Tips/Techniques Deck structural question - simple

[slow-poke]

See attached sketch (red mark up)

I'm replacing the 1" x 6" cedar cladding with Trex. The structure below is 2x8" on 16" centre.

Question relates to the support for the three centre vertical strips shown in red.

The span is 15" maximum (gap between 16" on centre joists). In theory there should be a joist under each of those vertical strips. So in theory the vertical strip could have one fastener in the middle but really needs two near the edges. So I will need to add a support on either side of the joist (36 total)

I'm guessing that a 2"x4" on edge would be adequate?

2x 6 or 8" seems overkill.

 

[quitereal]

2x4 on each side would be 3.5+3.5+1.5=8.5 - 5.5deck board=3 /2 =1.5 each side to fasten end of deck board. Id say thats fine, people often join deck boards on a joist where they only sit on .75

 

[Susquatch]

Question relates to the support for the three centre vertical strips shown in red.

It isn't obvious what your drawing shows. Is that a top view of the decking boards? If so, calling the subject boards "verticals" is confusing. Are they more like decorative laterals?

Also if so, can you provide a cross-section of your proposed support system?

LOT of cutting for the small decorative end boards on the narrow walkway (assuming that's what it is). The design is such that any errors in the mitre joints will be magnified to the eye. Looks nice in your drawing though!

3.5+3.5+1.5=8.5

I think you are adding the individual heights to approximate total strength. That doesn't work.

Beam bending strength is proportional to the cube of the height (depth) of the beam, so the extra height of a 2x8 gives it significantly greater strength than the same amount of wood used in two 2x4s placed side by side. A 2x4 has a height strength factor of 37. Two of them side by side is 74. A single 2x8 has a height strength factor of 422. Thus it takes over eleven 2x4s to equal one 2x8.

Even stacked on top of each other isn't equal unless the joint is glued to the same strength as solid wood.

That's the raw math but of course wood varies significantly so the code doesn't follow that ratio perfectly.

Then again, I don't really understand the drawing, so we might be talking watermellons and Hondas.

 

[slow-poke]

It isn't obvious what your drawing shows. Is that a top view of the decking boards? If so, calling the subject boards "verticals" is confusing. Are they more like decorative laterals?

Also if so, can you provide a cross-section of your proposed support system?

LOT of cutting for the small decorative end boards on the narrow walkway (assuming that's what it is). The design is such that any errors in the mitre joints will be magnified to the eye. Looks nice in your drawing though!

What I called vertical is as shown on the top view drawing. Lateral would better describe them.

The proposed additional supports would run the same direction as the 16' and 20' long Trex, except they would just span the distance between the joists so 14.5" long. The 5 decorative lateral pieces will be supported in the centre along their entire length by the joists, however to fasten those laterals closer to their edges requires the additional structure supports near the edge.

The narrow walkway is actually an eight foot wide staircase down to the lawn/garden.

Deck is 72' x 9'ish

 

[phaxtris]

I agree with @Susquatch, Your explanation is difficult to understand but i believe i have a grasp of what you are trying to achieve

Blocking is normally the same size as the joist, mostly to help with twisting of the joists, although not really a major concern here, that is typical standard framing practice, and how i would go about it. Cost difference is minimal.

The other thing, I would not cantilever the ends of the long boards in the middle of the deck like i believe you are going to, you may end up with soft spots and its likely the the ends will not all be the same height over time, especially seeing as some ends would be on blocking and some wouldn't, the more correct thing to do would be to add an additional floor joist where your joints will be, this would allow you to end the long board on 50% of the joist width, with your transition piece on the other 50%, then another floor joist that carry's the other side of your transition and long boards. Probably a few dollars more than blocking, but simpler and easier

 

[Susquatch]

The proposed additional supports would run the same direction as the 16' and 20' long Trex, except they would just span the distance between the joists so 14.5" long.

Again, I am lost. They run the same direction as the long deck boards???

The 5 decorative lateral pieces will be supported in the centre along their entire length by the joists, however to fasten those laterals closer to their edges requires the additional structure supports near the edge.

Ya, a 2x4 nailed into the 2x8 joist is adequate because they just transfer load to the joist which does the grunt work. But unless your lateral boards are narrow, I don't think that will reach the deck boards to support them too.

Deck is 72' x 9'ish

HUGE!

 

[phaxtris]

Ya, a 2x4 nailed into the 2x8 is adequate. But unless your lateral boards are narrow, I don't think that will reach the deck boards to support them.

I agree with both statements, you wont get the width assuming he's using 5/4x6 deck boards

 

[slow-poke]

Again, I am lost. They run the same direction as the long deck boards???



Ya, a 2x4 nailed into the 2x8 joist is adequate because they just transfer load to the joist which does the grunt work. But unless your lateral boards are narrow, I don't think that will reach the deck boards to support them too.



HUGE!

I don't understand why this is so confusing.

Drawing is a top view, showing the Trex boards are ~6" wide and are 16' & 20' lengths. Starting from the left a 16' length, followed by two 20', followed by a 16' for 72' total, ignoring the five decorative boards (red circle) that are at 90 degrees WRT the 16&20 footers.

The structure under that is a simple 2x8" frame on 16" center, obviously those 2x8 joists are at 90 degrees WRT the Trex top.

 

[Susquatch]

It's simple to you cuz you drew it. But it's a 2D drawing. That make it easy to misunderstand.

But I can JUMP TO CONCLUSIONS as well as the best of the ASSUMERS can.

A side view of the problem laterals would help a lot.

But assuming I see it correctly in my mind, the cross-section looks like this:

As you can see, there is nothing to screw the ends of the deck boards to.

 

[historicalarms]

If I understand what your trying to do here, substitute 2x8-10's with 2x4 because the 2x8's seam like overkill for the strength needed, the 2x4 are "probably sufficientl strong enough" when used on edge, but I would be suspicious about the warping that 2x4's are capable of with a bit of age, I've seen 2x4's turn onto basically an airplane propeller were as the 2x8's seem to resist this a lot better.

 

[Susquatch]

If you are thinking about moving the 2x4s out from the joist, then that is no good, they are not strong enough. They need to transfer their load to something that is strong enough.

 

[Susquatch]

If 2x4's were good enough, you could replace all the 2x8 joists with 2x4s. And truth be told, that would probably work. I've walked on decks built that way. But it would feel like you were walking on sponge. Especially as the 2x4 ages and especially for big guys like me. Kids ya, but 250 pounders, or a fridge on a dolly, no.

The other nuance is the feeling while you walk across it. Nice and firm across the joists but spongy at the 2x4.

The code specifies joist size for a reason.

 

[Downwindtracker2]

People have been injured when their sundecks collapsed during a party. I started out as a carpenter,Red Seal and all that, so I had a chuckle when it was on the news.

 

[slow-poke]

Perhaps some pictures will help.
Nothing is cantilevered.
I'm not replacing the existing structure or substituting any part of it.
The supporting structure is a 2x8" frame 16" on center, inside attached to the house and the outside supported with 6"x6" posts from the ground up to the bottom of the frame.

The one and only issue that I posted about is that the 5 Trex boards that run at 90 degrees to all the others will need some form of support along their edges, and those edges where they need a fastener will fall between (just a few inches from) the 2x8 floor joists that the Trex attaches to. So I was planning to just add 2x4s running between the joists just where the lateral decorative Trex boards are.

 

[slow-poke]

If I understand what your trying to do here, substitute 2x8-10's with 2x4 because the 2x8's seam like overkill for the strength needed, the 2x4 are "probably sufficientl strong enough" when used on edge, but I would be suspicious about the warping that 2x4's are capable of with a bit of age, I've seen 2x4's turn onto basically an airplane propeller were as the 2x8's seem to resist this a lot better.

Of course not, that would be ridiculous. I at no point said anything about substituting 2x8 with 2x4. No idea where this idea came from.

 

[slow-poke]

I suppose instead of running the 2x4s between the 2x8 joists, I could simply attach one on each side of the joists that fall below those 3 centre decretive Trex boards. The Trex is ~5.5" wide so it would overhang the joist by 2" on either side. So if the fastener is 1" from the edge of the Trex it would catch the outside 1/2" of the 2x4.

 

[Susquatch]

So if the fastener is 1" from the edge of the Trex it would catch the outside 1/2" of the 2x4.

Isn't it amazing how horrible the English language is!

Here is a close up of my earlier drawing. See - "This is the problem!!!"

 

[slow-poke]

Isn't it amazing how horrible the English language is!

Here is a close up of my earlier drawing. See - "This is the problem!!!"

View attachment 67470

There is a 6" post underneath every joint.

This was never a question about the 2x8" structure, except where the edges of those three center decorative top boards fall and how best to support those edges within that 2x8 structure.

 

[Elektrishun]

Couple of comments:

- beautiful house

- I always go "overkill" when it's my own projects

- does the deck get much use in the winter? I made a ramp leading up to my shed using Trex decking. Going straight up the ramp, approaching across the faux grain, no problem. If I approach the ramp at an angle, stepping on it more "with the grain" it becomes the "Stars on Ice" show. Even when it's really wet it's very slippery depending on the approach. Maybe this aspect has been improved since my installation? The shed doesn't see much action in the winter so it's seldom a problem.

 

[slow-poke]

Isn't it amazing how horrible the English language is!

Here is a close up of my earlier drawing. See - "This is the problem!!!"

View attachment 67470

Perhaps best solution is two parallel 2x4 on each joist underneath the three decorative pieces