Captions from our Drainage on a Sloping Site Video

The words from our Youtube video on fixing the drainage for houses on sloping sites are here for your reading convenience. You can watch the video on this page.

If you’ve got a house built on a sloping site and you’ve got drainage problems, then this video is for you.

Today we’re talking about where water comes from when houses have drainage problems and what you as a homeowner can do about them.

I’m Matt Cornell. I’m from Cornell Engineers and today’s video is about drainage for houses built on sloping sites.

To solve the problem with drainage on a sloping site we need to work out where the water is coming from.

So, we start with a site investigation or site visit and walk around and we ask them questions about where does the water come from when it rains?

Where’s the water go? What does it look like?

And we try to identify the water if there’s water coming from as far away as possible first before we work closer towards the house and solve the issues right against the house.

So, it’s easier and cheaper to work with water that is coming overland flow from an upstream neighbour’s place, for example, than to excavate down and re-waterproof a wall. That’s our last step. It doesn’t always work.

It hasn’t worked so far, obviously, when we’re involved in trying to fix drainage issues.

So, we’re going to work with the water that’s coming from upslope, upstream. Try and identify those water sources and work towards the problems that are closest to the house which are hardest and most expensive to fix.

So, the first place we’re going to look for water is from an upslope neighbour.

Now as structural engineers we tend to do a little bit of looking over people’s fences – whether it’s a camera or a quick look or an aerial survey but we’re looking for those contours, the lay of the land that’s running down towards our client’s house.

Don’t forget they’ve got this basement retaining wall, upstairs area at the back maybe looks about level with the ground area but it’s the water that’s running down the hill and getting in underneath the basement that we’re trying to solve.

So, we’re going to start up here on the other side of the fence to see what the lay of the land is. Is the stormwater that’s landing in this area, rainwater that’s landing in this area running down through the fence and then straight down towards our client’s property?

If that’s the case we’re going to try and solve the water entry or solve/catch some of this water up here and fix these problems up here.

We’ll come to all the solutions at the end, but this is our logic for what we’re doing to try and solve these issues.

So, there’s the upslope neighbour, there’s the upslope on own property.

When it rains does water land in our client’s yard?

Does that water then concentrate and run down towards the house that we’re talking about?

If that’s the case, then obviously we’re going to try and capture this water here and do something about that.

The next thing we’re looking for is roof gutters.

Does our client’s house have blocked gutters? Is there a lot of tree leaf litter filling up the gutters blocking them and causing those gutters to overflow?

When the gutters overflow and water lands straight on the ground, then that’s a pretty easy fix, for the sake of some gutter maintenance and leaf guards.

We can stop some of that water overflowing the gutters and landing on the ground which is then put potentially soaking into the ground and running in underneath our house.

So, we’re looking for the number of downpipes.

  • Are they connected to the stormwater system? 
  • Do they discharge downstream of the house, and are there enough of them?

The next thing we’re looking for is sandy ground and water that’s already in the ground.

When it rains does water soak into the ground, into sandy soil that’s been placed or is naturally occurring on the top layers of the ground – soaks into the ground, hits a hard clay layer down underneath and then that water runs down on top of the clay or even the rock towards the house.

This is quite difficult to identify.

Sometimes we’ll do a soil test and that’ll tell us what the soil layers are like, but it’s not always easy to identify these sandy soil layers.

That’s why we start on those upslope problems try and identify them and capture the water at that location before we start worrying about water that’s close to the house.

A sandy soil – sandy soil site – on clay underground, the water’s going to flow towards the house. We must try and pick up that water somehow.

The next thing is that basement areas, when they’re built, are built below ground and the ground has to be excavated back away from that basement area so they can build the retaining walls that are part of holding the dirt back.

That doesn’t happen without a little bit of excess soil being excavated or washing away or there’s soil in behind here and traditional logic says that this has to be a gravel backfill and that’s fine – gravel backfill is really good at getting water out that’s already in the ground but it’s terrible for allowing water into the ground that’s already on the surface.

So, gravel backfill behind a retaining wall is the correct solution for backfilling behind a retaining wall but in these drainage issues where water is getting into the basement garage or the downstairs area the gravel could be part of the problem and again we’ll come back to this and we’ll work out a way and we’ll go through some ideas on what to do to fix this.

Now that water that’s coming through the basement area is probably already coming through a retaining wall that has been built as part of the house.

These walls can be concrete masonry or Besser Blocks (which are the same thing) or it could be single or double skin brickwork.

Now the water that’s coming into the basement area of a house built on a sloping site is probably coming through a retaining wall of some sort it could be a concrete masonry Besser block wall it could be a single or double skin brickwork wall either way when this wall was built a little bit of extra dirt was excavated to allow the builder room to do his work and maybe it’s the, and maybe it’s the waterproofing on the back side of that wall that’s failed.

The area where the soil has been over excavated is traditionally filled with gravel and when this has been done well it collects the water that’s already in the soil and allows it to discharge and gets carried away by an ag drain.

But sometimes that gravel is part of the problem.

Finally, we’re looking at the waterproofing on the – so we’re looking at the waterproofing on the wall, the gravel backfill behind the wall – two of these – both of these are sources for water getting in either through the wall or under the wall and coming up through the slab in the basement area in the garage or storage area or whatever that room is being used for

The other places where water comes into a house are when water comes in and stays in the trenches.

So the trench beside or the backfill behind the wall and the gravel if that trench has been done wrong and these trenches are up and down then that water can sit there. Every time it rains finds its way into the bottom of the trench behind the wall builds up and then has plenty of time to soak through the retaining wall and into your basement area.

And then finally if the roof water from the gutters is being accidentally or even deliberately (by some plumbers) discharged into the ag pipes and into the drainage behind the retaining wall then that’s another obvious source of water in behind our retaining walls – an obvious source of water getting into the garage room, garage basement or storeroom and we obviously need to work out a way of solving that – and that’s probably one of the other easy ways.

These are the drainage problems that cause basement areas and retaining walls to leak:

  • upslope water from upslope neighbours,
  • water that lands on the client’s own property and runs towards the house,
  • roof gutters that are poorly maintained or there aren’t enough downpipes,
  • sandy ground over the top of clay ground
  • gravel backfill in behind retaining walls that allows surface water to soak into the ground (and don’t forget and some people actually forget that bit)
  • failed waterproofing on the back side of the retaining wall.
  • trenches that hold water
  • roof pipes connected to the ag pipes that are in behind the retaining wall.

Once we’ve done our investigation and worked out where the water is coming from and how it’s getting through the retaining wall, then the solutions become fairly straightforward.

We attack those water sources right where the rainwater lands and we try and do something about it there.

So water coming from an upslope neighbour we’re going to try and divert the water around the house. We can’t do it at the neighbour’s site so we’re going to have to do it on our client’s property.

We have a couple of options.

We have spoon drains that keep the water on the surface collect it and divert it around the house.

We have low walls maybe even a part of a retaining wall that does the same thing – that stops the water running towards the house, collects it and discharges it sideways.

Once it’s sideways the water can freely run down beside the house and we don’t have a problem with that.

It’s stopping the water coming in close to the house on the upslope side of the house.

Same thing for water landing on the night on the client’s own property or on your own property. You have to collect that water before it can soak into the ground and you have to take it sideways and discharge it around the house. So again we’re looking at things like spoon drains.

We’ve already done a video on spoon drains so check out that video for what a spoon drain looks like and maybe you can use that to stop some of this water coming down the slope towards your home.

So spoon drains we’ve got the other alternative for capturing some of this water but one that we like less are field inlet pits.

We’ve spoken briefly about field inlet pits on our waffle slab video so I’d encourage you to check that out. Field inlet pits are those boxes with grates on the top that allow water just to flow into the box and then leave it via a pipe.

These field inlet pits have to be done well. They have to be done at the right height so water can get into them. The pipes have to be maintained and they have to be kept clean of leaf litter and debris.

So if we have the choice we’re going to choose spoon drains over field inlet pits if we can.

So we’re still a little way away from the house but we’re capturing this water diverting it around the house with spoon drains and pits and pipes now as we get closer to the problem we’re looking at roof gutters.

So roof gutters when roofs overflow and the water soaks into the ground beside the basement area our solution is obviously going to be to keep these gutters clear.

We could use leaf guards. We want to make sure that there are enough downpipes and that they are actually connected and discharged downstream of the house.

There’s no point if the downpipes from the roof on the high side of the ground are discharging into the ground and onto the ground on the high side of the house.

If you have to take that water, carry it around the corner and allow it to run down the slope down beside the house but don’t discharge your downpipes on the uphill side of the house – that water is going to very quickly soak in behind the retaining wall and come through the garage and the basement area or this downstairs area that we’re trying to protect.

Don’t add to the problem by taking water from a roof surface putting it on the ground and allowing it to soak into the ground.

We have to capture that water divert it and allow it to discharge downstream of the house.

So the other thing we’re going to check when we’re checking downpipes is to make sure that they’re discharging downstream of the house that they are connected to the legal point of discharge so part of our investigation is often to run a hose into the roof gutter into the – or directly into the downpipe and ensure that the water is coming out at the street and that that water comes out fairly quickly.

If it doesn’t come out at all or if it takes a long time to get to the street after flowing a tap running a tap into the gutter for a long time then the water’s being built up in in the pipes or in the ground somewhere and that water really should be flowing freely straight to the legal point of discharge.

So that’s when you’re going to need a plumber to come and assess those pipes and make sure they’re connected all the way out to away from the house, downstream of the house at the legal point of discharge.

Next, we come to sandy soils built over clay soils.

This one’s a little bit trickier to identify and a little bit harder to solve.

So the water from the upstream area doesn’t look like it’s flowing overland flow towards the house but water is still getting in near the house.

So a soil test will identify the layers of the soil that there’s sand over gravel and now this is perhaps the first time we’re going to really advocate the use of an ag pipe.

Ag pipes and gravel pits are perfect for taking water out of the ground that’s already in the ground and this is ideal for sandy soil sites.

They’re not so great for clay soil sites where the clay comes right to the surface. There is no water in the ground so in this case the water is soaked in through the sand is running down towards the house on top of the clay layer.

We’re going to dig our ag pipe and capture that water before it finds its way all the way to the house the ag pipe and the agg pipes at the bottom it’s backfilled with gravel maybe a 20mm gravel and the important thing about this trench is that it doesn’t hold water.

It has to have a high end and low discharge end so the water when it hits that that rock can run along the trench and in the pipe around the house and downstream of the house.

It can discharge anywhere downstream of the house.

We don’t really mind. There’s going to be a legal point of discharge but we’re capturing the water that’s already in the ground and we’re going to use ag pipe and gravel-filled trenches for that.

The trenches have to be free draining not just the pipe! An aggi pipe, by itself, doesn’t magically make water run uphill.

So that’s our agg pipe and gravel pit and that’s going to capture any water that’s on a sandy soil that is overlying a gravel soil and that’s where the water is coming towards the house.

For the people who’ve already got gravel beside their house and water is overland flowing towards that gravel and then soaking into that gravel pit and still coming through the retaining wall then our solution is to capture that water at surface level and not allow it to soak into the gravel.

The gravel beside and behind a retaining wall is only there to capture the water that is already in the ground.

So yes we need gravel in the ground. We need it beside the retaining wall but we’re not using it to capture the overland flow water.

If we haven’t caught it upstream with those couple of other earlier solutions, the uphill neighbour’s place or water landing on your own property and there’s still water running towards the house then the ag pipe and the drain and the trench beside the house is not the place for that water to be flowing.

We need a spoon drain right beside that house and or a concrete slab that falls away from the house and that needs to be over the top of the aggregate, over the top of that gravel-filled trench, that backfill behind the retaining wall.

So a concrete slab with a spoon drain or some sort of drainage system or a spoon drain beside the house to capture that water that is still overland flowing towards the house despite our uphill attempts to capture all that water – that water then gets diverted around the side of the house and downstream of the house.

Just about our last solution to water coming through basement walls is to look at the way the wall has been waterproofed.

Quite often this is the hardest and most expensive part of the solution and often houses are built with concrete slabs in the back area, with concrete slabs already over the top of the retaining wall so access to the waterproofing on the dirt side of the retaining wall is pretty much near impossible without ripping up concrete slabs and digging right down beside this wall to reapply the waterproofing.

Sometimes it’s required but this is our least favourite way of solving the problem.

So if there’s a concrete slab there we have on occasions been able to gain access through the wall, through the wall to waterproof the back side of the wall but solving these water entry issues right at the retaining wall is our least favourite way of achieving these – of solving these problems.

That leaves us with the last issue which we’ve already touched on slightly and that is roof water from the roof gutters finding their way or being discharged deliberately into pipes that are in the gravel backfill behind a retaining wall. Ag pipes are slotted pipes – they’re full of holes.

By putting roof water into that pipe the water is going to straight away seep out of it and find its way down to the bottom of the trench.

If that trench isn’t free draining then this is the source of the water that’s coming through your wall or potentially is part of that problem.

Roof water into slotted pipes in my opinion should only be done downstream of a house if at all. Should never be done on the upstream side of the house. It could be contributing to that water that’s in the soil that’s coming through the retaining wall.

So to solve this issue we’re really in the hands of a good plumber to identify where these pipes where these downpipes run and to ensure that they are continuous pvc pipes that run all the way downstream of the house and discharge to the either the legal point of discharge or at least downstream of the house.

We don’t want that water to find its way into the trenches on the uphill side of the house.

So we’ve gone through all the problems:

  • Water’s coming into the basement area on a sloping on our house on a sloping block.
  • There’s water coming through the brickwork walls or the block wells.
  • There’s efflorescence potentially on the site.
  • Water is running across the slab.
  • There’s that dank musty smell in your basement.

We have looked at what you can do to solve these problems.

  • We’ve talked about where does this water come from.
  • We’ve looked at our upslope neighbour’s place. We’ve looked at the land on your own block to see if that land is draining towards the house.
  • We’ve talked about whether or not your roof gutters overflow or whether or not they need maintenance or if you have enough downpipes.
  • We’ve thought about whether or not sandy ground over clay soil is allowing water to run through the sandy soil towards your house and soak into the ground beside your house.
  • We’ve thought about whether or not gravel backfill beside a wall is part of that problem and if water from the overland flow sources is running into that gravel and yes it is part of that problem.
  • We’ve talked about how difficult it is to waterproof a retaining wall after a house has been built and has been backfilled potentially even underneath another part of the house because access is so difficult – this is one of our least favourite ways of solving the problem.

We have discussed that the problem that trenches sometimes hold water and we don’t want them to – we need them to be free draining and the base of a trench to be sloped just as the pipes inside those trenches are so the water can’t sit in those trenches and build up and then come through the basement retaining walls, and finally, we’ve made sure that roof water from the roof gutters is not being discharged into the aggregate behind retaining walls or into the agg pipes on the uphill side of the house.

If the roof water is going to be discharged anywhere it should be discharged downstream of the house maybe into a slotted pipe downstream of the house but definitely should be piped all the way to the legal point of discharge if possible.

I’m Matt Cornell. This has been drainage around houses on sloping sites. Let me know if you have any questions or comments and I hope this has been of benefit to you. Have a great day.