To sister or not to sister
 

I'm new to this forum and I'm a DIYer. That last bit got me forever banned on ContractorTalk. :wink:

But it led me here and I'm desperate for some guidance that I don't seem to be able to get on the 'other' forum.

I've been working on a remodel of our home's powder room and foyer. This is mostly tile work but has included plumbing, AC, and other finishing tasks to make it complete.

In any case, the powder room is just about done. Tile is down, sheet rock is ready for paint, and baseboards are cut and coped. Just waiting for arrival of urethane grout that needed to be ordered. If interested, you can read more about it here -
http://www.johnbridge.com/vbulletin/...d.php?t=104690

While waiting I've gotten started on the foyer. Removing this slate is so much fun. The slate is bedded in with 1/2" mud and wire mesh.

https://dl.dropbox.com/u/84516293/20...2006.37.58.jpg

I will be setting 12" x 12" porcelain tile. The current subfloor is 1/2". I intend to replace it with 3/4" OSB with a yet to be determined underlayment - either Ditra or NobleSeal.

However, I know I have some structural issues. As the following floor plan will point out:

https://dl.dropbox.com/u/84516293/Foyer.jpg

For most of the foyer, I'm good with a joist span under 10.5' but at the far right I have two floor joists that extend 14', which is too long according to Deflecto on the John Bridge forum.

My question is, do I need to sister these two joists or will this be okay as is, because of the support provided by the adjoining joists? I imagine this section is stiffer than it would be if the full foyer had a 14" span but I'm no structural engineer. I'm just guessing. Additionally, the existing slate has been in place for over 40 years. Yes, there are a few loose tiles but this section of the flooring seems to have held up very well given the substandard construction.

I'll sister these two joists if that is the recommendation but I'd rather save the effort if not needed.

Thanks!

Louis

For regular flooring materials?
Or to provide the rock solid no deflection substrate that tile requires?

This is for tile. I'm laying 12 x 12 porcelain over 3/4" subfloor with Ditra or NobleSeal as the underlayment.

Louis

Louis,

Yes you should sister the joists or build a supporting wall under it, like a closet

You can use smaller joists or even 3/4" ply glued and screwed.

Jaz

Jaz,

Thanks for your reply! I hadn't considered the ply. That could work too. In any event, I'll sister the joist. A supporting wall would interfere with the basement living space.

Louis

Jaz,

I've been doing some more Internet research and starting to question my choice of OSB for the subfloor.

I used OSB in the powder room but I also followed that with 1/2" ply and NobleSeal. I'm trying to keep the height down as the adjoining rooms are oak flooring and I'd like the height difference to be as small as possible.

My original though was to lay down 3/4" Advantec OSB topped with NobleSeal or Ditra. I prefer Ditra but the NobleSeal will keep the height down. In either case, I won't have the second layer of ply causing me to question if I want the OSB between the membrane and the tile?

Would 3/4" ply be a better choice in this situation?

Thanks again.

Louis

I don't have a problem with OSB as long as it's a high quality product like Advantec and it's t&g. I don't like the cheapo OSB you find at the big box stores. Most of them are just too porous for me.

Matter of fact, high quality OSB are actually stiffer than plywood subfloors as far as I know.

Jaz

Jaz,

Given these two options, which would you choose:

1) 19/32" ADVANTECH + Ditra

2) 23/32" ADVANTECH + NobleSeal CIS

I know neither of these are ideal but I'm trying to keep the floor height inline with the adjoining rooms. I'm inclined to go with option 2 although I could still choose to combine the 23/32" ADVANTECH with Ditra but that would put the tile above the hardwood floors. I know I shouldn't worry about this so much but I have to consider the WAF.

Thoughts?

Louis

what is the size of your floor joists and their spacing?

I'd use the 23/32" if possible

Joists are 2 by 8s, 16" OC.

not knowing your species of wood I calculated based upon spruce-pine-fir and by sistering (2)2x8 together you'll be right at the minimum deflection limit (about L/358) for total load. depending on your perception it may be a little bouncy.

The thing is, it doesn't feel bouncy at all with the current flooring. Quite solid and no cracks.

Louis

then you'll be better than you are currently

Wow, how close do you want it? If you want all the floors to be the same height. install the same floor in all the rooms like they do in most other places on earth.

When ceramic tiles were installed using the good old fashioned mud method, the bathroom threshold was about 1.5" higher than the hallway subfloor. Today an 1/8" matters? This is residential, not commercial where some people are looking for a way to trip.

Jaz

What can I say, I'd like it to be at the same height. :(

But I recognize I have to compromise so I'm trying to understand the trade-offs I'm making.

I've ordered the 23/32" ADVANTECH. Wish it would be delivered tomorrow but there is this little storm bearing down on us that will probably interfere with the delivery. So no work on the subfloor this weekend.

I still need to decide on NobleSeal or Ditra for the underlayment.

Louis