Concrete patio under potential new lawn...

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Afternoon all. :D

My rear garden was covered over with crazy paving. We would like to lay a new lawn for the boy to run about on so today I started taking up the paving to dig over and aerate the ground prior to seeding after the last of the frosts (hopefully early March).

My initial joy at the easy to dig earth under the slabs turned to dismay when I struck something hard underneath. I knew there was a sort of rectangle of concrete around the garden but assumed this was just where the original border for the flower beds had been.

Firstly I discovered that this was set as reinforced concrete with steel bars. By digging down it is about a foot deep in concrete and runs around the entire garden. I thought that maybe with enough topsoil covering it and earth either side, the grass would bed in ok over it. I've tried smashing it with hammers, pick axes and digging underneath to weaken it but it won't budge or even break.

However, when digging over the soil inside this concrete rectangle, I have discovered that there is what I can only presume is a former patio underneath. Until today this was covered over with various hardcore of old bricks and concrete, and also a good amount of earth. It's got a red tinge to it, I imagine popular of the 60's concrete fans.

My next concern was that even with enough topsoil on top, I would struggle to get much grass to grow as waterlogging could be a problem with this concrete basin in the middle of the lawn. However, I have since discovered a sort of diamond cut out shape in the middle of the concrete where there is just earth underneath. Presumably a central flower bed when the concrete was laid. The concrete appears to be around 6 inches deep laid on top of more hardcore.

So now i'm in pure optimist territory and hoping that this will provide enough drainage for the central grassed area, once I've sifted all the mortar, brick, loose concrete and other bits out and put down a decent topsoil.



So I basically had - crazy paving
3" hardcore/earth
6" concrete
a mass of hardcore (but can't dig deep enough to see)

I'm in London where rainfall levels are lower than most places but where there is a high content of clay in the soil. I have recently had building work done and the whole garden has been crushed by the weight of the bricks and associated mess in the garden. I have dug down and aerated to about 12 inches where the garden is earth and am in the process of digging over the entire content and sifting the central bit on top of the concrete patio with the hope that with topsoil I can still plant my grass.

Central concrete free area in old patio:


Reinforced steel concrete rectangle around garden:


Wide view showing rectangle of concrete and new border of bricks (laid by me today) outside it:


Close up of concrete free bit:


Another wide angle view of garden:


So, my questions are:
Is this central non-concrete area likely to offer enough drainage to the rest of the concrete basin?
If so, how deep should my topsoil go?
Will I be able to get grass to grow easily enough over the concrete rectangle up to the new flower beds?

Or do I have to bite the bullet and hire a big jack hammer and a big skip? To be fair i'd like to avoid this if possible for the obvious reason as it's a lot of work but also because my garden is already quite low, and having taken a load of paving and hardcore out, potentially taking out a further a further 8-10" of concrete and hardcore will leave me looking down quite low! The garden is already a step down of around a foot below the house!
 
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Its probably a question of cost and effort versus risk.

At the very least i think you should break the concrete up. you could leave it in situ if its well broken to allow water through. Adding as much grit to the soil will help.

A layer of 10mm gravel on top of the broken concrete would be good if its possible.
 
Its probably a question of cost and effort versus risk.
Ah....it always ends up being with these things.

At the very least i think you should break the concrete up. you could leave it in situ if its well broken to allow water through. Adding as much grit to the soil will help.
Would cracking it be acceptable. I was thinking if I fracture it in as many places as possible - or do I need to be breaking it into small bits even if leaving in situ? What sort of grit do I need to be adding?

Also meant to ask - how deep should my topsoil/soil base be before laying the grass?
 
I would be asking myself what the concrete was there for. Could it have been a septic tank? Or perhaps the remains of some old WW2 bunker?

Also I would be concerned about services and other pipes potentially in or close to the concrete. So be careful what you do without checking deeds, water board maps etc.

I have an old septic tank in my garden. There is ~12" of soil above, and I removed the top and filled it with flints and soil. It was a convenient way to dispose of the flints I dug out of the garden. Anyway, a couple of years later during a dry summer I noticed the grass was much darker, and longer, in one small area, and much shorter in a surrounding rectangle. Basically the grass was showing the outline of the concrete below. Where there was concrete, it was short, where there wasn't, it was long. I assume this is a case of moisture retention, the concrete depriving the grass above of a source of moisture that would otherwise be there.
 
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Would drilling a million holes right through the concrete allow drainage?
 
I would be asking myself what the concrete was there for. Could it have been a septic tank? Or perhaps the remains of some old WW2 bunker?
I did actually wonder about the bunker side of things with it being London but the house was built in the mid to late 40s so think it would have just avoided the need for a bunker. Prior to being built it was garages/fields (hard to imagine!)

Also I would be concerned about services and other pipes potentially in or close to the concrete. So be careful what you do without checking deeds, water board maps etc.
Thankfully this is one area that I know there are no services, having just had extensive building work completed.

Basically the grass was showing the outline of the concrete below. Where there was concrete, it was short, where there wasn't, it was long. I assume this is a case of moisture retention, the concrete depriving the grass above of a source of moisture that would otherwise be there.
This is my biggest concern. That once I've laid the lawn i'll end up with patches growing at different rates. I'll be putting down some topsoil but am unlikely to get to a 12" between the topsoil and the concrete. Unless I buy a lot of topsoil I suppose?

alan333 said:
Would drilling a million holes right through the concrete allow drainage?
I'd wondered this. It would possibly solve any drainage problem as the water would be able to percolate through. But I also think that as mentioned above, the grass with concrete below would end up much shorter than that in the surrounding rectangle without concrete.

So, I think my plan at the moment is to hire a breaker to drill it up then i'll remove some pieces and leave others. I would remove the lot but my garden is getting lower and lower and so I want to leave some as in fill to save it being so much lower than my neighbours. However, I had hoped that maybe 6" soil above the concrete/leftover concrete would be enough but reading the post above it sounds like even 12" isn't enough.
 
Ok, so I've removed and sifted all the earth that was on the old patio to remove the brick, tile, rubble and other junk.



Underneath is a full size old school patio as suspected. I will be hiring a breaker and removing the concrete from the garden before putting back the soil. However, I'm wondering about the hardcore (or similar) that will be under this patio. Could this also give me a potential drainage problem?

If so should I try and break it up/dig it over at all? Ideally I don't want to remove any more material from the ground if possible as my garden is getting lower and lower so my plan at present is once the concrete is out, to rake over and level the sifted soil that was between the two patios and put as much topsoil as I can afford on top of that. I'm aiming to have 6-8" of soil (half of which will be topsoil) on top of whatever is underneath the patio. Is this likely to be enough?
 
Grass will grow on almost anything it. its the quality that will be sacrificed by a shallower soil depth.

Hardcore and stones are not a problem but must be thoroughly dug over to remove the massive compaction.

6-8'' will be enough for a normal lawn but it may need a bit of help in dry weather.

Be prepared to feed it a couple of times a year and water when necessary
 
I only noticed the different grass grwowth rates in summer and then only when I'd not cut it for a while, so regular cutting should do the job. It is a small lawn, so you can always water it in high summer as mentioned by someone else earlier.
 
Grass will grow on almost anything it. its the quality that will be sacrificed by a shallower soil depth.

Hardcore and stones are not a problem but must be thoroughly dug over to remove the massive compaction.

6-8'' will be enough for a normal lawn but it may need a bit of help in dry weather.

Be prepared to feed it a couple of times a year and water when necessary
Thanks. I think it is likely to be extremely compacted. It's had this current patio on it for maybe 60 years, and another patio on top of that for probably another 30! I'm currently on a water meter but not really saving as much as i'd hoped so was thinking about leaving it anyway before my time is up. Will at least make watering the garden easier. ;)

Feeding it a couple of times a year and regular cutting shouldn't be a problem with it being roughly 4m x 4m.

WalksWithTurkeys said:
I only noticed the different grass grwowth rates in summer and then only when I'd not cut it for a while, so regular cutting should do the job. It is a small lawn, so you can always water it in high summer as mentioned by someone else earlier.
Sounds like regular watering will be the key if it ends up being another balmy summer in London (I can hope).

The plan is to get the concrete up on Wednesday and dig over whatever is underneath, rake everything over and put the topsoil on top if I can get it by then. I think someone on another post suggested putting some pre lawn fertiliser down. How long does that go down before seeding? I'd like to seed it around beginning of March, weather permitting.

Also would it still require regular watering when seeding at this time of year (with the SE being that little bit drier) or do I just leave it to it?
 
Fertilise at the same time as sowing but use a specific seeding fertilizer.

Watering little and frequently is essential for seeding and where most lawns fail. seed and the top layer of soil must be kept moist but not over watered.

You will need to water it everyday (if there is no rain) for at least 3 weeks. Once its well up you can reduce a bit.

theres plenty of good advice online. Have a look at lawnsmith
 
Fertilise at the same time as sowing but use a specific seeding fertilizer.

Watering little and frequently is essential for seeding and where most lawns fail. seed and the top layer of soil must be kept moist but not over watered.

You will need to water it everyday (if there is no rain) for at least 3 weeks. Once its well up you can reduce a bit.

theres plenty of good advice online. Have a look at lawnsmith
Thanks for the advice.

I've had the old patio out.


Then the earth underneath has been dug over with a fork. It actually seemed in surprisingly good health though I had to fig out a few more bricks. It is fairly dry though.

I've raked it all over, added some fertiliser and then put the topsoil on top, flattening it with my boots.


I've left it as it is this evening as its supposed to rain heavily. Am I ok to seed it tomorrow or am I best leaving it a couple of days. Advice seems contradictory (does it basically not matter then?!)...
 
The fertiliser needs to be at the surface really, it won't really have any effect if its buried under more than an inch.

Seed when your ready but a dry day will make it easier to get a good fine tilth on the soil.
 
So the grass is now growing quite nicely (in patches). Not surprisingly it looks best where it is getting plenty of sunlight and is taking longer where there is less.

There are also a couple of bare patches - i sowed more seed last week to try and get the grass to grow in these areas.

I think the longer bits may be long h for cutting (i'll try and include some pics when I get a chance) but should i wait until the rest comes up and if not do i just make an attempt at cutting the longer bits?

As always it looks good in side profile but you realise how bare some bits are when youre directly above!
 
You can give it a light trim when it needs it but make sure your mower blades are really sharp.
 

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