How Many Millimetres Is a 1/100 Drain Slope? Standards and Measuring for Drain Pipes and Surface Falls

1/100 means "drop 10 mm per metre"

A drainage slope of 1/100 drops 1 unit for every 100 units of horizontal run. With lengths in millimetres, that's a 10 mm drop for every 1 m (1,000 mm) of run. Likewise, 1/50 is 20 mm per metre, 1/200 is 5 mm per metre, and 2/100 is 20 mm per metre (the same as 1/50). As a percentage, 1/100 is 1%; as an angle, it's only about 0.57°.

The drop can be calculated as pipe length ÷ n. When running pipes under floors or above ceilings, you need clearance equal to this drop, so the longer the run, the more important it is to check early.

For indoor drain pipes, diameter sets the minimum slope

In Japan, the slope of drain pipes running horizontally inside a building (horizontal drain branches) commonly follows the values in the plumbing standard of the Society of Heating, Air-Conditioning and Sanitary Engineers of Japan (SHASE-S206). The thicker the pipe, the more water flows and the more readily it flows even at a gentle slope, so the minimum slope gets smaller.

When moving a kitchen or washbasin in a home renovation, first check whether the floor would need raising by the distance from the new position to the existing drain pipe multiplied by 1/50. For example, moving it 4 m needs an 80 mm drop, so if there isn't enough height under the floor, you'll have to raise the floor or change the pipe route.

For outdoor drain pipes, about 2/100 is the guide

Outdoor drain pipes running from the building to the public sewer's inspection chamber follow the local government's drainage facility standards. Many Japanese municipalities set values like the following, based on national guidelines.

Falls for balconies, concrete slabs and flat roofs

Floor and roof surfaces also have a slope (fall) to collect water at the drain without leaving puddles. Even when they look almost flat, falls like the following are normal.

If puddles remain on a balcony after rain, the likely causes are an insufficient fall, a clogged area around the drain outlet, or a sagging floor. First clear leaves and mud from the drain outlet; if water still collects in the same place, have a specialist look at it along with an inspection of the waterproofing.

What happens when the slope is too shallow or too steep

• Too shallow: water flows slowly, and grease, hair and waste are left on the bottom of the pipe, making blockages more likely. It's also a cause of bad smells.
• Sagging midway or a reverse slope: if the pipe isn't supported properly and dips partway, water and dirt keep collecting there. If flow gets worse after a renovation, suspect this first.
• Too steep: it's said that water runs ahead and solids are more likely to be left behind. Outdoor sewers may also set an upper limit on slope so the flow speed in the pipe doesn't get too high.

What matters for household plumbing is securing the specified minimum slope and running the pipe at a constant gradient without changing it partway. Where the height difference has to be large, rather than making the slope steeper, methods such as installing a chamber to create a step are used.

How to measure a drain slope on site

1. 1

   Measure with a spirit level and tape

   Place a 1 m spirit level (or a straight length of timber plus a level) on the pipe or floor, lift the low end until the bubble is centred, and measure the gap with a tape. A 10 mm gap means 1/100; 20 mm means 1/50.

2. 2

   Measure with a string line

   Stretch a level string line above the start and end of the pipe, and measure the distance from the string to the top of the pipe at both ends. Divide the difference by the pipe length to get the slope. A 50 mm difference over 5 m is 1/100.

3. 3

   Measure the angle with a phone

   1/100 is about 0.57° and 1/50 about 1.15°, such small angles that placing a phone directly on the surface tends to get lost in the error. If you put the phone on a long piece of timber and correct for offset by measuring twice in opposite directions, it's good enough for a rough check of which way the slope runs and whether it's around 1/50.

Calculate the drop from the distance instantly

In the "Height, length & ramp" tab of the Slope & Angle Meter (Phone Level), enter a slope such as 1/100 and the pipe length (horizontal distance), and it calculates the drop in mm and cm. The "Convert slope" tab converts between 1/n, %, per mille and degrees, and also shows whether a slope meets the 1/50 and 1/100 drain pipe guides.

How slopes relate to percentages and angles is explained in Converting slope percentages and angles, and tips for measuring small tilts with a phone in How to measure floor tilt with a phone.

FAQ

How many millimetres is a 1/100 drain slope?

It's a slope that drops 10 mm per metre: 30 mm over 3 m, 50 mm over 5 m and 100 mm over 10 m. That's 1% as a percentage and about 0.57° as an angle.

Are 1/50 and 2/100 the same?

Yes, they're the same slope. Both drop 20 mm per metre, which is 2% or about 1.15°. 1/50 is often used for thin indoor drain pipes and 2/100 for outdoor drain pipes.

How much slope does a kitchen drain pipe need?

Kitchen and washbasin drain pipes are often around 50 mm in diameter, so the guide minimum is 1/50 (20 mm per metre). In a renovation that moves the sink, first check that you can get a drop of the distance moved × 20 mm.

Does a steeper drain pipe flow better?

Not necessarily. Shallower than the minimum slope makes blockages more likely, but an extremely steep slope is said to let water run ahead and leave solids behind. What matters is running the pipe at the specified slope, constantly, without sagging partway.

How much fall does a balcony need?

1/50 (20 mm per metre, about 1.15°) or more is the common guide. If puddles remain after rain, possible causes are a clogged drain outlet, insufficient fall or a sagging floor.