Even though I am a woman and should not be interested in articles on building I read yours each month and find it very interesting. I hope that you can solve my long standing problem with the drainage to my property. The problems are two fold; I often smell sewage in my en suite shower cubical and also the water in my down stairs bath waste bubbles vigorously when the toilet upstairs is flushed. I’m afraid that one day the pipes will block leaving me with an expensive, smelly problem.

Kind regards,

Sandra – Alcalali

Hi Sandra,

Unlike the recently introduced and over the top requirements for reinforced concrete single storey structures, the Spanish drainage system still remains in the dark ages. It is not that many years ago that when visiting Spain you expected to smell sewage, in fact it was part of your holiday, a talking point you might say. Although the Spanish are progressing slowly in this field they are still light years behind the U.K. Only a few weeks ago whilst completing the drainage on a new under build apartment in Denia we were visited by the technical architect responsible for the project, he found it highly amusing that we should want to install some form of inspection chamber and necessary access to clean the drain. He explained - that if the drainage does get blocked you simply need to dig it up and lay another one - sounds easy when you say it quickly.


The problems that you are encountering are common and could be solved by simply ventilating the drainage system. When you flush a toilet a large volume of water enters the main drainage pipe very quickly; this water forms a hydraulic plug (a solid mass of water) which compressed the air in front of it and also leaves a vacuum behind it. The air in front of the plug needs to escape to allow the water to continue downwards and the vacuum behind the plug needs to be recharged with incoming air to prevent self siphoning of the water traps (U bends below each bathroom/kitchen fittings). The depth of water in a Spanish shower trap is very shallow, often only 25mm., unlike the traps on the normal sink, wash hand basin and toilet which is much deeper, usually around 50mm. Air trying to enter or leave the drainage system will find the simplest route, the route of least resistance, normally the shower waste as in your case. In a typical U.K. installation air would be allowed to enter and escape the drainage system via a 110mm soil and vent pipe (s.v.p.), this ventilation allowing the pressure within the system to equalize. In Spain s.v. pipes are very rare only usually seen on fully designed drainage systems in prestige’s high rise buildings. The smell of sewage coming from your en suite shower cubicle is the direct result of the water trap below the shower becoming dry and in doing so allowing the ventilation of your sewage system through your shower. This trap would only normally become dry for one of two reasons; either the water in the trap has been siphoned due to the flushing of the toilet as explained above or the water in the trap has evaporated, this being a common occurrence in the summer months. To temporally stop the smells simply pour a small amount of water down the shower to replenish the trap.
The reason that the water in your down stairs bath waste bubbles vigorously when the toilet upstairs is flushed is that the compressed air in front of the hydraulic plug is trying to escape to allow the water from the toilet to continue on its way.

The only sure way to stop the traps from bubbling and possibly emptying when the toilet is flushed is to introduce ventilation into the system. There are two ways of achieving this; firstly as mentioned above provide a s.v.p., usually consisting of a 110mm vertical plastic pipe which can be taken up in the inside or the outside of the property.  The s.v.p. would then be connected to the under ground drainage at the bottom and extended a minimum of 1m above any opening window at the top. Each bathroom and kitchen fitting would be connected into the s.v.p. via its own waste pipe and trap (there are a few rules governing waste pipe sizes and pipe lengths that would need be followed) the end result would be a perfect and fully functioning system.


The second method and the least expensive way to solve your problem would be to install an air admittance valve; these are one way valves allowing air into the system but not allowing the smell of sewage out. They were first developed in the U.S.A. but received a British Standard Kite Mark about 15 years ago. Primarily used in the conversion of existing buildings (apartments, houses in multiple occupation etc.) were it was difficult to alter or extend the original drainage. These 110mm. diameter valves need to be installed as high as possible on the main vertical drain either inside or outside the property. They cost about 100 euros to buy but are not readily available in Spain; you may need to order one from the U.K. I have seen smaller diameter valves in Spain that could be used on individual wastes which if installed would help, but being a smaller diameter will not always allow sufficient air into the system quickly enough to be 100% effective.

I have tried to explain in simple terms the reasons for and the remedy to overcome the problems with your drainage, however I do realize that this is a complicated subject and you may feel that from such a simple question asked you seem to have receive a technical and complicated answer, therefore I am more that pleased to visit your property if you wish, to help you resolve the problems in the least expensive way.

John Phillips (F.I.A.S. F.A.B.E. M.I.B.C.O. F.F.B. M.R.S.H. C.M.W.O.B.O. M.B.I.M)
Construction Buena Vista

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Construction Buena Vista
Urb. Palmeria. Orba 03790


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Buena Vista Villas s.l.

Urb. Palmeria. Orba 03790


Please call us on


0034 965 577 464 (office)


0034 629 554 561 (mobile)


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