My new rear garden has had concrete slab paving for decades, resting on heavily packed and deep London Clay soil and a small amount of odd rubble. The paved area measures about 6x7m. The soil under the slabs is solid with weed roots, even where there's been no light for many years, and weeds (of all kinds) come up between the slabs continually.

I want to convert the paved area back to a nice garden and grass. The options I have heard about are:

  • Bulk replacement: excavate existing area some inches down, remove old rubble/soil/weeds, bring in new topsoil, compact a bit, lay new turf or seed. Concerns - Guaranteed success, straightforward, completed quickly, but probably quite costly as work on this scale probably would need mechanical hire and grabber removal.

  • Remove paving only (much less bulk), break up packed soil some inches down, sift manually for weeds, mix in compost/whatever, compact a bit, lay turf/seed. Concerns - no idea how much work involved and time required (might drag on a long time), soil may end up poor condition than if new good soil was introduced and need regular prevention against deteriorating, and introducing new soil implies bulk removal and cost as above so will there be any net benefit compared to the 1st solution.

I haven't had to do garden landscaping on this scale before and have no idea what I would actually have to do, nor the amount or work and cost and any practicalities involved, nor what mechanical equipment might exist could offer trade-offs between time, cost, final quality, and physical effort. I am open to hiring a contractor but will probably end up doing it myself, especially if a contractor would add a lot to the bill as expected.

I don't expect sorting it out to be cheap, but I would like to feel once it's done, it mostly stays done (other than annual maintenance) and is "low maintenance". For example, if it will help that the soil drains well, or retains water and nutrients, I'd prefer to ensure the soil the turf grows on actually has those characteristics for at least some of its depth, instead of needing to be "propped up" each year. I'd also like to ensure that any depth I work to is sufficient for the soil to retain water and nutrients enough for the grass to cope with future hot summers (ie if I only sort out the top 3 inches will there be issues with drying out/nutrients washing out). I'd like to "do it once and do it properly" as we hope to enjoy it for a long time to come.

The house is a terrace property so the only access to the garden is through or over the house itself. Large scale material movement would involve renting a conveyor, a narrow mobile digger, or at an extreme I could hire a mobile crane and hoist bulk material over the property roof (all of which could be acceptable options). Because of the residential neighbourhood and the terrace nature of the house, I'm doubtful that large scale manure and similar material (or chickens!) would be a good idea socially :-)

I need to dig up and remove about 15-18 cu.m. of packed soil and rubble and get it to the front and taken away, to get the basic ground level nearer the back of the house as I want it, so heavy bulk moving of material is already going to happen to some extent. I haven't yet decided how this will be done so that's open to being combined with the return of the paved area to turf.

Naturally I'd like to minimise effort or cost. How much work is actually involved and how deep do I have to work? What are the options I should consider and how do they stand regarding time, money, effort and final quality?

Update

The garden is mostly exposed (a few smallish trees, no overarching shade). It's north facing but raised (the house has a cutaway as it was on a hillside) so it doesn't get much shade from being behind the house. There will be beds and planting in places, and there's a small "island" in the middle (maybe 8 ft across) which has some small shrubs/trees and that will stay, but I'd like to put the bulk of the paved area back to turf. Below the paving seems to be almost entirely compacted soil and stone, any concrete or rubble layer I do find anywhere, when I get going, I can deal with. The pics show some of the area where a little paving has been lifted.

The garden is about 3m of extension + 11m of paved area (almost all raised behind the small retaining wall). The overall plan is to level the first 2.5m of that paved area level to the back door of the house so that the when it's done, the garden is "split level" - the first 2.5m (about 1/4) of the current paved area "behind" the retaining wall" is dug away and level with the back door, the current retaining wall is removed, and a new retaining wall (or gentle grass bank) replaces it, about 2.5m down the garden. About 6m of the remaining elevated 8.5m of the paved area will be mainly grassed apart from about 2.5m at the very back which will be left as-is (shed etc). The 2.5m that is at the same level as the back door will have about 50% new paving for a small rear patio, and the other 50% grassed like the rear part. So the final garden will be about 2.5m lower level (50% grass, 50% patio) + retaining wall or grass bank + 6m elevated (grass apart from the 8ft island) + 2.5m left as-is. Plant beds and so on will largely stay where they are but might be reworked or improved later, but for now I'm hoping to sort out the turfing and landscaping, not the beds. I am reckoning I might have to lower the surface level a bit to lay more topsoil, but don't know if this is necessary or not. I'm not tied to any especial ground level so if the level rises by a few inches that's not an issue.

Photos (including 1 of the paved area with most weeds cleared-for now anyway)

  • Please confirm whether your use of the term 'London clay' means you are actually in London, UK? Which bit of London, south, west? And do you only want lawn, no planting? Is it shady or sunny? – Bamboo Jul 29 '16 at 12:02
  • If you can upload a photograph of the area which shows which weeds are growing, that would be very useful too... – Bamboo Jul 29 '16 at 12:13
  • Yes, actual London actual clay :) North, Harringey sort of area.. Will try to find a pic as well, and edit in the other info asked. – Stilez Jul 29 '16 at 13:03
  • Photo awaited then. One other thing, if you've lifted some paving already, does the soil beneath look different in colour and texture from that in the planted 'island'? And it sounds like there's no concrete or tons of aggregate beneath the paving, more like compacted soil and stone, is that right? – Bamboo Jul 29 '16 at 13:15
  • photos linked and OP updated at the end. Used a photo host that allows images not to be resized, so clicking/downloading will show the full 20mpx image on at least one of them. – Stilez Jul 29 '16 at 13:31
up vote 1 down vote accepted

I'd hazard a guess that paving's been there longer - it seems to be largely crazy paving, and not laid on anything other than soil, though there may have been some sand initially. If it was done 50 years ago, there wouldn't have been sand underneath either, people used to just lay it straight onto soil if it was heavy clay soil, but there might be some areas of clinker or cinders yet to be revealed.

I'm not seeing any pernicious weeds such as bindweed, Japanese knotweed or equisetum, which is a good thing. I note also the steps seem to be fairly solid, and that the area is raised up, so I'm assuming they're going to stay, along with any retaining wall.

It's not as bad you feared, first thing to say - remove all the paving, that's the first step, also taking out any clinkers or cinders you find - if you find a patch of concrete (particularly from an old path) you might find it's very deep, but if it's not, a pickaxe should break it up. If there are the remains of a concrete pad anywhere, usually a good six to nine inches thick, I'm afraid that's a job for a kanga drill.

When you've cleared all stonework, next thing to do is dig it over, to a spade's depth, removing all weeds, paying particular attention to things like docks and dandelions, which are deep rooted. When you've loosened the soil, pick up a handful, squeeze it, see if it sticks solidly together, or just sticks a bit, with some crumbliness, compare it with a handful of soil taken from the border to see how similar it is. Examine the colour, if it's orange, that's not good, means its subsoil. I don't think it will be, it looks like there's still topsoil there. If you don't want to dig it all by hand, (use a garden fork, not a spade, it's easier with London clay) then just dig out the weeds by hand first, then turn the whole lot over with a rotovator or cultivator you've hired. If its very stony or full of pebbles, they need to be screened out, but small fragments of grit or stone are fine.

There isn't sufficient depth above the retaining wall to add a couple of inches of topsoil, so instead, use composted animal manure (from the garden centre), roughly a bagful per square metre. If you're planning on turfing this year, you'll need to turn it into the soil either with a rotovator or by hand with a fork, then leave it to settle for a good week, preferably two. Leaving it allows for settlement and consolidation, and also gives time for any bits of weed root you've missed to show themselves by starting to grow again, so a second chance to make sure you got all the weeds out. You could buy topsoil, but its expensive for the top grade, and if you don't buy top grade, it comes complete with weed pieces, weed seeds and plenty of stones, and is often clay, in other words, motorway spoil. Even if you did get some topsoil, you'd still need to add humus rich material (the composted animal manure) to up the fertility.

After that, mark out (with string or spray a mark onto the soil with water based aerosol paint) the areas where you want turf and where you want planting. For the areas you're going to turf, you may need to lower the soil level - you don't want the lawn higher than any planted areas, nor higher than the retaining wall at finished level, and turf is usually about 1 to 2 inches thick, so work that out before proceeding. That decided, rake the area to be turfed level, then walk all over it closely, on your heels, to get out any soft spots or invisible sunken area. Re rake and level. The best time to lay turf in the UK is actually October, but only if the soil is not waterlogged; late August and September's a reasonable time to do it, so long as you have a sprinkler to keep it well watered after laying. The soil should be damp with a friable, loose surface on top prior to laying turf - I suggest you get yourself a copy of D. G. Hessayon's The Lawn Expert, which tells you everything you ever needed to know about lawns, including how to lay one. It's not expensive and you might even find one in the local library you can borrow - if its a very old copy, some of the chemicals it suggests using may well now not be available, but otherwise, the method and practice recommended in there is still good.

To answer the last part of your question, in terms of labouring time, if you're on your own, you can reckon on probably a week to ten days for clearing and some preparation, depending on your level of fitness. If you paid someone to do it and they had two men working, I'd certainly expect it cleared, dug over, manured and levelled off within five days, assuming they don't come across something unexpected under the soil. In terms of final results, because the soil looks to be topsoil, even if it is clay soil, the addition of humus rich material such as composted animal manure will help to improve the clay - however, if it is really solid clay, the kind you could make pots with, and goes into a solid, sticky ball in your hand, it would be best to add in horticultural grit at a rate of a 40 litre bag per metre along with the composted animal manure.

Ongoing, ordinary, proper lawn maintenance will be necessary if you want to keep the lawn looking good, and any planted areas will do better given humus rich materials every year or couple of years. This is not because you've got clay soil, clay soil is usually more nutrient rich, but because a good garden is only as a good as the care that goes into the soil ongoing. 'Low maintenance' is certainly achievable, but that doesn't mean no maintenance - it's a term usually applied to the type of planting rather than lawns, so choosing low maintenance plants is a good idea. Which wouldn't roses (I see one in your pictures)...

UPDATE

The only reason you might need to sieve is if there's a high proportion of medium and large sized stones or pebbles. As for 'turning', if you mean digging, then of course you need to dig the area over. When it comes to incorporating composted manure (and grit if necessary), if you extract the weeds individually first, then spread over the manure and grit, then dig through that, it all gets mixed in at the same time, but will need to be left to settle for a fortnight. We're only talking about a spade's (or in this instance, a full sized garden fork, not a shorter tined border fork) depth (around 6 to 8 inches max), not double digging.

However, having now read of your desire to change levels nearer the house and build a new retaining wall, I recommend you call in a landscaper to have a look, draw up plans and give you a quote. Stripping out the retaining wall and steps, changing the levels to give a flat area outside the house is a lot of earthmoving, and the drainage directly outside the extension in the newly paved patio area will need professional consideration, particularly if you're considering a slope of lawn down to it.

  • I've added a paragraph to the update, to clarify some of the intentions. A kanga is mundane, needed one for the front concrete and the 2x4m concrete slab. So I hire it again :) What worries me is if the work required turning mixing and sieving the whole volume of soil - for example trying to mix sand/grit/compost into a 3D volume of soil a foot deep would be more work than just digging it up directly. So can you clarify that aspect and whats needed - is this "scatter on top", or "mix in throughout the depth"? That's the physical work aspect of remedying it that would mainly be a concern. – Stilez Jul 29 '16 at 15:14

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