Damp issues in my new flat - French drain or reduce ground level or both?

[cammy0102]

Hi everyone,
I have some damp issues (though not huge still annoying and I see paint bubbling up in some areas) in my ground floor Victorian flat (I think - built in early 1900s). When I had the building survey done, it highlighted the ground level at the back of the house being too high. I have a patio made up of bricks on one side and concrete blocks on the other side of the building.

Recently I decided to get my garden done and at the same time do something about the damp issues. I've had a few quotes from landscape gardeners for following options:

1. Leave the current patio but dig a trench around the house (remove a row of bricks from patio to do this) and install a french drain

2. Remove the current patio, dig out further and build a new patio with sandstone plus a french drain around the house

Option 1 costs about 2K and option 2 costs about 5.5K (both options include clearing the garden, laying turf and creating flower beds).
Obviously option 2 will look much nicer and it will add more value to the house but having done a full refurbishment, I am still under a bit of financial pressure so it's not an easy decision.

However if I go for just a french drain (option 1), I don't want to later find out, it hasn't solved the problem.

I know there's a lot of people here with experience/knowledge in damp problems and how to prevent it so would love to hear your opinion on what would work in my situation.

Will I be able to get away from having just a french drain (but higher ground level)?

If I go for reduced ground level and new patio, do I still need a french drain? Could I get away from not having a french drain? I wonder not having to install a french drain will save much money?

Thanks in advance for any input.

PS: I can't see the DPC level on the building. Either the ground level is above DPC or there was no DPC installed. I can see that the floor inside is either level with ground level outside or slightly lower.

 

[Pennyviz]

Hi
Just to understand better - the patio is on the other side of the property to the damp problems? How much higher is the ground level? What is your current flooring/pointing like?

Penny

 

[88v8]

Hello and welcome to the forum.

1900.... there might be a dpc, or not. Look for slate, ideally two thicknesses. If it's been rendered over, that would be unhelpful.
Initially, the dpc should have been 9" or so above ground level.

French drain might help if there is somewhere for the water to go.
Assuming the drain is properly constructed.

But high ground level is a no-no. Should be at least 6" down.

Also, paving up to the wall is a no-no as it creates rain splash onto the wall. Especially bad if the ground is already high. There should be a gap, flower bed, 18" or so, or at the very least gravel because that deflects the rain and does not bounce it onto the wall.
If you create a French drain and then plant over the top, make sure it's well wrapped in ground membrane and amply protected by pea shingle, or eventually roots will get in there.

I'd start with the ground level and creating a gap around the house.

If the damp is localised to one or two spots, also look for bad rainwater goods - overflowing gutters or hoppers, blocked downpipes - or leaky plumbing or drains.

Whatever you do, it will be a good while before the success or otherwise becomes apparent. It takes perhaps six months to a year for a saturated wall to dry out. You may be back here for Stage II in a year's time.

Good luck.

Ivor

 

[cammy0102]

Wow you guys are fantastic! Thanks very much.

To answer the questions, the patio is at the back of the house (the house has been extended) and it's going around the back of the original house and and extension. It's L shaped. One part of the patio (the long part) is all bricks and the other side of the patio (the shorter part) is mostly concrete slabs with a row of bricks right next to the building. Patio is built right up to the building and I'm guessing this is causing significant problems with water splashing against the wall plus the ground level is already high.

I think the patio is sloping away from the house.

I talked to the landscape gardner and he siad hte french drain would consist of a a trench filled with shingles but no perforated pipe. He said it's illegal to connect a pipe to the existing drain in the garden (man hole cover is on the patio). If I need a pipe to take the water away then he'd have to connect it to a soakway which he reckons will be expensive to build.

As for reducing the ground level, I am not sure if he can reduce by 6" because the existing main drain that's under the current patio so there's only a certain amount he could lower it by. He mentioned lowering ground level 100mm - 150mm in his quote.

I think If I have french drain, I'll just leave it plain with gravel and no flower beds. How wide should this french drain be? Is the width of a brick (15-20cm) enough? Also is a french drain without a perforated pipe be OK?

I'm leaning towards forking out the extra money and getting the level lowered and getting a new patio with a french drain around the building. Will there be any adverse effects with this solution? Will it work so well and make the foundation too dry?

Thanks in advance

 

[Flyfisher]

I'd echo Ivor's comments above, especially starting with the ground level and creating a gap around the house.

Regarding French drains, I've no practical experience but from what I've read about them I've gained the impression they can certainly help to drain saturated ground but are likely to be less effective for normal 'damp' ground piled too high against a wall, hence my initial comment.

This topic concerning a similar issue might be useful reading: http://periodproperty.co.uk/forum/viewtopic.php?f=1&t=14035 and describes how Zebra created a small trench alongside a wall where the ground level was too high. The trench was then covered with a suitable grating to allow good ventilation to the newly exposed wall.

I think this is a better solution than a gravel filled trench because although gravel will certainly allow surface water to flow freely, it will also prevent good ventilation and thus effectively retain moisture. We have some gravel areas at the front of our house (fortunately well below the inside floor level) and I've been amazed to find how wet the gravel can be just an inch or two below the dry surface, even on a warm summer's day.

 

[Zebra]

A french drain will not solve penetrating damp where ground levels are too high. Although actual liquid water will drain away from against the wall, it will not be dry. Scrape away the top couple of centimetres of gravel, and it will be damp. I tried it, and took it all out again a year later and left an open gully. No, you don't need both, not unless there is water cascading down the garden towards the house in large volumes.
Thank you Flyfisher, I see you've already posted a link to my efforts!

 

[Pennyviz]

I took off built up levels and put up an area of decking over top. Seems to be working.

 

[MatthewC]

I had concrete path alongside the lounge wall at floor height. I removed the concrete up to about 10" from the wall, dug down 8" and covered with two inches of pea gravel. Being so close to the wall, there is no real danger of anyone putting a foot in it. It might not be a final solution but it has certainly helped.