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Old 02-12-2010, 03:19 PM   #1
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Deck Building: Does a 16 x18 Crawlspace Foundation require intermediate piers/ posts?


I looked at a deck design site online (Fiberon) & got some ideas about a screenporch we're thinking about. However rather than piers & posts, we want a foundation / crawlspace to match the rest of the house.

If we do get a foundation poured, can I just span joists across the foundation - or do I need intermediate pirers / posts between the foundation walls?

The joists would run parallel to the 18 foot length & tie into the house via a ledger board, FWIW.

So I guess the question is: do 18 foot joists need to sit on a beam at the midpoint of the span?


TIA

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Old 02-12-2010, 03:57 PM   #2
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Deck Building: Does a 16 x18 Crawlspace Foundation require intermediate piers/ posts?


You can span 18 feet without an intermediate pier. You have to size the joists based on the load, which can be obtained from your code book or perhaps from your local code enforcement official. Make sure you apply the loads for the appropriate type of structure, i.e. the loads for a deck are different than the loads for a porch, which in turn are different than the loads for a room inside a house. Your code official can tell you which loads are appropriate for your particular addition.

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Old 02-13-2010, 07:02 AM   #3
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Deck Building: Does a 16 x18 Crawlspace Foundation require intermediate piers/ posts?


Engineered lumber products can usually span that far easily, but they are not intended for an area that may get wet. I would use intermediate supports with PT lumber/beams.
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Old 02-13-2010, 07:37 AM   #4
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Deck Building: Does a 16 x18 Crawlspace Foundation require intermediate piers/ posts?


Off topic a bit--How are you going to keep water from flooding the crawl space?

I do like the permanence of a poured foundation --I just wondered how to keep it dry.
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Old 02-13-2010, 02:25 PM   #5
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Deck Building: Does a 16 x18 Crawlspace Foundation require intermediate piers/ posts?


Thanks guys!

@mike

It's going to be roofed & screened, so at most the water infiltration will be limited to blown rain that gets past the screens. Also, see the drawing below... Properly vented any incedental water should evaporate. Since I'm planning to spread any dirt we don't put back in around the blocks back under the deck, and grade the earth around the deck away from the foundation, I figure we'll be in good shape.

Unless anyone sees a problem with my plan (?!?) I'd be very interested in any suggestions.

TIA
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Last edited by DyerWolf; 02-13-2010 at 02:30 PM. Reason: *** For some weird reason, the image is not as large as intended... hope you can read it
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Old 02-13-2010, 02:29 PM   #6
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Deck Building: Does a 16 x18 Crawlspace Foundation require intermediate piers/ posts?


The expense of running that open span versus a mid span beam and 2X 6 joist would be huge thats why 90% of deck have more intermediate supports than floor systems no need in most applications to have to have long spans. Make sure to lag that ledger in nail wont cut it. Put a 3 inch pipe though your footing at a low spot the water will drain to the outside to help it you can put some drain rock by it when you backfill

Last edited by Say What; 02-13-2010 at 02:31 PM.
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Old 02-13-2010, 02:34 PM   #7
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Deck Building: Does a 16 x18 Crawlspace Foundation require intermediate piers/ posts?


Quote:
Originally Posted by Say What View Post
The expense of running that open span versus a mid span beam and 2X 6 joist would be huge thats why 90% of deck have more intermediate supports than floor systems no need in most applications to have to have long spans. Make sure to lag that ledger in nail wont cut it.

Oh yeah, the ledger will be lag bolted / screwed. I'm not even looking at 2x6 joists, btw... 2x8s if I have to install a set of intermediate 2x10 beams, or 2x12 if I try to span the whole thing. Probably go the 2x8 / 2x 10 route.

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