Polythene against wall instead of French drain?

[adrianth]

Deal All,

We're moving into a ca. 1900 brick-built house that has some rot in the ground floor sub-floor timbers. Blocked air bricks and gully are probably the main reason. While yanking up the Corydalis that has grown against the wall, I found clear polythene between the soil and the wall, though the inch of polythene sticking up was looking ragged. There is a lot of efflorescence on the surface of the first course of bricks above the soil level and polythene.

I've spent a lot of time on the website, so I get the idea of letting old buildings breathe. But is this polythene helpful or not? The house is prone to subsidence and leaning down the hill, so there may have been a reason not to make a proper French drain. The wall in question is the footing of the font bay, which is not integral to the structure of the building, but is hinging away from it a bit.

So! How does it work out? Is breathing important even below soil level? I suspect it must be, otherwise eveyone would do this polythene trick instead of making French drains. Can anyone comment?

Thanks very much for ideas, and for all the posts before,

Adrian.

 

[middi]

Hi,
Your 1900 brick built wall would have had a row or two of welsh slate to act as a damp proof course. Is it there??.

plastic against the wall below soil level would make matters worse because it will act as a capillary matt and trap water against the brickwork and take a lot longer to dry out.

If your bay wall is coming away from the house it sounds that someone made a bad job when built. It may mean a bit more time and money to get the job done done properly and rebuid it.

in the mean time check that all the ventilation below floor level is working correctly, put some more vents in if necessary.

 

[adrianth]

Thanks Middi! We really appreciate someone with your knowledge taking the time to help- very useful advice. The plastic will not stay.

There is no sign of a damp proof course of any kind that I could see. The next-door neighbour, born there 70+ years ago, was of the opinion that there is no DPC... said the houses are "a bit damp, but then they're old houses" indicating that it's manageable if the ventilation is OK. I'm not aware of damp damage to the decorations, its just under the floor that seems to have been a problem. We'll see...

Thanks again,

Adrian.

 

[NT]

Underground is different to above ground, and water only flows one way down there, from wet soil into the brickwork, and out the other side by evaporation. A polythene barrier will much reduce this water flow, thus be helpful, as long as water is not funneled into the gap between poly and brick.

Poly is not normally used because it is not durable. It is neither tough nor long lived. The best grades have a life of 4-5 years outdoors. While it will last better underground (no UV), there it has to contend with worms, animals, sand and grit and so on, some of which will hole it.


NT

 

[adrianth]

Ah! Interesting.

It was slack of me to call it polythene: I have no idea. Clear plastic sheeting, maybe 0.5 to 1mm thick. I dug down to about a foot depth this evening, and it looks tough and intact. The inch or two sticking up above ground is a bit frayed, which would fit with UV degradation. (This whole area was covered with weeds, and probably previously garden plants, and is North facing, so perhaps not subject to much UV.)

The top of the plastic had curled outwards, and some soil had collected between it and the wall: indeed a funnel. This could be fixed.

I don't know how much further down the sheeting goes, but at the very least there must be water wicking up the walls from the foundations (whatever they are). The usefulness must depend on the moisture vs. depth profile of the soil, coupled with the base level of moisture at the bottom of the wall.

I'm slightly more abivalent now, but still swayed by my perception that this practice is not common. Any more information would be appreciated, but thanks very much to you Middi and NT for your advice so far!

Adrian

 

[Matt Green]

Blocked air bricks and gully are probably the main reason.

Really?

Exterior ground levels should ideally be 150mm lower than finished internal floor height. This ensures the moisture content of joist ends and plates isn't raised high enough for fungal decay due to lateral penetration by ground water. I'm guessing this isn't the case as it sounds like the ground height is playing a part is subsidance reduction (which is fair).

As NT says, a physical barrier to prevent lateral penetration is good thing in this situation.

When you replace the damaged timbers be sure to isolate them from the surrounding masonry to prevent decay.


Matt Green

 

[adrianth]

I was assuming that the original arrangement was satisfactory, so that the excessive damp would be due to what had changed: obstructed air bricks, and overflowing rainwater drain. But you're right - I shouldn't assume that! Presumably these things were ok at the stage when someone thought it necessary to install the plastic sheet.

I'll be lifting floorboards this weekend, which might provide more information.

I don't know the role of the ground height in stabilising the wall, but since things have moved in the past it seems wise to leave in any support available. My caution regarding a possible French drain is based on reading
http://www.ihbc.org.uk/Technical Papers/French Drains.pdf

However, my knowledge is entirely from web surfing: I don't know what I'm talking about! So your coments are very useful indeed.

Thanks again,

Adrian.

 

[NT]

adrianth:

> I was assuming that the original arrangement was satisfactory, so that the excessive damp would be due to what had changed

Above ground this is almost always true. But below ground its different, period properties usually have very damp cellars not fit for habitation, and modification is the normal way to make them fit.


NT

 

[adrianth]

We don't have a cellar. Interestingly, the terrace was continued after the First World War in a matching style, and those later ones were given cellars. Your point is well taken, though: thanks, NT.