My husband and I are deep into installing new flooring in our home. We decided on a combination of laminate flooring and ceramic tile. The tile work is much harder than I’d thought. We’re installing the large tile directly on top of the concrete slab. I’m using plastic spacers to keep the grout lines straight, but I’m having great difficulty. I’ve never grouted before and am worried about messing that up. Help! I don’t want to ruin this project.

— Marilyn H., New Bern, N.C.

I learned long ago that the plastic spacers that are used to keep grout lines straight don’t always work well. It’s not because the spacers are defective; it’s because the ceramic tiles might be slightly off. A variance of just 1 / 32-inch in a tile can create a 1 / 16-inch difference if two such tiles are installed side by side. Pretty soon, the grout lines will look more like a sinuous stream channel if you use the plastic spacers.

I prefer to create a grid on the floor using a chalk line. Accounting for the width of the tile and the grout line spacing I want, I calculate exact distances for either a 3-foot-by-3-foot tile grid or possibly a 4-foot-by-4-foot grid. I snap the lines, making sure they’re accurate, and lay the tile to the edges of the grid lines. The center tiles are installed by sight, making sure the grout lines are consistent and straight.

Before the grout is applied, the spaces between the tiles might appear somewhat less than perfect. Grout has a magical ability to mask those very slight imperfections. However, twisted or crooked tiles will still look twisted or crooked after grouting.

When it’s time to grout, don’t spoil all the hard work you’ve done up to that point. Rookie tile setters often make any number of mistakes when mixing, applying or finishing the grout. Buy some inexpensive floor tiles and install them on a 3-foot-by-5-foot piece of cement backer board; you might have one lying around in your garage. Practice grouting on that test area until you perfect your skills.

Mix the grout to the consistency of a bricklayer’s mortar. If it flows from the bucket like pancake batter, it’s too wet. Using too much water when mixing the grout or finishing the joints weakens the grout, which might cause it to crack or crumble in the months ahead.

Apply the grout with a hard-rubber float at a 30 degree or 45 degree angle to the grout lines. That diagonal stroke will ensure the grout is flush with the top of the tile. Don’t rub the grout with the sponge until it firms up enough so that you don’t remove grout from the joints.

Low humidity and high temperatures will cause floor grout to set up rapidly, making it hard to finish. You need to discover on your own how much grout you can install and finish at one time. Mix only enough grout that you can install and finish in one hour. You can always mix more.

Also, don’t add water to stiff grout in the bucket. Re-tempering the grout with water will weaken it.

Tim Carter is a columnist for Tribune Media Services. He can be contacted through his Web site at www.askthebuilder.com.