Countersinking Screws into CBU

[Verdeman]

I'm using CBU-rated screws, but they are difficult to countersink. I used a 3/8" masonry bit to "dimple" the cement board. Is there a better way?

TIA

[jadnashua]

Not all cbu screws are created equal. There's one brand that has little nibs under the screwhead that actually ream out the cbu sort of like a built-in countersink drill bit. If you are using HardiBacker, it is good to search out these. Another thing that helps are the McFearson head ones (square drive)...it is much easier to get enough torque to set these without stripping either the bit or the screw.

[Verdeman]

Thanks, JAD.

These screws have a conical screwhead, but don't seem to want to countersink at all.

Maybe you can help identify the cement board I'm using. I found the perfect sized piece in his tool shed, but it has no writing to identify it. The backside looks like a typical hardibacker, but the front has a nice orange peel texture to it. Any ideas?

[jadnashua]

If the bottom side of the screwhead is smooth, and doesn't have any nibs or raised ridges, they won't work well on HardiBacker, which sounds like what you have. A fiberglass and cement board will crush enough for the screw to set, but HardiBacker is denser and needs the nibs to ream out a recess for the screw, or do what you are doing, countersink the hole. The dust from that if you use a saw is carcinogenic...do it outside and use a dust mask. You can score and snap it.

[MudMaker]

Eric,
Just a thought but, what are you screwing into?
Second thought, how long are the screws and what relationship is there to the ply?
i.e. Is the screw long enough to pull the screw flush with the CBU?
If there is enough ply, this should not be too hard to screw flush with the CBU..

[jerseyreef]

also remember the screw shouldn't go all the way through underlayment and into the subfloor, it should just pierce it. So with a proper underlayment of 3/4" and a 1/4" Hardibacker, plus thinset, the 1 1/4" Hardiscrews would be the proper screw, the 1 5/8" would be too long.

[Verdeman]

I did buy the 1 5/8" CBU screws. I thought I needed the extra length because I'm also anchoring a toilet flange through a fairly thick floor tile, then through the Hardibacker, and into the underlayment.

3 more questions:

1. Why is it wrong to go into the subfloor more than 1/4"?

2. Is it important to always set the CBU in thinset before doing any type of tiling?

3. If you have a solid 3/4"-1" subfloor, can you simply install the 1/4" CBU over that and thus avoid having your bathroom or kitchen floor being an inch higher than the surrounding floor?

[jerseyreef]

Eric - To have a successful tile installation, you'll need two layers of flooring. One is the sub-floor and the second is the underlayment. The Hardibacker does not suffice as an underlayment.

Depending upon loading and type of material to be installed, the most recommended thickness of the sub-floor is going to be 3/4" -1", some might recommend 5/8". Me personally, I'd go with a sub-floor rated material at 3/4" and CC rated or better (if plywood).

For the underlayment, you'll get a mix of 5/8" being the minimum and 3/4" being preferred. Again at least CC grade or better (if plywood). The thicker the underlayment, the stiffer the floor will be, of course if you have a proper joist/support system.

The Hardiback has zero structural strength and shouldn't never go directly on top of the sub-floor. The Hardibacker only purpose is to provide a uniform (or monolithic) surface for adhere the tile and the hardibacker must be applied with a thinset material.

If you read the thread in the Liberry regarding how to install a sub-floor and underlayment and the associated documents, you'll pick up several pieces of advice

1. The underlayment should overlap the existing sub-floor in a different pattern, I believe the minimum overlap is stated to 4" inches
2. The underlayment is to be secured using screws no longer than 1/4" past the sub-floor and is not to be secured to the joists
3. The Hardibacker is secured the same way, ensuring to overlap the underlayment and not secure the screws past the underlayment. This I believe decouples the hardibacker from the sub-floor and limits any movement deflection if it should occur
4. The Thinset should be un-modified between the Hardi and the underlayment, although the manufacturer will state either can be used. Some of the pros recommend going with an un-modified thinset to further increase the decoupling and allows easier removal of the Hardibacker if required in the future.

I'm no pro and I'm just passing along what I've picked up from this board. I'm sure some of the other pros will come along if stating something incorrectly or pass other information along.

[Mike2]

Hi Mike - Jersey Reef.

I'm not aware of any such recommendation, from a reputable source, that backerboard fasteners should only penetrate the underlayment layer and not the sub-floor layer below. Where are you getting this?

[mdnky]

Surprised this hasn't been asked -- what are you using to drive the screws?

[ckl111]

Another thing that helps are the McFearson head ones (square drive)...it is much easier to get enough torque to set these without stripping either the bit or the screw.

Square head screws are Robertson screws. Sorry to be technical but I had to give credit to a fellow Canuck.


Are you using a drill to set the screws? Perhaps you are just not getting enough torque to countersink the screws. I never have any problems but I'm using an impact driver. They have the torque to pull the screw right through the stud. You may just need a slightly stronger drill.

Regardless, it shouldn't take that much force to countersink the screws. Can you do it with a screwdriver? If the screws are too long, most of the threaded part of the screw will pass through the sub floor leaving you with insufficient thread in the wood to pull the screw down into the CBU.

The alternative is to use galvanized roofing nails to set the CBU in thin set. Countersinking that many holes would drive me crazy.

You should ALWAYS set the CBU/hardiboard in thin set to eliminate any gaps between the CBU and the sub floor. Any gaps, no matter how small, will cause movement. Here a link to a demonstration of not doing this: Improper CBU Installation