Pan Slope, Mortar Bed, and Membrane Area

The 1/4 in per foot drop to the drain, the 1/8 in per foot limit inside 2 in of the drain, the drop at every point in the pan, the mortar bed volume, the waterproof membrane square feet, and the tile and drain detail.

IRC P2706.1: the shower finish floor slopes toward the drain not less than 1/4 unit per foot and not more than 1/2 unit per foot, and within 2 in of the drain the slope does not exceed 1/8 unit per foot. The bed runs 3/4 in at the drain and 1/4 in per foot out.

—Slope check against the code band
—Drop at the pan edges
—The 2 in drain rule
—Mortar bed volume and bags
—Waterproof membrane area
—Tile, curb, drain, and the test
How the math works
  • The pan slope (IRC P2706.1): a shower floor slopes toward the drain on a slope not less than 1/4 unit per foot and not more than 1/2 unit per foot, and within 2 in of the drain the slope does not exceed 1/8 unit per foot. The drop on a 5 ft run at 1/4 in per foot is 1.25 in, and the 2 in ring at the drain drops 1/4 in.
  • Mortar bed: a pre-sloped bed runs 1.5 in average thickness at a 1/4 in per foot pitch with a 3/4 in minimum at the drain. A 50 lb bag of pre-sloped mortar covers about 4 sq ft at that thickness, so a 15 sq ft pan takes about 1.9 cu ft and 5 bags with the waste. A site-mixed bed runs on a 4,000 psi mix with the 1.5 in minimum at the far edge and the 3/4 in at the drain, and the pitch is the 1/4 in per foot line, not a level line.
  • Waterproofing (IRC P2706.1, P2706.3): a shower assembly with a tile, stone, glazed tile, and a nonabsorbent finish gets a waterproof base, a membrane in the mortar bed with the 3 in wall lap and the curb covered, a membrane at the door opening with a dam, a shower wall base behind tile with the 78 in line at a shower head wall, and the tub-shower water line 6 in over the tub rim.
  • Drain and weep holes: a shower drain body is fastened to the subfloor with the gasket under the flange, the clamping ring, the weep holes open in the bed, a secondary seal at the subfloor, and the drain flange set 1/8 in below the finished tile face. Plugged weep holes are the water that stays in the bed, and the bed under a tile at the drain is the assembly the flood test proves.
  • Deflection and tile: a shower pan over a wood-frame floor goes to a deflection limit of L/360 under the tile assembly with the 3/4 in underlayment, a 2x10 floor at 16 in on center over a 12 ft span, the 1.5 in mortar bed, and the 1/4 in per foot pitch plane. A large-format tile pan goes to a linear drain, a single-plane slope, a 2 in ring at the drain, and a tile cut on a radius at the flange.
Planning tool on the International Residential Code shower drainage sections P2706.1, P2706.2, and P2706.3 as adopted locally, with the tile industry assembly practice (the ANSI A108 and A118 methods, the 2 in drain ring, the pre-sloped bed, and the flood test). The code edition, the local amendment, the plumbing inspection, the flood test requirement, the membrane manufacturer listing, the shower assembly listing with the 78 in line, the accessibility shower rule (a 60 in by 60 in roll-in shower with the slope at the 1/2 in per foot maximum and a barrier-free threshold), the curbless rule, and the 2 in curb rule in your jurisdiction are the calls. A curbless walk-in pan, a linear drain in a pan, a pan over a wood frame floor, a pan over a slab, a pan with in-floor heat, a pan with a 12 in tile, a pan with a four-way slope, a pan with a center drain, a pan on a second floor, a pan over a conditioned space, a steam shower with a fully bonded and fully waterproofed assembly with a steam-rated membrane, and a pan with 1/2 in cement board and no membrane are all assembly calls, and a flood test is the document.

What is the code slope for a shower pan?

IRC P2706.1: the shower finish floor slopes toward the drain on a slope not less than 1/4 unit per foot and not more than 1/2 unit per foot, and within 2 in of the drain the slope does not exceed 1/8 unit per foot. A 5 ft pan at 1/4 in per foot drops 1.25 in at the far edge, and the 2 in ring at the drain drops 1/4 in.

How thick is a mortar bed for a shower pan?

1.5 in average thickness at a 1/4 in per foot pitch with a 3/4 in minimum at the drain: a 5 ft by 3 ft pan is 15 sq ft, so 1.9 cu ft of mortar and about 5 bags of 50 lb pre-sloped mortar at 4 sq ft per bag with the 10 percent waste. A site-mixed bed runs on a 4,000 psi mix with the 1.5 in minimum, and a bed under 1 in is a bed that cracks.

Do I need a 2 in ring of small tile at the drain?

The code caps the slope at 1/8 unit per foot within 2 in of the drain, and a 2 in ring of small tile at the flange is how the field build meets it. A 12 in tile at 1/4 in per foot drops 3/4 in across a single tile, so the flange sits 3/4 in low under one tile and the tile cannot land flat: the ring is the small-tile band, the tile cut on a radius, and the flange set 1/8 in below the finished face.

Do you flood test a shower pan?

Yes, and the field build tests twice: 24 hours with 2 in of water on the mortar bed before the tile with the water line marked, the check at the room side of the curb, the wall base, the valve plate, and the drain, and a second test on the finished pan. A flood test is the document, and a membrane assembly with no flood test is an assembly on faith.

Related tools

Tile, thinset, and groutThe tile side of the pan Fixture units and pipe sizeThe drain and the vent the pan feeds Water heater sizingThe shower the pan serves Deck boards and framingThe floor deflection line for tile