Fireplace Renovation: Part 3 – Installing Backerboard

Just when we think we’ve got it all figured out, we realized that we haven’t.

Our grand plan was to patch the fireplace and pour the concrete (for our hearth) directly on top. Apparently this wouldn’t work because we’d be pouring the concrete over existing cement and part of the wood subfloor. Since concrete doesn’t enjoy changes in materials – particularly from a super hard surface like cement to soft, flexible surface like wood – we had to install some backerboard.

Backerboard is basically a sheet of cement that you put down with construction adhesive and/or screws. Someone recommended that we use cement grout because it would give us the “squishiness” that we needed to even out the floor underneath.

So, back to Home Depot with the cargo van we went!

We don’t live in a square house. The width varies about an inch or inch and a half from one end to the next. This non-squareness means that we have to be very careful about how we line up our flooring. Using the wall as a guide isn’t an option because it simply won’t look straight. Since we’re going to use our hearth as a guide for the flooring, it meant that we had to figure out where our “straight” line was tonight.

We decided to use the existing subfloor boards as our guide simply because they look pretty damn straight in the room and that’s all that matters.

We measured the width of the fireplace and squared it up with the floorboard to figure out how to cut the backerboard. After some careful measuring (a few times), we made a line of where we needed to score the backerboard.

We bought a carbide scoring knife to score the backerboard. I think we could have gotten away with a utility knife, but we weren’t chancing it.  

We were very careful not to move the level (which I was using as a ruler), and successfully scored a straight line!

We placed it down in front of the fireplace and it was a perfect fit! Yay!

The next step was to figure out the next cut. This one was a bit trickier because the “straight” reference line was not parallel to the fireplace. 

We simply put the board down and marked the edge to match the floorboard underneath. We then drew a straight line between the two and our new edge was created!

When we placed it down, it was aligned perfectly with our “straight” floorboards, but it looked weird. I could really tell that the right side of the board was wider than the left. Maybe I’m crazy, but Alex saw it too.

We spent a lot of time walking around the room, trying to gauge whether it was off or not. Then we decided to cut a second piece that was a perfect rectangle to compare.

When we put down the perfect rectangle, the hearth area looked much better. But when we put our plywood subfloor against it to see the new “straight line” it created, it was really crooked and it looked terrible.

We made a call that it was much better to have a straight-looking floor than a straight hearth. Here is a comparison. The left picture is the perfect rectangle, the right picture isn’t.

Now that we were convinced we were making the right decision – and our first real decision, by the way – we had to install the board.

Mixing the cement grout was tough. And I’m not strong.

It took a bit to figure out what consistency I needed, and I was slightly panicking because I could see that it was starting to dry. It wasn’t pretty and I’m not sure I did a great job…

I’m working very hard here, trying to get it to reach the edges. I really should have added more water.

Once I more-or-less covered the area underneath, we put the board in place. Then Alex screwed it down using tapcon screws (designed for concrete).

Here’s the finished product! Hopefully it sets and doesn’t move!