Deck extension project...

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Old 01-12-20, 09:20 AM
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Deck extension project...

Hello all - first post in this forum. It seems to be a good one with a lot of experienced folks.

About me? An almost retired mechanical engineer, have done quite a few projects like this deck extension, but one thing we all know for sure - you can never gain enough knowledge. It's always good to ask people that have been there before. Hoping this forum will be a good resource.

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Bought a home where they built a deck from the first floor out over a walk-out slab, But the deck does not extend all the way to the corner of the adjacent brick wall, it needs another 2.5 feet.

Deck extends 15-16 feet from the main wall of the house, and is about 12 feet wide. It has 2 posts that go through the slab to the frost line, with joist ends fastened to the house.. The edge of the current deck is already cantilevered 18" or so past the last post. I would like to extend it another ~2.5 feet, about 42 - 46 inches total cantilever.

One option is to sister all 12 2X8 joists with 8 foot 2X8 joists, and cantilever the whole extension distance without any new supporting posts. I have seen the limits for cantilevering joists vary from place to place, some say 1/3 the primary joist length (which would be 5 feet) - other say 1/4 the length (which would be 3-4 ft) and others say even less. Depends on the size of the joist, the species, and so on. From a purely structural viewpoint, a 42" extension using sisted 2X8 joists would be fine in most situations. I guess if you had twenty 200 lbers standing right on the end of this platform it might creat enough uplift and bending moment to be noticeable but not really concernced about the structural inegrity. Sale of the home if it isn't code is another story.

Second option is to construct as described above, and place 2 posts under this 2 foot extension (which would place the posts only 4 feet outboard of the existing posts). Easy way would be to use a deck post support on the existing slab. The axial load this post would take would be very small. The underlying slab goes to the frost line, and these posts would be right over that part of the slab.

Either one of these options is structurally OK. Question is, are either of them code? Cutting a hole in the slab to extend the post into the ground would actually weaken the slab (and might not even be possible since post is over the footer).

Thoughts on it?

Here is a sketch of the ideas.

 
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Old 01-12-20, 06:11 PM
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Correction to the earlier sketch. Posts are actually 6X6 not 4X4.

 
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Old 01-12-20, 06:52 PM
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I dont think anyone would approve option 1, a sistered cantilever like that, at least not without an engineer's approval. R507.5 refers to 1/4 of the actual joist span. The span for this calculation in your drawing is the distance between your 6x6 POSTS. Option 1 is definitely out.

As far as option 2 is concerned I don't know if they would approve that either. The pad might have a perimeter footing below frost but the interior of the pad can still freeze and heave... which is why your other posts continue through the pad and have their own footing.

Only way to know for sure is to ask your local building inspector. Dont know what good another 2.5 ft does on such a large deck, but I guess that's your business. The mention of brick nearby concerns me. Deck ledgers should never be attached to brick. If it is, it's already flunking current code requirements.
 
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Old 01-13-20, 04:31 AM
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Thanks for the helpful reply.

Joists are not attached to brick. they extend through brick with connection as in FIGURE R507.9.2 "ledger board connection to solid foundation" . It's a well constructed deck.

Looking at RB264 which seems to be based on the R507 regulation, there is a little more information. Looks like in there they define the joist span as the total length of the beam back to the joist hanger. Then, they allow 1/4 of that length as the cantilever. So if you have a 16 foot joist span, you would be allowed a 4 foot cantilever according to RB264. Link below.

RB264

Agree it is a good idea to contact the local trustee and county inspector about this one. Structurally, it seems to me to be a very low risk, particularly with option 2. This slab has little frost/heave potential, as most of it is covered by a second story roof. Actually when I first moved into the house I had a structural engineer out to survey the foundation and this slab - the footers are substantial.

Reality is that there is a lot of conservatism built into this regulation, I will be curious to see what they have to say. Will let you know if I get any assessment on it.

Anyone else have some thoughts or previous experience?



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Old 01-13-20, 06:57 AM
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That's good that they extend through the brick.

But they will not approve 1/4" of joist span as the cantilever once it reaches the upper limit of what is allowable for your joist size and spacing. (See DCA6 table 2.) Where under the heading "Joists" it states: Overhang length is the lesser of allowable overhang, LO, or one fourth the joist span, L/4. In your case I think you would have to go by LO, not L/4.
 
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Old 01-13-20, 10:10 AM
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Got it - that LO cantilever limit is super conservative. They assume that someone will do something ridiculous on the overhang like putting a hot tub out there. And they probably have to assume the joint hanger connection could be damaged or deteriorated at some point.

Did some simple hand calculations, for a 4' overhang at this length, a static 1000 Lb point load at the end will create about 500 lbs of up-force at the joist hanger, at the wall. That will get distributed across several joists, let's say 6 of the 12 react this load. So the up force might be 50 - 100 lbs per joist hanger for this 1000 lb load, and that assumes the intermediate 6X6 takes 0 upload.

Also just for grins, had a look at the dynamic loads a human generates when jumping. That might range up to 500 lbs or so max at the end of the cantilever for an average person, so 25-50 lbs per joist hanger.

So from any realistic practical point of view there is very little structural risk. Still I agree that either of these ideas do not meet the letter of the code. My plan would be to take these rough calculations and these two options and see what building inspector says.

Anyone been down this path?
 
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Old 01-13-20, 01:56 PM
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they built a deck from the first floor out over a walk-out slab, But the deck does not extend all the way to the corner of the adjacent brick wall, it needs another 2.5 feet.

sorry, I'm confused, in your pic, there is another wall (brick) on the right side? if so why not just ledger and joist hanger into that.

No 44 foot cantilever! 2 ft max by code usually, regardless of what codes you may be reading. even with hurricane clips and without a party of people on the edge taking a photo (which is often how decks collapse on youtube I've seen), I mean, I'm sure it'll hold but you might have probs down the road with it not being to code.

Why not just do it the right way, rent a wet saw for ~$60 and cut some clean squares out of the 6" slab, jackhammer/sledge/chip them out and dig proper footings there then re fill the squares with new concrete?
Also assuming hot tubs etc should also be in the plans and not allowed on a cantilever, will usually need extra/doubled up joists plus 18" (not 12) minimum and 4' deep (not 3') footings and possibly posts every 4ft or close like that.
 
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Old 01-14-20, 04:52 PM
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Thanks for the ideas gunner666. I think you meant a 4 foot cantilever, not a 44 foot cantilver. I am pretty sure a 44 foot cantilever won't fly!

The wet saw might be the way to go. The problem though is the the right place to place the posts is over the footer, which is 30" deep at least in this area, Would have to destroy the footer to place the post so not excited about that. Could come inboard of the footer but then the new post would be very close to the existing posts.

Sorry I didn't show all the deck or all the views. This is just the joist structure not the ballisters.

Below is a terribly out of scale plan view but it gives the general idea. Just showing one joist.

Have no interest in a hot tub and would never own one, but of course some future owner might.

Another idea I came across was to use steel C-Channel to support the cantilever. I am not worried about the structural strength of this entension but it could sag or creep.

 

Last edited by Midwesterly; 01-14-20 at 04:53 PM. Reason: addition
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Old 01-15-20, 04:28 AM
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I do not know about passing code but cannot see any problem with the safety of the overhang especially with option 2.

I would glue and stitch nail or screw the sister.
Also the decking is holding the sister from moving.
 
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Old Yesterday, 08:18 AM
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Thanks manden. This is exactly what I am concluding too. I plan to make the extension with 8' sistered 2X8 joists.

Then install 2 6X6 posts under that extension that will be anchored directly on top of the slab footer which is 1 feet thick. It is not going anywhere and in fact is probably overkill, but better safe than sorry. Not worried about the structural integrity of the deck w/o the posts but it could sag over time. And some future owner might put a hot tub there, who knows.

The last question I have is regarding the material of the post anchors. Some say that the galvanized steel Simpson anchors are incompatible with the pressure treated posts and galvanic corrosion will corrode them over time. Of course that might take a decade or more, and I may not be around to worry about it, but would like to choose the right material for the anchor.

If I get this thing done I will post some pictures here.
 
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Old Yesterday, 08:25 AM
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Most anything you buy will be Zmax. Which is acq compatible.
 
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