Update on damp

[lagori]

I recently asked about the damp that's appearing in our house on this thread.

I've looked at a solution, but want to check it off here, to ensure I don't cause myself more trouble:

The plan is to dig up the concrete, install a perforated pipe on the two sides of the property affected, and dig down as low as possible without compromising the required fall (there is no foundation, but hopefully, that far...). Backfill with clean stone and point the pipes at our storm/rainwater collection tank, which then gets pumped away.

This idea might be considered terrible - in which case, please let me know - but I wanted to check two aspects suggested by the installer. One option is to apply a damp-proof membrane to the building's face and backfill with 40mm stone. Instead of leaving the top open, as I've seen most often with this, though, he proposes that they would want to reconcrete the surface.

These make sense to me in principle; he reasons that the DPM on the building face minimises any water sitting around the pipe from seeping into the walls. Concreting back over is his preference to avoid surface water complications. Given appears to be a subterranean issue that presents itself entirely during periods of very dry weather; it does not seem to be a surface rainwater issue. In his experience, leaving this open would likely clog horribly and quickly (oak trees drop all over this alley at the back of the house), and it is asking for more complications.

A very reputable firm, have done work for us in the past around drainage and have been superb in those instances, but I'm coming here to check others' thoughts - appreciate this making sense in principle doesn't mean it makes sense.

 

[lagori]

Just bumping this in case anyone might be able to tell me why cementing this back in might be a terrible idea.

 

[Cubist]

Installation of land/French drains adjacent to perimeter walls is a fairly simple and inexpensive approach and perforated tubes are frequently used for this purpose. Provided that a 'fall' (slope) is provided to direct the water away from the property such drains will usually allow the water to dissipate through the topsoils downhill if at all possible but, as in your case, can be routed toward a storage tank or cistern for use in the gardens or onward pumping to other drains or ditches. Few people would actually bother attempting to install a damp proof membrane against the walls of the house and particularly where a DPC is a feature of the property - its not unusual for the footings/foundations of walls to be a little 'foot wet'. Provided that the land drain is properly installed and is kept free of blockages then little more should be needed. A concrete cap or cover would be entirely counter productive - it's always wise to leave the drain buried in say a loose gravel to help rapid run off of surface water into the drain below. This loose cover is also helpful should there be any future need to examine or perhaps clear blockages/silt build ups within the pipe. Frankly your contractor is over-engineering the job.

 

[lagori]

Installation of land/French drains adjacent to perimeter walls is a fairly simple and inexpensive approach and perforated tubes are frequently used for this purpose. Provided that a 'fall' (slope) is provided to direct the water away from the property such drains will usually allow the water to dissipate through the topsoils downhill if at all possible but, as in your case, can be routed toward a storage tank or cistern for use in the gardens or onward pumping to other drains or ditches. Few people would actually bother attempting to install a damp proof membrane against the walls of the house and particularly where a DPC is a feature of the property - its not unusual for the footings/foundations of walls to be a little 'foot wet'. Provided that the land drain is properly installed and is kept free of blockages then little more should be needed. A concrete cap or cover would be entirely counter productive - it's always wise to leave the drain buried in say a loose gravel to help rapid run off of surface water into the drain below. This loose cover is also helpful should there be any future need to examine or perhaps clear blockages/silt build ups within the pipe. Frankly your contractor is over-engineering the job.

Thanks - appreciate your insight. I believe the goal of covering the solid surface is to reduce the impact of mud, leaves, and sticks, which are abundant in the exact location where we need the drain, surface water not being such a problem.

Where we have gravel on the other side of the house, it becomes abundant with weeds very quickly, and as a result, it becomes soil and gravel quite quickly.

Assuming the damp is being caused by water flowing underground in the direction of the house, it's possible that this isn't the problem, naturally.

 

[Cubist]

I should have mentioned it before but... if installing perforated pipes to create a land drain its wise to drape a water permeable garden membrane over the pipe before infilling with gravel. This works to limit the amount of fine sediments that would otherwise wash into the body of the pipe where it can accumulate to cause a blockage or otherwise limit its performance. It also needs to be said that such land drains will require some maintenance in the future so its always sensible to install them in such a ways as to ensure that the full length of the pipe can be exposed and, if need be, lifted so that it can be replaced or washed out.

Weeds in gravel beds of any kind are an inevitable fact of life. Dusts, soils, pollens, seeds and the corpses of insect and animal life will be washed into the gravel beds to create a rich environment for new growth. As a consequence it will ultimately become necessary to excavate the old gravel and replace it - or perhaps wash it out if you can. A point worth making here is that if you intend to use the water run-off from these drains for use in your garden you need to be careful about your choices of any weedkillers.

You don't describe the topography of your property but given you're in Cornwall theres a good chance your property stands on hilly and rocky land with fairly shallow beds of clays and top-soils. Water run-off above those rock beds may be the root cause of your issues and there is precious little you can do about that other than other than to intercept and divert that water with the land drains as planned. In your place I would not be overly concerned about the accumulations of leaves and twigs. Short of cutting down the nearby trees there is no real option but to accept that it will be sometimes necessary to rake up the detritus and toss it on the compost heap.

On the plus side for gravel beds - honest, there is one. You'll find that these can be the source of new plants and trees that can be transplanted elsewhere. The gravel beds around our old place have, over the years supplied me with something in the order of a 1000 trees that have subsequently, with a little help from yours truly, been migrated into the hedges and fields here. The only trick being to recognise them as plants you want to keep.

Finally; again a concrete cap is absolutely pointless and in fact would become a liability when it becomes necessary to inspect and repair the drains.

 

[Stevers]

I thought it was the practise to line the trench to keep fine material out of the gravel, thus presenting a greater area for filtration. I would use rigid slotted pipe not ribbed pipe and fit rodding eyes for cleaning and inspection. Even then I would be averse to concreting it all in.

 

[stuart45]

I thought it was the practise to line the trench to keep fine material out of the gravel, thus presenting a greater area for filtration. I would use rigid slotted pipe not ribbed pipe and fit rodding eyes for cleaning and inspection. Even then I would be averse to concreting it all in.

You should line the trench, but you can wrap the pipe as well. Non woven is much better than woven. The first one I ever did for a customer I used a woven one as recommended by Bradfords, it was also the only one they stocked. I was a bit worried after I finished the job when I tested an off cut and found that it's not as permeable as I thought it would be.