Correct Tape for Joining Concrete Backerboard in Hearth

[BillGriggs]

I need to join two panels of durock for my hearth extension pad.

After looking at joint tape, I selected this one because it is listed as fire resistant. However, after doing some more research, it sounds like this kind (specifically made for cement backerboard) is the only kind to be used in that application due to it being alkali resistant. (Adfors (broken link removed to http://www.adfors.com/us/building-products/wall-finishing-tapes/fibatape-cement-board-tape) it's "only tape that should be used for cement applications with dry or thin-set mortar.")

I didn't pick the cement backerboard tape because it was not listed on the HD website as being fire resistant, and I've seen this repeated elsewhere.

So, what tape should I use? Does it even matter? Do I even need tape?

 

[jetsam]

I don't know what the correct answer is, but a whole roll of that tape contains a few ounces of fiberglass. Fiberglass melts at 600° rather than burning, and it'll be buried in thinset with a big heat sink on top and behind. If you manage to melt it, you probably don't have a house left for it to be a fire hazard to.

So I wouldn't worry about it, but I'm sure someone who does this stuff for a living will be along shortly with a better opinion.

 

[BillGriggs]

I don't know what the correct answer is, but a whole roll of that tape contains a few ounces of fiberglass. Fiberglass melts at 600° rather than burning, and it'll be buried in thinset with a big heat sink on top and behind. If you manage to melt it, you probably don't have a house left for it to be a fire hazard to.

So I wouldn't worry about it, but I'm sure someone who does this stuff for a living will be along shortly with a better opinion.

Hmm great point. I figured it would probably be safe from burning because it's fiberglass and sandwiched in all sorts of thinset, but I did worry that the heat would weaken the bond at least and then I wondered if it's even worth having in the first place.

With how many combustibles conversations have happened around here, I'm surprised this hasn't come up! (Or maybe it has and I'm just bad at searching)

 

[begreen]

Fiberglass holds its strength well above 600º. http://metricfelt.com/high_temp_fiberglass.html
Regardless, If the hearth gets anywhere near 600º you have much greater problems than grout integrity. The hearth will probably never see over 130-160º.

 

[BillGriggs]

Fiberglass holds its strength well above 600º. http://metricfelt.com/high_temp_fiberglass.html
Regardless, If the hearth gets anywhere near 600º you have much greater problems than grout integrity. The hearth will probably never see over 130-160º.

Yes I hope that fiberglass tape never sees anything near that. That all being said do you think the best bet is the alkali-resistant cement backer tape? If there's one thing these forums have taught me it's to take all this stuff seriously and not cut corners!

 

[begreen]

That all being said do you think the best bet is the alkali-resistant cement backer tape?

Alkali-resistant glass fiber tape is what Durock recommends.