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Twister 07-05-2009 02:03 AM

Adhesive for Marble Threshold
Hello all -

I have a cement subfloor in a front door entryway. I just glued down some Bruce cherry wood engineered flooring (BEAUTIFUL, BTW) up to the middle of the front door jam/casing. The Bruce engineered floor is 3/8" thick. Notice I said it was glued up to the "middle" of the door jam, ie, where the door sits. I bought a new metal threshold to install in the doorway. Before I installed the wood flooring I realized that if I just screwed down the metal threshold right into the cement subfloor the metal threshold would not sit down flush on the cement subfloor. Remember?, the wood flooring is 3/8" thick and it only goes halfway through the door jamb. What to do? A local flooring company then gave me a 3/8" thick piece of marble (2" x 36") to install below the metal threshold. This way the 3/8" thick marble sits below the outer portion of the metal threshold facing outdoors (the 2" marble will extend beyond the metal threshold by 1") and the 3/8" thick wood flooring sits below the metal threshold portion that faces indoors making the metal threshold now level. There will be a small gap between the marble piece and the Bruce wood floor so I can screw the threshold screws right into the cement subfloor.

(sorry for the lengthy explanation but hopefully that will bring an accurate answer from you guys).

OK, here's my question: what can I use to adhere the 2" x 36" marble strip to the cement subfloor?

Thinset? If so do I mix it w/ anything else? Somebody recommended gluing it down with plain silicone but I'm not sure the silicone won't give/flex which would break the marble the first time it's stepped on. Liquid nails? I'd envisioned some type of cement-to-stone glue but now that I need to do it I have no clue what that would be.

Surely this is not the first time someone has had this question since engineered floors are not always run all the way through an outer door doorjamb.

Any knowledgeable solution would be greatly appreciated. Thx!

cabinetman 07-05-2009 04:10 AM


OK, here's my question: what can I use to adhere the 2" x 36" marble strip to the cement subfloor?
I've used thinset with no problems. Embed evenly.

Twister 07-05-2009 11:19 AM

cabinetman -

How thick should the layer of thinset be?

Also, is there any particular brand you recommend?


Bud Cline 07-05-2009 03:16 PM

Silicone is the worst idea for the reasons you mentioned.

Liquid Nails would work if you spread 100% of the marble and 100% of the concrete floor area the marble will consume. Then, stay off of that marble for about two weeks. The objective is to have no voids anywhere under the marble.

Thiset (modified) would work the best but mostly comes in 50# bags. What would you do with the remaining 48 pounds?:)

Thinset is available in smaller boxes if you can find it.:) If you do use thinset also use a small "v"-notched plastic trowel and coat both surfaces thoroughly. That will determine your thickness.

Twister 07-05-2009 08:18 PM

Bud -

What do you mean when you say "modified" thinset?

Is there any particular brand you recommend? The only big box store here in town sells a product called "Henry MarbleFlex 316". Ever used it?

Bud Cline 07-05-2009 09:06 PM


What do you mean when you say "modified" thinset?

Is there any particular brand you recommend? The only big box store here in town sells a product called "Henry MarbleFlex 316". Ever used it?
Henry's is one of the kings of quality adhesives - use that.

"Modified" simply means that the powder in the bag already contains all the additives you could ever need to create a high strength mortar adhesive.

rusty baker 07-05-2009 10:38 PM

Wonder if you could glue that down with Kool Glide glue sticks? I have glued tack strip to concrete with them and powerstretched off it.

Bud Cline 07-05-2009 11:15 PM

You need to have the bonding-area covering 100% of the marble surface. Any voids anywhere could result in cracks later. Holding power of the adhesive isn't as important as covering the entire surface with adhesive, not only for bonding but also for support. Any viscous adhesive would hold the marble in place even with only a few dots of adhesive but that isn't the point. Bonding strength is second to support in this case.:)

rusty baker 07-06-2009 09:03 AM

You are the tile expert. I was just kinda throwing that out there.

Twister 07-06-2009 10:37 AM

Guys -

OK, thx for your recommendations. I'll use the Henry MarbleFlex thinset today and will let you know how it turns out.


Twister 05-14-2011 11:16 AM

The Finished Product....nice, eh?
4 Attachment(s)
All -

Have been meaning to post these pix for almost 2 years. Well here they are. It turned out SUPER and so far there has been absolutely ZERO cracking of the underlying marble strip. I remember when I started asking around about how do floor guys install a threshold on top of a wood floor in such a way that the ends of the wood floor would not be exposed to the weather on the outside of the exterior door that I got a lot of blank stares at first. Well, if you have the same issue, here's one way to do it. Thanks so much to cabinetman and Bud Cline who gave me the good advice about thinset type mortar. Make sure you have ZERO voids/open pockets under the marble strip and your marble strip will be as solid as a rock (no pun intended!). :wink:

Bud Cline 05-14-2011 01:20 PM

Wow! That was quick!:laughing:
Glad it worked out for you.:)

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