Countersinking Backer-On screws in Hardiebacker?

Discussion in 'Shower & Bathtub Forum & Blog' started by ironspider, Jan 26, 2009.

  1. ironspider

    ironspider Member

    Greetings all,

    Finalyl got hte tub/shower alcove framed in and now we are moving on to putting up our hardiebacker 500 (1/2inch) to the alcove.

    I am using backer-On screws (square drive) to attach the hardibacker 500 (the 1/2" stuff) to the studs.

    My question though is that when I drive the screw as it begins to come up flush the area around the screw's head pushes up creating a little "bump" that consists of the pushed up hardibacker and the screw head. Thinking about basic physics this must be normal right? because as the screw head goes into the board the head presses down, displacing the board that was there--forcing it to rise up around the head. But it just seems that this makes the surface not flush anymore. It also seems like the screws don't countersink that well--if at all.

    So I guess my question is whether or not the screws should be countersunk, or more flush, or more "just above" the board.

    In any situation, should I be sanding down the raised board around the screw head? Or does the thinset even it all out?

    I've also read that maybe I should back out the screws now, clear out that raised and broken up backer board from the screw hole path, and re-drive--does anyone do that method?

    Thanks in advance!

  2. GabeS

    GabeS Remodel Contractor

    Brooklyn NY
    I drive the screws flush with the board. Make sure you drill is set on the drilling option so it doesn't limit the power (don't have it set on a number).

    I wouldn't countersink, but if you did you should be able to just scrape off the excess that came out. Shouldn't need to unscrew first.

    You did put the tub in first right? If not, you might have a problem with the tile because the tub has a flange.

    This detailing is important. Two ways. Both ways have tub in first.

    1) Hardibacker up top of tub flange. Silicone space in tub flange. Tile.

    2) Make frame 1/2" small. Notch studs to get tub in. Hardibacker goes over tub flange keeping it a quarter inch off of tub with shims to prevent wicking. (This is my preferred method, but may be too late for you if already framed).

    You can always build the studs out with quarter inch strips of material.
  3. ironspider

    ironspider Member

    Thanks for the quick reply Gabe,

    Yes we furred the studs and then put the backer over the flange sitting it up off the tub deck about 1/4".

    Good to know about the scraping though! I don't think I had the clutch set (I think I was in drill mode) but maybe I didn't put enough pressure on--and maybe I could drop the drill speed to amp up the torque a bit.
  4. hj

    hj Moderator & Master Plumber Staff Member

    Cave Creek, Arizona

    Any builder who made the tub recess 1/2" undersized would be getting a bill for the extra time installing the tub, and it would not be a cheap one.
  5. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    New England
    Some of the cbu screws have nibs on the underside of the screwhead that act like a countersink. Don't remember if those do. You do want the head flush with the plane of the's a pain spreading the thinset evenly if they stick up. You don't want to sand it, there's a lot of silica in the stuff, and the dust is not good for you. Make sure you don't have any tenting (the screw pushing the board away from the studs). Also, make sure you use the alkaline resistant mesh tape on the seams. you can do this while you tile unless you are going to use something like Redgard, then you need to do it first, let it cure, then apply the waterproofing.

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