How to install a patio level with top of DPC & drain it properly

[David9876]

Hello

I'm hoping to get some advice on possible options for installing and draining a patio in my garden to replace the existing decking. I've researched this for a long time through forums and Paving Expert etc and it seems replacing with new decking is the only way to maintain the existing ground height and not contravene building regulations regarding height vs. DPC and drainage (at least not without extensive additional costs), but I wanted to see if anyone could help advise if that is the case or if there is a way to do it that is generally accepted and wouldn't cause issues with the surveyor if we sell the house?

I'll try to explaining what I know (or maybe misunderstand) so far as well as ask questions.

Current garden pictured below, with existing decking level with the top of the DPC. I want to maintain this level so a) we don't have a large step down from the back door, and b) so I don't have to drop the level of the rest of the garden to match (even though it is only small garden). The house was built in 2000, and the conservatory added in 2005 by Anglian Home Improvements, walls of both buildings being approx 300mm deep so I assume they're cavity walls. We bought it 5 years ago and haven't noticed any issues with the current arrangement.

I'm planning to makeover the garden similar to my design below, and believe I can put new decking against the house walls and have it drain in the same way the existing decking does now (i.e. slight fall away from house and gaps between decking with 180mm gap underneath to a gravel base), before leading into a circular paving area that can be sloped to drain into the lawn and/or the decking.

However we would ideally like to have the whole area as paved patio rather than a mix of decking and patio, for cost, aesthetic and safety reasons (the current decking is like ice in the wet and anti-slip composite decking costs a fortune). My research suggests there are two main roadblocks to achieving this, which may have suitable solutions but I don't fully understand if they're viable in my circumstance, 1) draining the water off the patio without bridging the DPC, 2) getting rid of the water somewhere once it has drained off the patio.

1) draining water off the patio without bridging the DPC.
Due to the neighbours fence along one edge of the patio (which I'm not legally allowed to drain towards?), and the total length of the patio towards the lawn being 8-9 metres, I believe my only option is to drain the patio towards the house I believe this is ok as a last resort providing there is adequate drainage that stops water sitting against the bricks above the DPC?

Having a 200mm stone french drain between the patio and Wall B isn't desirable (I don't mind so much on Wall A) so I'd like to avoid the proposal from Paving Expert shown below

I've read people propose using a linear drain instead of the french drain proposed above, i.e. this proposal below from Paving Expert but level with the DPC instead of 75mm below. Their argument being that this would be enough to stop the DPC being bridged and 'splashing' isn't a concern because all bricks above the DPC get rained on anyway without any issues.

So it would be like using these threshold drainage designs but along all of the wall (albeit with the paving flush with the underside of the door sill and not the top surface). Is this an adequate solution, and if so could I use a slot drain along all of Wall B or would it have to be a linear channel to manage the amount of water (the width of patio draining towards Wall B is 2.75m and Wall B is approx 4m long, so approx 11sqm area to be drained)?

One added complication is an air brick along Wall A, the top of the air brick is the same height as the DPC so would be below the level of the proposed patio. Am I right in thinking neither the linear drain or the stone french drain solution would work here because they would block the air brick?

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Therefore would something similar to this shown below on Wall A be a sensible alternative and if so would I need to measure the 150mm down from the bottom of the air brick rather than the bottom of the DPC?

Assuming there is a suitable solution to draining the patio towards the house at the DPC level, and that anyone has been kind enough to still be reading this far into my War and Peace post how would I drain the drains?

2) getting rid of the water somewhere once it has drained off the patio
I'm probably more confused about this part. Am I right in thinking that all of the solutions above would need the water to then be moved and finally drained somewhere else? Either into an existing surface water drain, a french drain in the lawn, or a soakaway? If so, a soakaway seems very impractical due to cost and getting the required machinery into my back garden, so I'm discounting this idea for now (although happy to reconsider if it's easier and cheaper than I'm assuming).

The most straight forward solution would seem to be to use an existing surface water drain. The drain in the corner between Wall A and Wall B (shown in the image further up the post) seemed the perfect solution where I could run both linear channel drains into it. In fact my neighbour (in a symmetrically opposite house) had a patio installed in a similar configuration and had a linear drain connected up to this very drain. "So why are you even bothering with this post?" I hear you say. Well there are a few differences in his conservatory and garden size/shape, his patio runs away from the walls towards the lawn. There's also a question about the integrity of the people he hired to do his patio and whether they did it correctly. I'm 99% sure the drain is also connected to the kitchen waste pipes, I filled my sink and emptied it in one go and saw the water moving at the base of the external drain. My understanding is it's against regulation to empty surface water into a foul/grey water drain, is this correct? I'm still holding out some hope that I can drain a patio into this drain, otherwise why would it have been installed with an inlet from the garden/original patio if it wasn't allowed to take surface water, can anyone advise? He has since sold his house but I'm still concerned if I go down this route it will be picked up by the surveyor if we sell our house as an infringement.

If I can't use this drain the nearest guttering drain is at the front of the house and there's not enough fall to run a pipe from the patio to there without some serious excavation along the side of the house and the base of the existing downpipe. So my only other thought is installing a french drain in the lawn and connecting to this, similar to the image below. Is this an option or have I misunderstood how a french drain works? Also the corner of the lawn marked with a red hashed oval is prone to getting very wet when it rains, would the french drain help this or just make it worse by discharging more surface water in this area?

Thank you for taking the time to read this and I hope people are able to offer me some advice to close the gap between everything I've read on this topic and what I can actually apply to my circumstances.


David

 

[Halitosis]

Hi David, You've clearly done a lot of research already and have most of the answers. I'm no expert but had a similar experience when I raised my patio above DPC to door threshold (plus a similar air brick issue).

I'd suggest you can run your patio fall away from the house and even towards your neighbour's fence AS LONG AS you have suitable drainage in place - run a linear/clark drainage channel in front of the fence that drains to a soakaway under your lawn/Luna's toilet. The cost isn't much, just a fair bit of digging required. You're right that you aren't allowed to connect to a foul water drain. The french drain idea would be a "poor man's" soakaway but probably still sufficient given the size of your patio. However, if you already have drainage issues in the red zone, why not bite the bullet, do the job properly (as appears to be your way) and install some proper soakaway crates. Hire a micro-digger that can fit through your gate for a day (and driver).

On the airbrick and DPC, while not a solution you seek, you could leave a gulley between the house and patio (I have one 150mm wide) except for at the patio doorway thresholds where it is usual to breach the DPC with doorsteps anyway. We honestly have forgotten the gap is there. Many folk fill such with decorative stones but I think such might concern a surveyor. Leaving it empty makes visual inspection easy (checking its dry and draining) and prevents a build up of crud under the stones.

 

[David9876]

Hi Halitosis

Thanks for your advice. I'm more convinced on the soakaway than I am the 150mm trench but it seems some sort of trench is going to be the only viable option. Is the whole of your patio above the DPC or just at the door threshold? How deep is your trench below your DPC/air brick? Are you suggesting I connect the trench to the linear drainage channel so it too drains into the soakaway?

 

[Halitosis]

The whole of our patio is at door threshold height (so above the DPC). The trench goes down to the original ground level before we built up the patio, and what little rainwater goes into guttering I put down that connects into a downpipe/drain - however I think I would have been fine simply letting what little gets in there soak away naturally. I didn't need a soakaway as I was also able to connect my linear drainage to the same drain. Pictures here if it helps: https://www.diynot.com/diy/threads/garden-levelling-retaining-walls-and-patio.436119/

 

[David9876]

Thanks, certainly looks like a great project and top quality result. I think the 150mm gap would be a lot more noticeable in my case as my patio is so much smaller than yours and the gap along wall B is a lot more in the eye line.

Does anyone else have any more thoughts or advice?

- is connecting a linear patio drain to the already existing outside drain that also takes the kitchen sink waste water not allowed? The reading I've done suggest this is not allowed, as Halitosis also confirms, however my neighbour's patio does this and I've seen a number of houses with guttering downpipes connecting to outside drains that also take the kitchen sink waste water, so am I misunderstanding something or is this allowed if done before a certain date when new legislation was brought in? Or is installing a soakaway or French drain the only option?

- can I have my patio level with the DPC as long as I install linear drains (like below) and ensure they are kept clear of debris so as not to breech the DPC? Would this be sufficient for either the patio falling toward or away from the house wall? Or only if falling away from the house?

- if the patio does fall away from the house walls into a drain along the fence would I still need a linear drain against the wall or could I just run the patio slabs up to the bricks? I've seen this done on many patios but maybe this is only when they are below the DPC?

Thanks


David

 

[Habibmuraj]

Hi David

Hope your project is now complete

Please can you advise what did you end up doing

Because I have a similar situation the only difference is that I don’t have an Air Brick

I am installing a patio with the door level. The DPC is Approx 30-50mm above the finished Patio level

I am planning to put a slot drain and the slope is not towards the house

Anyone else has comments please advise as well

Thanks

Habib

 

[sh4d0w]

i am interested too

 

[Texas]

To be honest I think you are over thinking it!
House exterior walls get rained on in the UK all the time - above and below DPC particularly lower down where there is no protection from overhanging gutters and soffits then dry out again when it stops.

A DPC is a horizontal barrier built into the walls of a building, typically near the ground, to prevent moisture from rising up through the masonry via capillary action, a process known as rising damp.
As long as the DPC isn't bridged, for example a blob of concrete or mortar mix which starts below the DPC and ends above the DPC or any other type of material with a capillary action then the DPC should carry on doing what it is supposed to do.
Many houses are built where the soil goes right up to the house walls, albeit under DPC level. Rain hits the walls drips down over the DPC and into the ground without issue. Why all of a sudden would you now want to start installing expensive drainage systems? You could in theory just leave a gap between the raised patio and everything would continue as it has been for years before.

Bearing in mind people usually do raised patio areas after having patio or bi-fold doors installed to bring the finished floor level up to somewhere closer to that of the room floor you step out from. These types of doors usually have decent size thresholds on them which would probably cover most of the gap anyway.

Typical bi-fold door threshold:

Any rain hitting the glass would run down the glass, off the threshold onto the raised patio slabs/tiles etc. As long as there is the correct amount of fall on the raised patio floor to make the water run away from the house there shouldn't be an issue.
I would be more concerned in diverting the water from the patio area itself, particularly if it's a large one into some drainage channel and back to the the rainwater sewers.

Alternatively there are thresholds with shallow drainage channels for use above dpc:

If the gap looked unsightly between the raised patio and the house wall, you could fill it with decorative pebbles, as pebbles do not generally exhibit significant capillary action due to their large pore spaces. If you wanted to be really anal about it you could install some aco or clarke drainage below DPC draining to the nearest rainwater sewer with decorative pebbles on top (bigger the better, easier to clean out and less if any capillary action).

Another alternative would be to treat it like a brick chimney or a wall abutment with lead flashing. You could chase out the mortar a brick or two above the DPC level or in line with the underneath of the door threshold, bang some lead flashing into the brickwork, then beat the lead down past the DPC into an aco or clarke drainage channel. A vertical piece of lead would have no capillary action at all... Finish with decorative pebbles and seal the lead where it goes into the brickwork with a nice neat line of lead mate silicone sealant. Any driving rain would simply run down the wall, down the lead into the drainage channel.

Going back to the threshold thing on the patio/bi-fold doors example above. If you previously had a 6m width house WITHOUT a raised patio or drainage channels running all over the place and everything was hunky dory then decided to install some 3m bi-fold doors with raised patio to boot. As mentioned above the threshold would probably cover any gap left between the raised patio and the house wall and rain would run off a well laid raised patio to fall, so essentially you've already reduced any driving rain hitting the house and dripping down the walls to the foundations by 50%. Your just diverting it further away from the house... Hopefully to drainage and back to the sewer.

For airbricks you'd simply extend them: Google air brick extending.

 

[Ivor Windybottom]

That was a long and detailed answer to a dead thread!

 

[Texas]

That was a long and detailed answer to a dead thread!

Not necessarily a dead thread. People come onto forums all the time looking for information.