Building Construction, Renovation, Maintenance & Advice

The Design Of Drains and Drainage Systems

Good MEP design, drainage design and installation - 1

Good MEP design is essential to allow maintenance and avoid ongoing problems. here we look at some basic rules for drainage and plumbing design.


See the full Fixed Abode article here "Pass Me The Powdered Unicorn Horn"


MEP Design

Properly designed buildings should allow for the ongoing maintenance and repair of the essential plumbing and electrical systems that keep your home operational.

Electrical and plumbing design and installation

Two key factors are required here: a sensible design and correct installation. Installation is very important and only 2 days ago I saw a beautiful brand new tiled ceramic bathroom wall broken down because a “plumber” had “forgotten” to put a bit of PVC solvent cement in a water pipe joint. But let us assume that you have found a tukang who actually knows his Tees from his R’s.

It is important that the MEP (Mechanical, Electrical and Plumbing) systems are designed correctly.

The routes for electrical cables, water pipes and drains should be carefully thought about to allow faults to be easily found and repaired.

Design for maintenance

Clever design will allow access to the back of bathroom and kitchen walls so the expensive wall finishings do not have to be removed to effect repairs, you can go through the wall from the rear.

I recently came across a case of an embarrassed woman who lifted the cover off a bathroom floor drain and stuffed a used hygiene item down the drain to dispose of it. The drain was, of course, blocked and the bathroom flooded. This could have been a very difficult problem to solve requiring, at worst, the bathroom floor to be taken up. Fortunately the drain was a straight run from the bathroom to outside the building so the end of the pipe could be dug out and a drain rod pushed up the pipe to clear it.

Good drain and wastepipe design

It is a fact of life that drains can become blocked from time to time. Here are some simple guidelines for installing drains, whether renovating or building new, to make them easy to unblock.

  • Drains should be large enough - 1.25” dim from washbasin to U bend then into a large 2 “ pipe with straight run to outside the building. Washbasins and floor drains 2 “, toilets 4”.
  • Drains should also slope sufficiently to get a good flow of water to carry solids away and help keep them clean. The normal slope for a sewer or grey water drain pipe is 2% (2cms per metre).
  • Wherever possible use straight runs of pipe, pipes under floors should always be straight lengths until they are outside the building.
  • Do not install 90 degree elbows, these are “sharp” bends and drainage rods cannot get around them. If you look around you might find what we call “slow” bends. Better still use two 45 degree bends to turn through 90 degrees. This is so that drainage rods can be pushed through the pipes and will get around the bends.
  • Install “clean outs” at critical places along the pipes. What is a cleanout? No it is not a particularly hot vindaloo, it is a branch taken off a drain with a screw cap on it placed in such a position that a length of pipe can be cleaned out using a drain cleaning rod. A clean out allows a plumber to push drain cleaning rods into the pipe. Clean outs are often placed at the ends of straight pipe lengths.
  • Install Y connections rather than 90 degree tees where pipes join. Y connections in pipes allow smoother flow and do not block as easily and they also allow drainage rods to be used to unblock the pipe.

Bad smells from drains

Make sure that drains have U bends in them, the U bend provides a water trap to stop bad smells coming up the pipes. Take particular note of washbasin drains with screw fittings in the pipe so that the U bend water trap can be cleaned out.

If you have a smelly drain the first thing to do is check that there is a properly designed floor drain cover in place. This should have a water filled cup built into it to stop smells coming out.

If you do not have an effective water seal in your pipework you can dig out the pipe outside the house and install a U bend. Make sure the U bend is 4” diameter and has a screw cap top on it to allow you to get in and clean it out if you need to.

Electrical installations tend to be a lot easier than plumbing but a basic rule is that cable should be installed in conduit (plastic pipe) which is set into walls and allows cables to be removed and replaced if necessary without having to break out the wall.

It is a good idea to keep your eye on your plumber. If he uses silicone anywhere on the pipework this is a sure sign that he doesn't know what he is doing and it might be an idea to send him back to his satay stall.

Just as an afterthought it is probably a good idea to put away a few spare pieces of that beautiful zebra striped marble (you might also choose to stash away the odd Athenian Virgin, some donkey’s milk and of course some powdered unicorn horn) they might come in handy one day if you get a blockage.

Copyright © Phil Wilson 2014
This article or any part of it cannot be copied or reproduced without permission from the copyright owner.

17 July 2017 Copyright © Mr Fixit,
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