Another French Drain

[frustin]

Hello everyone, this is my first thread and I'm glad to have found you.

I have a Grade II Georgian house with NHL (lime fat) rendered timber framed walls. Down the side of the house is a block paved driveway. I think the house foundation is bricks. There's a boot room where the boiler is and at it's lowest level you can see bricks.

I used a listed building surveyor and he wrote, "Within the ground floor accommodation where tested, there was dampness within the floor slabs around the edges of the room and in the lower wall surfaces."

Quoting from survey "When this house was originally built, there would have been no made-up highway in front of the property. When public roads and pavements were constructed, road levels will have been raised up often between about 12 to 24-inches. Thereafter, the grounds within the curtilage of a property would also be builtup gradually, to coordinate with pavement level. This resulted in ground levels at the base of a building being too high against the plinth and significantly reducing the proportion of exposed plinth brickwork below the timber framed structure.



To the right-hand side of the house, a shared driveway has now been constructed leading down to parking and drive areas. This has significantly increased ground orientation to the right-hand side wall of the property. The render finishes continue to ground level and therefore render finishes will absorb moisture into the plinth and will also trap moisture in the plinth and the wall surfaces above.

Along the entire right-hand side flank wall, block paving is too high and in some areas between the blockwork at the base of the walls cement infill has been provided. Hardscaping should not be built-up against the wall and is now enabling dampness to occur. Ground level is also too high and should be lowered – and ideally a French drain constructed at the wall bases.

The French drain would be about 200mm to 250mm wide. The bottom of the French drain would be at least 150mm below internal floor level. The drain can then be backfilled with large grade stone to encourage moisture evaporation. The grade of stone should be not less than 35mm to ensure an airgap for moisture evaporation to occur. Shingle or small grade stone is inappropriate and does not provide an adequate airgap. This is a priority repair." End quotes from Survey



Do I needed listed building consent before carrying out the work?
Does it need to go down as far as they have stated?
I think backfilling with stones down the side wont be in keeping with that area of the house. Wont something like this type of french drain suffice?
Dont get me wrong, I dont want to do things on the cheap, I just want to understand what i'm realistically looking at.
I'm getting a drainage expert in. What sort of cost should I be looking at? £500? £1000?

 

[WetWales]

Search “ Zebcolm trench” on here - much better than a french drain and should sort you out.

There’ll be lots of posts with people with a similar problem
To yours and you can see how they tackled it,

Zebcolm trench so named after 2 users on here that invented their way out of damp

 

[frustin]

Trouble is, a Zalcolm trench is going to stick out quite far at the chimney breast area therefore encroaching on the driveway and thus potentially vehicle access. Regarding the drive way you can just about get a 3 ton truck down there as it's quite narrow.

Aco drains are about 100mm so not so bad.

I got quoted 2k today:
1. Dig out and remove all the waste from the site
2. Fill with gravel and lay iron drains along the house wall
Total cost for all the labour and the materials for the above £1950 inc vat

 

[Cubist]

May I ask please; have your surveyors and other professionals identified any clear sources of the damp found at the edges of the interior slabs during the inspection? Was this on just the side adjacent to the driveway or was it detected all around the rooms? In other words; how sure are you that the issue is penetrating moisture from the driveway alone?

 

[frustin]

May I ask please; have your surveyors and other professionals identified any clear sources of the damp found at the edges of the interior slabs during the inspection? Was this on just the side adjacent to the driveway or was it detected all around the rooms? In other words; how sure are you that the issue is penetrating moisture from the driveway alone?

"Within the ground floor accommodation where tested, there was dampness within the floor slabs around the edges of the room and in the lower wall surfaces." and goes on to say, "To the right-hand side of the house, a shared driveway has now been constructed leading down to parking and drive areas. This has significantly increased ground orientation to the right-hand side wall of the property. The render finishes continue to ground level and therefore render finishes will absorb moisture into the plinth and will also trap moisture in the plinth and the wall surfaces above. Along the entire right-hand side flank wall, block paving is too high and in some areas between the blockwork at the base of the walls cement infill has been provided. Hardscaping should not be built-up against the wall and is now enabling dampness to occur. Ground level is also too high and should be lowered – and ideally a French drain constructed at the wall bases."

 

[Flyfisher]

Trouble is, a Zalcolm trench is going to stick out quite far at the chimney breast area therefore encroaching on the driveway and thus potentially vehicle access. Regarding the drive way you can just about get a 3 ton truck down there as it's quite narrow.

Aco drains are about 100mm so not so bad.

I got quoted 2k today:
1. Dig out and remove all the waste from the site
2. Fill with gravel and lay iron drains along the house wall
Total cost for all the labour and the materials for the above £1950 inc vat

If the trench is covered with a suitably rated steel grid then it would be able to sustain vehicle traffic over it. As I understand it, the point of the trench approach is to allow free movement of air, thus providing maximum ventilation to the trench wall, thus providing maximum moisture evaporation.

Filling a trench with gravel will significantly reduce the movement of free air, ventilation and evaporation. Test it for yourself - dig two holes somewhere convenient and cover one with some sort of wide mesh and fill the other with gravel. Leave it for a couple of months, remove the gravel and see which hole is the driest one.

But even if you decide to limit the ternch width to 100mm as described, why not just omit the gravel?

 

[frustin]

If the trench is covered with a suitably rated steel grid then it would be able to sustain vehicle traffic over it. As I understand it, the point of the trench approach is to allow free movement of air, thus providing maximum ventilation to the trench wall, thus providing maximum moisture evaporation.
But even if you decide to limit the ternch width to 100mm as described, why not just omit the gravel?

Thank you for your reply. The drain doesnt need to sustain vehicle traffic - if you check out the plan view diagram i created, the channel needs to run along the side of the house, not across the drive. I think the drains referred to are something like these. I'm not clear what he means by fill with gravel, since you wouldnt fill those plastic channels with gravel.

 

[Flyfisher]

I was referring to your comment about a Zebcolm trench encroaching on the narrow driveway, especially around the chimney breast. Even if it encroached by 30/40cm, it wouldn't impede vehicle traffic if a strong enough grating was used.

If you limit the trench to 100mm then I agree that traffic encroachment would likely not be a problem. The issue then is whether that would be sufficient to ventilate the trench.

If the proposed drainage is as per your link then I wouldn't regard that as a Zencolm trench as it will not allow any ventilation at all. It would be effective at collecting and removing surface water but I can't see how it would provide any below-ground ventilation, which I believe is the whole point of the Zebcolm trench.

 

[Cubist]

"Within the ground floor accommodation where tested, there was dampness within the floor slabs around the edges of the room and in the lower wall surfaces." and goes on to say, "To the right-hand side of the house, a shared driveway has now been constructed leading down to parking and drive areas.

Thankyou. To me this indicates the house has damp rising from directly from the ground below BUT the level of damp detected is higher on the side abutting the shared drive. This, confirms that some relatively minor works to improve drainage would make some contribution to, or even completely eliminate, the imbalance only unless the topology of the site suggests that water run-off from the drive is making a significant contribution to the moisture content of the earth under the house.

Both a French Drain and the plastic bodied and metal grilled gulleys, suggested will do the job but the latter would only prove superior if the water can be easily channeled to lower ground, perhaps via the channels already spanning the driveway as shown in your photos, or alternatively maybe routed into the surface water drains fed by the down-pipes from the roof gutters. From an economic stand-point the French Drain would be less costly and ultimately less of a nuisance to maintain.

 

[frustin]

 

[Kearn]

What your surveyor seems to have omitted is that your walls appear to be masonry painted, whether it’s over lime or not I don’t know how he’d tell?
NHL isn’t the ideal finish anyway, and depending on the brand and grade and mix used it may be as bad as having cement render from a moisture management perspective.
The plastic paint can be seen bubbling, as the moisture can’t get out. There’s the tell tell black painted plinth. The lower few courses certainly look sodden - is this splashback from rain or coming through internally?
Equally the chimneys are both pointed in inappropriate sand and cement, again limiting their ability to moisture manage.
If it were me, I’d be starting with these areas before digging up the ground as it doesn’t appear the correct materials have been used on this building.

The fact that you say it’s timber framed would concern me even more, given those external coverings on a structural frame.

Ground levels do rise, but how are they compared to your internal floor levels and finishes .
Hard standing isn’t great from a rain a splashback perspective and needs to fall away from the building, but isn’t necessarily they end of the world.

Dare I ask what the surveyor ‘measured’ dampness with? Particularly floor slabs as mentioned…

 

[frustin]

That ACO channel i had installed in the last two days. I believe that the chimney bricks dampness is a mixture of what was splashback and absorbing water since the driveway had cement or dried kiln backing. In some cases the (bottom pic on the far left) the driveway bricks were pushed under the plinth which meant water would just drain directly under the house.

The plastic paint can be seen bubbling, as the moisture can’t get out. There’s the tell tell black painted plinth. The lower few courses certainly look sodden - is this splashback from rain or coming through internally?

If you mean the 2nd from bottom picture, that was picked up in the survey and the render is definitely some sort of modern render. I will look at getting that removed and re-rendered with fat lime in the summer.