Shower Slope / Pre-Slope Calculator

How the shower slope calculator works

A tiled shower floor, or the mortar pre-slope under a liner, has to fall towards the drain so water never sits. The usual way to build it is to screed the edges level all round and slope everything down to the drain. With a level edge, the wall farthest from the drain sets the height: fall = run to the farthest wall ÷ R, for a slope of 1:R. Every nearer wall then ends up steeper, because the same height drops over a shorter distance.

That is why the calculator gives three slopes, not one. The target slope runs to the farthest wall. The steepest runs to the nearest wall, and it has to stay under the maximum. The valley runs from the drain to the far corner, along the fold where two slopes meet, and it is always the gentlest.

The limits: 1/4 to 1/2 in per foot, or 1:80 to 1:35

In the US, section P2709.1 of the International Residential Code says the finished floor shall slope uniformly toward the drain not less than 1/4 in per 12 in, nor more than 1/2 in per 12 in. The code calls those 2 and 4 percent, but 1/4 in 12 is really 1:48, or 2.08%. A 1:50 slope is 1.96%, a touch shallower than the minimum, which is why the US default here is 1:48 rather than the rounder figure. In the UK, BS 5385-4, the code of practice for tiling in specific conditions such as wet rooms, recommends falls between 1:80 and 1:35, and warns that anything steeper than 1:35 can be inconvenient and unsafe underfoot. The metric default of 1:60 sits between the two. Other countries set their own figures, and a membrane or drain maker may ask for more.

Choosing the drain position

A centre point drain puts the farthest wall half the longer side away. A linear drain makes the floor a single plane, so there is only one slope to check: along a short wall it falls the full length of the floor, along a long wall only the width. In a 3 × 5 ft alcove with the drain along the 5 ft back wall, the run is 3 ft and the fall 3 × 1/4 = 3/4 in; put it along a 3 ft end instead and the fall becomes 1 1/4 in. A corner point drain puts the far walls a full side away. Pick the one that matches the drain you will fit. For a drain set somewhere else, measure from it to the farthest wall and work the fall out the same way.

Worked through: a standard shower

With the US defaults, an 8 × 6 ft floor with a centre drain at 1:48 has its farthest wall 4 ft away, so it needs 4 × 1/4 = 1 in of fall. The nearest walls are 3 ft away, so the same 1 in there works out at 0.33 in per foot, inside the 1/2 in limit. Along the valley to the corner, 5 ft away, it is 0.20 in per foot. In metric, a 2.4 × 1.8 m wet room at 1:60 needs 1200 ÷ 60 = 20 mm, with 1:45 to the nearest wall and 1:75 along the valley.

Why long, narrow showers need a linear drain

Stretch the floor to 3 × 8 ft and keep the centre drain. The farthest wall is still 4 ft away, so the fall is still 1 in, but the long walls are now only 1.5 ft from the drain: 1 ÷ 1.5 = 0.67 in per foot, over the 1/2 in limit. The calculator flags it. Move to a linear drain along the short wall and the whole floor becomes one plane at 1/4 in per foot, falling 8 × 1/4 = 2 in from end to end. Metric works the same way: 0.9 × 2.0 m with a centre drain at 1:60 gives about 1:27 to the long walls, steeper than 1:35, while a linear drain needs about 33 mm over the 2 m length.

From fall to mortar bed

The fall is the difference in height between the drain and the wall, not the thickness of the bed. The Schlüter shower system installation handbook tells installers to slope the mortar bed at 1/4 in (6 mm) per 12 in (305 mm), using the drain's bonding flange and a perimeter screed as guides, and asks for at least 1 in (25 mm) of mortar at the flange over a wood subfloor, or 3/4 in (20 mm) over concrete. So a 3 × 5 ft stall with a centre drain, 5/8 in of fall, needs a bed about 1 5/8 in deep at the walls over wood. The same handbook lists its linear drains as suited to single-plane slopes and large tiles, which is the trade-off above.

What this tool does not do

It does not size the drain, check how much water it can take, design the waterproofing, or check that the floor structure can carry the screed. It assumes a level perimeter; a curbless shower that ramps into the rest of the bathroom needs its own set-out. Follow the drain and membrane maker's instructions, which win over any general figure here.

Using this calculator with other BuildMetricLab tools

The shower pan calculator sizes the liner, the waterproof membrane calculator covers the tanking, and the bathroom tile calculator counts the tiles for the finished floor. All BuildMetricLab tools run in your browser with no sign-up, and the formula is on the page so the maths can be checked.