Q: I’m retrofitting a curbless shower with a linear drain in an existing home with an I-joist floor. What’s the best way to drop the floor for the mortar bed?

A: Michael Byrne, veteran tile installer and consultant, and moderator of JLC’s ceramic tile online forum, responds: Retrofitting the slope for a tile shower is a common problem for builders. Curbless showers, which have been growing in popularity, present an even greater challenge because of space limitations. A curb allows the slope to be added directly over existing subflooring. But while many people don’t mind stepping over a curb to enter the shower, there seem to be just as many who object to stepping up and onto the floor of the shower.

Click to enlarge

Drain at Shower Entrance

The first question to ask is where in the shower the linear drain will be placed. If it’s located at the entrance to the shower, then no supplemental or retrofit construction is needed to provide slope for a curbless shower floor. In this configuration, readily available manufactured sloping panels can be literally dropped into place, or the slope can be added with a single-plane mortar bed.

Either way, sheet-membrane or liquid-applied water­proofing is required only on the surface. As I see it, the only two concerns for this installation strategy are the potential for waste water to run past the drain and out onto the bathroom floor (which should also be waterproofed regardless of the type of curb or drain) and the need to provide stability for a wheelchair that naturally wants to follow the slope and roll out of the shower.

Drain at Back of Shower

Sloping the shower floor away from the entrance—from front to back—allows gravity to stabilize a wheelchair against the back wall, but it brings us back to structural issues with wood-framed floors. In these cases I recommend dropping the entire shower area and then using manufactured panels or a mortar bed for the slope.

Creating a dropped area in an I-joist floor means removing sections of I-joists and usually framing the dropped area with dimensional lumber or an LVL type of engineered lumber. The trick is to ensure that the dimensional lumber part of the equation—the part supporting the tile installation—ties into and is at least as strong as the surrounding I-joist floor. Also, the dimensional-lumber part of the floor must comply with the tile industry requirements for deflection: no more than 1/360 of the overall span (uniform load), and no more than 1/360 of the span between joist members (concentrated load).

As a caveat, the simplest way to drop a floor—regardless of the framing—is to have access to the framing from below. Because curbless showers are most often installed for accessibility, and because accessible bathrooms are usually on the first floor, let’s assume that the shower you will be installing is on the first floor with an unfinished basement or crawlspace below. Let’s also acknowledge that if this work is being done above a finished space, you most likely will need to remove a large part of the finished ceiling to gain access to both the framing and the plumbing.

I start by cutting out the subfloor in the shower area, as shown in the illustration above. From below, add spacer blocks that fit between the flanges of the I-joists just outboard of the hole. These spacer blocks pad out the I-joist webs and need to be installed to manufacturer specs. Next, sister 2-bys or LVL material onto the joists that you padded out. These sistered members should extend back to some type of bearing, such as a carrying beam, post, or foundation wall. (If you’re working in a crawlspace it may be possible to add footings and posts to support the framing). Attach the new framing to the padded I-joists with 1/2-inch bolts at least every 8 inches or so in a staggered pattern.

On both sides of the opening, temporarily support the I-joists that run through, and then cut them short enough to give you room to install headers across the opening with a pair of 2-bys or LVLs on each side. The headers will need to carry the cut I-joists on hangers after they’re cut, so mount the I-joist hangers on the headers before slipping them into place, if necessary.

To fill in between the headers, start with a double joist on either side of the opening that consists of a full-height joist to catch the edge of the existing subfloor and a shallower joist at the height of the dropped shower floor. Then it’s just a matter of filling in the joists at the dropped height. You may be able to get away with spacing the joists 16 inches on center, but 12-inch spacing will give you a stiffer floor. Finally, glue and screw down at least 5/8-inch-thick subflooring. Then you can add either a manufactured sloped shower pan or a sloped mortar bed with the appropriate waterproofing.

After you install the tile—and I assume the bathroom floor will also be tiled—lay out the tile so that a grout joint lands directly above the junction between the dropped area (where the shower floor begins to slope) and the original I-joist floor. Fill that transition joint with a resilient material such as a silicone or latex sealant.