The slope of your drain pipe is an important part of any new plumbing installation. If the slope is too gradual then your drain will be prone to clogging, if it is too steep then the water drains away from the solids and again becomes prone to clogging. Typically, this range is from 1.5° to 18° which equates to 20mm to 250mm drop in a meter or ¼” to 3” drop in a foot.
Measure the Drain Length
The first thing to do is measure the drain length. This is the distance from where the drain connects to the basin/sink/tub/shower etc to the point where it connects to the main drain or existing plumbing. Make sure you measure the total length if it needs to go around walls, corners or pillars. If you have difficult areas to measure then try run a pieces of string along the line you wish to run the drain and then measure the length of the string. Take the measurements horizontally along the same height.
It sometimes helps to map out your drain route on a pieces of paper. Mark each small distance on the paper and then add them all up to get the total drain length. This drawing will also assist you when you go through to the hardware or home store to buy plumbing fittings and piping.
Calculate The Drain Slope
There are two methods to calculate the drop that you need to get to the correct slope, one involves trigonometry and one straight multiplication.
You know that you want your drain to be in the range of 1.5° to 18° so choose a value around the middle, say 10°. Multiply the length by tan of the angle and that will give you the required drop.
For example, if your length is 5m (5000mm) or 200″ and you want an angle of 10°:
5000 x tan (10) = 882mm
200 x tan (10) = 35″
So the drop you need is 0.88m (882mm) or 35″.
The multiplication method also uses the length of the drain along with the ranges we gave you in the beginning.
Say your drain length is 5m (5000mm) or 16′. In the introduction, we said that the drop of the drain needs to be 20mm to 250mm drop in a meter or ¼” to 3” drop in a foot.
5 x 20 = 100mm and 5 x 250 = 1250mm
16 x 1/4 = 4″ and 16 x 3 = 48″
So your drain needs to drop between 100mm and 1250mm or 4″ to 48″. Choosing a value in the middle of these gives you 675mm or 26″.
When installing your drain pipe, you can either use a digital level in order to measure that each section of pipe you fit is at the same angle (10° or so in these examples) or you can measure the drop out and make sure that between the beginning and end of your drain, you reach the required drop.
Cover image: Sink Drain by Mark Lea used under CC BY 2.0.