The Brew Cabin

I sincerely hope you can help us with this. We have discovered, since buying our house a couple of years ago, that we have sufficient dampness below (and in) our suspended wooden floor to be causing wet rot. Our 1930's house lies at the bottom of a long, steep field and our boundary lies across the slope - with a field drain which is probably ineffective as it's horizontal. After having read your excellent pages on drainage we suspect our field drains are insufficient to cope. Although we have a DPC the walls are coated to ground level with render and are also then coated with Sandtex paint. Suspect DPC being bridged! The house is cut into the hill so there is a bare minimum of 150mm below the DPC at the back - should we dig it out more? I anticipate a few days of chipping off Sandtex and exposing the DPC from under 1 inch of render.

We want to do the work ourselves but want to know what's the best plan of attack to catch all surface and below surface water. Just how low should we go? I'm not convinced a perforated pipe at 1m in a heavy clay soil will catch surface water in a heavy storm. Your advice would be invaluable.

The render to ground level sounds particularly naughty and that would be one of the first things I tackled, if it were my property. I suspect that, once you've got the rendering sorted, some form of dry channel arrangement (see DPC might be what you need.

What you need to catch all the surface water is an interceptor drain. The depth of the pipe is immaterial - it's the fact that the granular backfill comes to the surface that makes them so effective. If this is within a paved or hard-surfaced area, then a linear drain would perform the same function, but, as with all drainage issues, bear in mind that you must have a place where you can outfall the collected water. Do you have existing drains?

Hi, thanks for the quick reply! We have old outbuildings with various pipes leading off who knows where and the nearest point is about 30m from the edge of the house. Husband has been trying them out with a hose and they appear to run freely, so they are our probable destination for the field water rather than our septic tank which is closer.

Thanks for the tip about the dry channel - had missed that page on your wonderful site! Would you recommend a DC and an interceptor or would the latter suffice. I am concerned about the depth of the interceptor as I don't want anything coming underneath and undoing all our good work. Would you run an interceptor all the way to the outlet, or could it just run into a drainage pipe part of the way - once it has done its job.

After further reading on your site - I was up late!- am also suspicious of the solum being too low in relation to the hill at back and allowing water to penetrate both courses of brick below the DPC. The moisture level readings from our Rentokil survey even above floor level were at 'dangerous' - so there's a distinct surplus of water getting in somewhere! How high should the solum be above ground level?

Should mention we will be increasing ventilation too!

A million thanks for your time!

You can use drain tracing dye to help you figure out which pipe runs where, but, the general rule is that surface water is never sent to a septic tank - that would just overload the system and could present a health hazard. So, it's more likely that the SW drains go to a ditch or a soakaway somewhere - it may not even be on your property!

It's difficult for me to say what would be your best option, as I'm not familiar with the property and drainage is one of those jobs that can only be properly weighed-up when you can see the ground, and even then, there's a risky element of guesswork involved, as you never know what's buried down there. However, the purpose of an interceptor drain is to intercept surface water and groundwater in the top 300-450mm or so. If the collector pipe at the base of an interceptor drain is lower than the foundations of the house, then it must be a boon, as groundwater will be collected and diverted before it can threaten the house itself. To be honest, as long as the base of the interceptor is at least 600mm below dpc, and a couple of metres away from the house, it can only have a positive influence.

So, would I use a full-length interceptor, or switch to standard drainage? Again, it depends on site conditions. No matter which option you choose, you have to dig a trench, lay a pipe and backfill it. With standard drainage, you can cover the pipe with a sprinkling of pipe-bedding material, and then backfill the rest of the trench with the excavated spoil, whereas with an interceptor drain, you have to back fill with granular material (pipe bedding) AND get rid of all the surplus excavated material, so, from a cost point of view, a standard drain is the better option.

What's this 'solum'? I'm not sure that what I call a solum is the same as what you call a solum. The standard definition of a solum (in the construction industry, anyway) is 'a piece of ground on which a building stands'. Geologists have a slightly different definition, but it still doesn't help me understand your last paragraph. Do you mean Floor Level?

The solum I refer to is the finished level (with tar DPM) below the suspended wooden floor. I have been searching around the net and can't get details except that it must be above ground level. Have now also discovered that Sandtex - as I suspected - causes lots of moisture problems by trapping moisture behind it, above and below DPC, which can't dry out except into the house! ....Oh joy!

Hopefully if we get this drainage right it won't be such an issue. Having weighed up everything I think this is the best starting point for tackling our problems.

Many thanks for your fab site - I have been scouting around it for measurements and spec. for the interceptor drains. Also keen to price the Terram 1BZ(?) -non permeable on one side- as it looks like it would be easier. Can't find anything on the net - so will trying phoning around for prices on Monday - unless you can give me a ballpark figure per m - you seem to know everything about everything!!

Thanks again - we feel able to tackle the job ourselves with confidence now we know exactly what we're doing! We're hoping to start digging soon - will let you know how it goes!

Aaaah! We call that a sub-floor, but then, it's a bit out of my field so maybe it is called a solum - I've just looked it up my Dictionary of Building (rather than my Dict Civ Eng) and it says...

The ground below the lowest floor of a building. If for any reason this is not covered with oversite concrete, it must be damp-proofed in accordance with the Building Regs., in Scotland with hot pitch or asphalt, or south of the border with thick plastic sheet covered with 50mm of fine concrete (BS 2832)

Terram 1BZ is one of those products that is priced according to quantity required. Try 'phoning Terram in Pontypool on Monday and getting a direct sale price. They may have a stockist/distributor up in your part of the world.


Good luck!