roosting support for birds, drains well, and does not only rely on fasteners shear values to work. I'd look seriously at duplicating the old detail.

My home was built in 1951, and has the same sort of bracing in all the outside walls. No stucco cracking in half a century, with some minor quakes thrown in.


(post #101314, reply #7 of 15)

IMO, decking installed diagionally goes well with contempary homes but visually, not so good with traditionally styled homes. 


(post #101314, reply #10 of 15)

I think the bracing you descibe does the same thing that "X" bracing from post to post does. The only difference is that the lateral force is applied to the ledger rather than the piers/posts, but I don't think you really talking about enough force to effect the ledger connection.


Jon Blakemore

Fredericksburg, VA


Jon Blakemore

Fredericksburg, VA