DYI Hearth pad - In order: 5/8" plywood, 1/2" Micore 300, 1/2" Durock, and ceramic tiles

[Gmarcil]

I am planning to build a large 115" x 64" hearth pad for a Pionner Princess cookstove (needs a R-valued of 0.6) in our kitchen. The pad is extended for esthetics to accommodate a wood box and word stove accessories.

Does anyone have an installation guide or the proper procedures to build the pad on top of an existing vinyl floor?

Questions:
1- Should the vinyl flooring be removed?
2- Should the 1/4" plywood be removed to be left only the existing 5/8" plywood floor?

Then I plan to install the 1/2" Micore 300, 1/2" Durock, and ceramic tiles.

3- Can the Micore 300 be simply layed down snug or do they need to be glued, screwed or cemented to the plywood floor?

4- When installing the Durock boards on top of the Micore 300, should they be glued, screwed or cemented?

Thanks for any tips and professional advices

 

[begreen]

It can sit on top of the vinyl floor as long as it meets or exceeds the required insulation value. The cement board requires being screwed every 8". Micore is too porous for cement to adhere well to it. It's ok to use longer screws to make a sandwich of the cement board on top of the micore with no cement, screwing every 8" with 1.75" screws. If you can't find cement board screws long enough, you may need to use drywall screws for this purpose.

 

[Gmarcil]

Thanks Begreen.

The R-value required for this stove is R-value 0.6 and the total R-value of the material here is:
1.03 - 1/2" micore 300
0.25 - 1/2" Durock cement board
0.02 - Ceramic tile

Total R-value - 1.3 which exceeds the thermal requirements of 0.6

I certainly intend to use cement board screws as I have been told that if one uses drywall screws with Durock boards, they will commonly:

• Break when driven
• Corrode over time
• Lose holding power

A concern I have is the thickness of this hearth pad floor which amounts to almost 1.5" thick
(1/2" micore + 1/2 durock board + thinset and porcelain tiles)

A concern This thick hearth sub-floor might cause accidents if people trip on the edge of this hearth while accessing to the cookstove for cooking!

Do you think it would be acceptable to use 1/4" Durock boards instead of 1/2" to reduce the tickness of the hearth floor?
It would still meet the thermal protection requirements. However, I wonder if the hearth would be strong or rigid enough to support the weight of the cookstove ( 630 pounds)? I am concerned about tiles potentially cracking!

How do you folks think?

 

[EatenByLimestone]

What will you border the hearth pad with? Your height might be hard to find trim for depending on your plans.

 

[Gmarcil]

Not sure yet. I will probably need a custom made oak trim cut with at no more than 45 degrees.

 

[begreen]

I would not use 1/4" cement board. Find a piece of micore and push your thumb into it. The product is soft. It's full of air.

Let me know if you find 1.75 or 2" cement board screws. They are not available at the big box stores or our local lumber yard. Once captive and the mortar has dried, I am not too worried about drywall screws rusting out. But one could use stainless screws.

 

[Gmarcil]

Thank you again Begreen.

Thanks for confirming the durock boards thickness!

There are 1 5/8" cement screws for cement panels available at home depot:
#9 x 1-5/8 in. Star Drive Serrated Head Cement Board Screws (575-Pack) or
#9 x 1-5/8 in. Star Drive Serrated Head Cement Board Screws (140-Pack)

Have a great day!

 

[begreen]

Thank you again Begreen.

Thanks for confirming the durock boards thickness!

There are 1 5/8" cement screws for cement panels available at home depot:
#9 x 1-5/8 in. Star Drive Serrated Head Cement Board Screws (575-Pack) or
#9 x 1-5/8 in. Star Drive Serrated Head Cement Board Screws (140-Pack)

Have a great day!

Better, but I'm not sure if they will be long long enough, especially if on sheet vinyl flooring.

 

[Gmarcil]

Oups, you got me thinking about the exiting floor.... Yes 1 5/8" screws would be too short indeed because I also have 1/4" plywood under the vinyl.

So a better choice would be: #9 x 2 in. Star Drive Torx Bugle Head R4 Multi-Purpose Wood Screw (110-Pack)

Highlights​

• For use in wood, particle-board, cement fiberboard, plastic, sheet metal
• Star drive eliminates stripping for improved productivity
• Zip-tip point eliminates the need to predrilled and provides easy starts into wood
• CEE thread enlarges hole to reduce splitting, thus eliminating rework
• W-cut thread design acts as a saw blade providing clean cuts through all wood and fibrous materials
• Zip-tip allows for no-pre-drilling and 1-step application
• Both zip-tip and w-cut thread design help eliminate splitting of material
• Cutting teeth cuts wood fiber and provides flush seating
• CEE thread reduces torque and friction during screw-in action
• Common project applications: remodeling, fencing, storage, cabinets, framing
• ESR-3201 approved for structural applications
• Cutting pockets underneath head allow screw head to countersink flush with material
• Ground contact lumber approved for worry free use in pressure treated lumber
• Climate coating resists corrosion

Cheers!

 

[begreen]

Yes, that's more like it.

 

[jalmondale]

In case it's a useful data point, I have a very similar 3x5 hearth pad: 2 layers of micore (staggered seams), 1/2" cement board, and 3/8" porcelain tile (large format, 1' x 2'). I've had it for 5 years and haven't run into any issues with shifting, tripping, or toe stubbing (and as an exceedingly clumsy person, that's saying something). I trimmed the whole thing out with some door casing stained to match the floor.

Some notes:

- Technically, it's floating, as I didn't attach it to the underlying wood floor. With a 500 lb stove on top, though, I wasn't worried about it going anywhere
- I don't have any screws going through more than one layer (they're all 3/4" - 1"), probably out of an abundance of caution about thermal transfer
- IIRC I used cement board screws
- I don't have any screws directly under the stove (ditto from the above)

I've found the clearly raised aspect of the hearth pad is helpful for avoiding tripping - I think something under 1" would actually end up being easier to stumble over, while mine is high enough that it registers as a "step" rather than an "uneven floor".

 

[Gmarcil]

In case it's a useful data point, I have a very similar 3x5 hearth pad: 2 layers of micore (staggered seams), 1/2" cement board, and 3/8" porcelain tile (large format, 1' x 2'). I've had it for 5 years and haven't run into any issues with shifting, tripping, or toe stubbing (and as an exceedingly clumsy person, that's saying something). I trimmed the whole thing out with some door casing stained to match the floor.

Some notes:

- Technically, it's floating, as I didn't attach it to the underlying wood floor. With a 500 lb stove on top, though, I wasn't worried about it going anywhere
- I don't have any screws going through more than one layer (they're all 3/4" - 1"), probably out of an abundance of caution about thermal transfer
- IIRC I used cement board screws
- I don't have any screws directly under the stove (ditto from the above)

I've found the clearly raised aspect of the hearth pad is helpful for avoiding tripping - I think something under 1" would actually end up being easier to stumble over, while mine is high enough that it registers as a "step" rather than an "uneven floor".

Hi Jalmondale
This is great information. Thank you very much for sharing this precious information!

Your the floating pad design truly appeals to me. I guess that it is not a requirement to have it secured to the floor to meet the WETT inspection?
As you say the weight of the stove plus the flooring material is preventing the pad from moving.

A couple of questions if you don’t mind me asking…

A) Did you put in your 3/4 screws every 8” square as suggested?

B) Nice to hear that you never tripped on your raised pad. What kind of bevel/slant did you use? Less than 45 degrees?

C) I am curious as to why you use 2 x 1/2” layers of micore? Was this required to get the required R-value or for other reasons? This increases the hearth thickness! So your hearth pad thickness would be almost 2” thick?

Cheers!

 

[Murray01]

I guess that it is not a requirement to have it secured to the floor to meet the WETT inspection?

No mention of any structural details for the hearth pad. You might want to consider leaving an edge exposed so the inspector can see the radiant heat protection.

 

[Gmarcil]

Thank you Murray 01 for sharing the hearth pad requirements and confirming that is no need to secure it down to the floor.

However given the rather large pad width (115”), I will probably screw down the outer edges only to prevent pad shifting , especially where my wood box will roll up/down the pad.

 

[jalmondale]

My hearth pad is waaay over engineered (actual story: I read the manual, decided there was no way it only needed R 0.6 and therefore I would YOLO something that would definitely be fine, then I got the stove with the heat shield underneath, ran it and saw that the tiles under there barely got warm, and realized that 0.6 would have been plenty *facepalm*. At least the extra insulation doesn't hurt anything). Not sure what you mean about the beveling - here's a photo of the edge detail.

I took a bunch of construction photos in case an inspector ever wanted to verify what was going on there, but no one ever checked anything about it except the dimensions. ETA: Now that I say this, I went back and found those photos. Apparently I only put 4 screws in the top layer in total, although IIRC I put a few more in the micore since it wasn't one solid piece. Photos are of the two layers of micore (before screws), layer of backer board (with very few screws), and test fitting some of the tile. As you can see from the actual finished photo, I didn't end up using the bullnose tile trim and went with wood instead (I think the bullnose was ~1/16" too tall, but I ended up liking the wood better anyway).

 

[Gmarcil]

The wood trim looks great.
Thanks for the details and the pictures.

I had a wett installer/inspector come in today for a consultation and he was not familiar with the micore 300 product. However he said that I would need to secure the board to the floor to prevent wrapping or bending of the hearth pad. So I’m not sure what to think about that recommendation but if I want a pass of his inspection I will need to secure the micore boards to the vinyl (or plywood if I remove the vinyl flooring) using some kind of adhesive or screws.

 

[EatenByLimestone]

I’ve never seen Micore warp. I googled it to see if warping is a thing that I just hadn’t heard about and apparently it’s not.

Feel out your inspector. Maybe search the micore literature for something specific about warping. It’s laying on a flat surface and has lots of weight holding it flat. He may just be unfamiliar with the product and wants to err on the side of caution for your safety. Worst case, sandwich it between a piece of ply on bottom and Hardie or durock on top.

In the end, you’ll have to go with what he wants to see, but presenting the right info might get you somewhere.

 

[begreen]

Usually, one is making a sandwich of the tiled cement board, micore, and substrate which is typically 1/2" plywood. The cement board needs to be screwed every 8" in each direction. Without the substrate to anchor the screws, the cement board needs to be anchored every 8" to the subfloor under the vinyl. Anchoring to just the corners isn't going to work without the substrate.

 

[jalmondale]

> Anchoring to just the corners isn't going to work without the substrate.

I'm curious about this, since it seems to run pretty counter to what I've experienced with mine (0 shifting in 5 years). It doesn't seem like the hearth pad should experience much if anything in the way of lateral loads (just people/pets walking on it), all the materials involved (backer board, micore, tile) are resistant to warping, and there's a significant amount of weight involved from the tile and the stove that would provide stability.

Obviously, if an inspector requires something, then there's not a good way around that, but is a shifting hearth pad something that has come up for other folks with an insufficiently anchored cement board?

 

[EatenByLimestone]

I haven’t seen it, but every situation is different I suppose. My dog gets excited on the couch and can send it sliding across the floor. That’s not really associated with lateral load either. I wonder if a rubber mat glued to the bottom would satisfy the inspector. It all really comes down to them.

 

[begreen]

> Anchoring to just the corners isn't going to work without the substrate.

I'm curious about this, since it seems to run pretty counter to what I've experienced with mine (0 shifting in 5 years). It doesn't seem like the hearth pad should experience much if anything in the way of lateral loads (just people/pets walking on it), all the materials involved (backer board, micore, tile) are resistant to warping, and there's a significant amount of weight involved from the tile and the stove that would provide stability.

Obviously, if an inspector requires something, then there's not a good way around that, but is a shifting hearth pad something that has come up for other folks with an insufficiently anchored cement board?

This is not about the hearth pad moving. The screwing to the substrate every 8" is a requirement of the cement board. This is what creates a level and stable tiling surface. It would be necessary even if there was nothing on top of the tile and it's even more important with a spongy interlayer like micore.

https://www.usg.com/content/dam/USG_Marketing_Communications/united_states/product_promotional_materials/finished_assets/usg-durock-cement-board-with-edgeguard-installation-guide-en-CB321650.pdf