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Unread 10-22-2006, 11:43 PM   #1
kayshav
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Join Date: Aug 2006
Location: Silicon Valley
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Tile on 24" OC joist with 7/8" subfloor?

i'm planning to tile my bathrooms but I noticed in the crawl space that my joists are 24" OC. The subfloor is 7/8 plywood and my plan is to use 1/4" hardibacker underlayment. The house is single story, built in 1982 in Northern CA. The piers are 4x8 spaced every 48". There is one row of these supports in the middle of the crawlspace and one more close to the foundation and the distance between the two seems to be 25'-30'. Looks like the builder has originally tiled the entryway area (5'x 12') with 12'x12" tiles and when I look underneath the entryway I don't find any exra blockings or sister joists foe extra support. Here are some pictures of the joists and piers.

[IMG]joist

[/IMG]piers

I'm not knowledgeble enough to tell

a. is this construction good enough for a tiles floor? I can't tell what type of construction it is.
b. if not, what do i need to do, minimally to shore it up for tiles?
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Unread 10-23-2006, 05:42 AM   #2
bbcamp
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The Hardibacker folks won't accept your floor construction for their underlayment. The joist spacing is too wide. You'll need a membrane of some sort. The one I'm most familiar with is Ditra, from the Schluter company. They'll require another layer of 3/8" plywood, though.

Your joists are really open-web trusses. They are very good, for supporting long spans, but without further information, I can't tell for sure yours are good enough. The depth (measured top to bottom of the truss) and the maximum span (measured from one supporting structure to another) needs to be measured carefully before I can even make a guess. While you are down there measureing, look for manufacturer's markings and write down anything you can find. The manufacturer is the best source for information on these products.
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Unread 10-23-2006, 08:38 AM   #3
kayshav
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tile on 24" OC joist

Bob,

The height of the truss is 12" and the supports are spaced 48" apart. As I mentioned above, there is one long row of supports perpendicular to the joists almost in the middle of the house and one more about 30' from the first one. Therefore, 2 rows of piers only for the entire house.

-k
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Unread 10-23-2006, 11:14 AM   #4
kayshav
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updated pictures

I have annotated the pictures for clarity
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Unread 10-23-2006, 05:22 PM   #5
John Bridge
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Hi K,

You'll still need another layer of plywood. Might work as is, but we won't take the hit.
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Unread 10-23-2006, 05:52 PM   #6
kayshav
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Another Layer of plywood - what thickness

John,

Thanks for the pointer. So, can I just add another layer of 3/8 Exterior Grade plywood and then 1/4" Hardibacker or Wonderboard?

Hardibacker says min of 19.2" spacing and therefore I'm wondering if I should use Wonderboard instead.

-kayshav (yeah, that's my name
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Unread 10-23-2006, 06:40 PM   #7
bbcamp
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Hardibacker says 19.2" maximum joist spacing.

What I was looking for is the maximum span of one of the trusses. The 48 inch spacing on the beam supports is good information, too.
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Unread 10-23-2006, 09:09 PM   #8
kayshav
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Truss Span

Bob,

These trusses run from the front of the house all the way to the back, which is a distace of ~50 ft. There are two support beams (the 4x8 shown in the picture above) about 25 ft apart, one almost in the front of the hous and another in the middle. On the ends, they are supported by the foundation walls. Therefore, I would say the maximum span of these trusses is 25 ft.

-k
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Unread 10-24-2006, 05:25 PM   #9
John Bridge
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I'll give you a bump. See if Bob picks up.
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