Does anyone have a few pointers for a 20′ x 20′ x 4″ slab over 7/8 decking and TGIs for an exterior deck in a moderately dry location? This combination seems to be more popular each year and actually looks pretty interesting with good quality stamping and a high quality wood railing. The elevated deck has been structurally over engineered by the architect, has flashing/drainage/ 1/2″ air gap around all posts penetrating the slab, 2 layers of 30 pound felt over the osb, flashing from under siding to top of felt, and the railing system has been designed to span over slab and only contact wood posts that protrude through slab.
Under the TGIs we have green 4x10s sitting on top of pt 6×6 posts. As the green beems shrink I’m figuring about 1/4″ shrinkage in verticle hight. What kind of cracking have people been having with this type of deck? Should we be using squash blocks where the TGIs cross the beams or will a little give help to make up for the drying and settling of the deck? Should we be configuring our slab cuts in a pattern that falls on the beam? What humidity level can TGIs hold up to?
To be perfectly honest, concrete over TGIs is common in our big summer homes since hydronic heat is almost standard equipment in the mountains these days, but my exposure to this combination for exterior decks has been very limited.
Cheers, Don
Replies
4" thick seems alot... i just poured 75ft x 150ft at all less than 3" on top of wood... stained & scored.... or scored and stained.... used flat 6x6 #10 wire in the whole thing... and lightweight 4000lb mix
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Our 4" actually slopes away from the house 1/2" down to 3-1/2", the minimum thickness allowed by local code.
Seems like a lot of work. Why not just pour concrete piers with sonotubes and designa floating slab? Use corrugated metal decking for the pan, add rebar and pour. Won't shrink. Once the concrete cures the metal pan is superfluorus. You go into detail describing how to deal with shrinkage ... use a systenm that won't shrink.
"When asked if you can do something, tell'em "Why certainly I can", then get busy and find a way to do it." T. Roosevelt
Rather than the felt, I would be using peel'n' stick ice and water shield. At that small slope , The thirty pound would do absolutely nothing to keep water out of the decking if the crete lets water through, which it will. Thje only function the 30# is serving is as a separation slipsheet the way you describe. The wood deck should have a slope minimum of 1/8" per foot.
As for the crete, it should be poured as dry ( low slump0 as possible to work. The wetter it is, the more shrinkage cracks it will have to let additional water in.
BTW, it is TJIs ( Truss-Joist International) or genericly - wood I-joists
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BTW, it is TJIs ( Truss-Joist International) or genericly - wood I-joists
He may be refering to G-P I-joists (TGI's)
Terry
I thought it was a mispronunciation/typo. The originals were TJIs but now anybody can make them since the patent is off. Would make sense that Georgia pacific would be common in Idaho
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But I did not think that you could uses on the exterior?
Use what on the exterior?
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The TGI's."To be perfectly honest, concrete over TGIs is common in our big summer homes since hydronic heat is almost standard equipment in the mountains these days, but my exposure to this combination for EXTERIOR decks has been very limited."
Then I don't know. Never had cause to use them outside
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I agree ......the 30# does not cut it....
don't like this whole deck detail...sounds like a pain when it
eventually does leak.....
but if I had to do it,I think I would start with a torch-on...
with special attention to the deck / house /siding/replacement for
when it does leak down the road....
Sounds like a great candidate for a torchdown to me.I used to hate usijng the spud bar for gravel roofs to fix a leak. I can just imagine the guys trying to fix aleak in this thing. A little sealer. a little caulk, an epoxy paint job, and then in ten years when the rot shows through, a little Jack Hammer
Welcome to the Taunton University of Knowledge FHB Campus at Breaktime. where ... Excellence is its own reward!
used to hate usijng the spud bar for gravel roofs to fix a leak. I can just imagine the guys trying to fix aleak in this thing. A little sealer. a little caulk, an epoxy paint job, and then in ten years when the rot shows through, a little Jack Hammer
It would be nice if who ever is insisting on this deck detail had to fix it
when(not if)it springs a leak.....
A carpenter can dream, can't he?....:)
Not to be picky, but TJI stands for Truss Joist I-beam. Let us sing from the same page whenever possible.
Chris
They tell me I can't carry a tune in a bucket
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Concrete slab outdoors over PT+ TGI = have architects gone nuts!
Is this so they can put radient heat in the deck? If so, then really nuts.....
everything rots or rusts sooner or later, & all engineered structures are a compomise between cost & quality, but why make entropys' job easier.....
I joists are not designed for wet areas. Period.
It takes much less moisture to rot an I joist than conventional framing lumber. The webs are made from OSB which rots as soon as you spit on it.
Slabs like this always tend to weep and that weeping is going to kill your system.
When it leaks, how are you going to find the leak ?
When you find the leak, and it is in the middle of the slab, what are you going to do about it ?
Have you ever seen a repair on a patterned concrete slab ?
When you open up the slab to fix the leak, how are you going to remove the section of slab without damaging the whole membrane ?
There are so many reasons not to do this.
Don't do this.
Please.
carpenter in transition
My choice for a moisture barrier would be ice & water, but a co-worker in charge of the last concrete deck called our architect and he specified 2 layers of 30lb felt so that’s what was used. Regardless, I’ll probably use ice & water on future decks that are under my control.
The APA lists maximum moisture content of 16% for I joists in service, so I’ll have to poke some completed decks with a moisture meter when the rainy season comes.
Don,
concrete is not a roofing material
ice and water shield is not a low slope roofing material
30# felt is not a low slope roofing material
you say in your post that you need a moisture barrier, but what you really need is a roof membrane professionally installed.
once the concrete cracks over this framing system (within 30 days) water will then be flowing over your 2 layers of 30# felt. the concrete will hold it there so that it can back up in between the felt layers
if a roofer came to you and said he was going to give you a new roof on a flat deck and he was going to use 2 layers of 30# felt as the membrane, what would you say ?
crazy, right ?
contractor's insurance carriers don't provide coverage for mistakes like this. it would be a good idea to set aside at least $10,000 for each one of these decks that you do for future replacement.
i hope you don't take my tone the wrong way. i'm just trying to save you from the giant pissing match between the angry homeowners, the architect who will be distancing himself from this awful spec when the lawsuits are filed, and yourself the installer.
carpenter in transition
Edited 8/23/2005 10:55 am ET by timkline