Shed on a steep slope project - retaining wall question

[Mandrake]

I’m embarking on a fairly big project to landscape my garden which rises pretty steeply from the back of my house. It’s roughly a 20% gradient overall.

First aim is to get a timber shed built at the back which is why I’m here in the hopes of getting some advice. The site for the shed is the far right corner in the above photo. Here’s a shot closer in, the two green stakes are the rough position of the front wall of the shed, which will be 12’ wide and 10’ deep.

In order to get the shed where I want it I need to cut into the slope and build retaining wall close to the boundaries at the back and side to retain the foundations of the fence and wall. I did consider using pier and post construction but it would make the shed too high.

My question here is about the amount of space required behind the walls ie between the retaining wall and the boundary. I have read it should be as wide as your wall is high (walls will be 90cm), but I’m hoping to get a bit closer than that if at all possible. Due to the layout of the final garden every 6 inches I save here will give me an additional 18 sq ft of lawn.

I’ve dug out the area a bit to see what I’m working with, here’s a closeup of the corner which is the highest point of the garden:

You can see the uncovered pier for a fence post on the left and the wall foundations in the right. I’d be looking to have the ground behind the retaining wall just high enough to cover the pier and foundations.

So how close can I build the wall safely? I’m currently planning to use standard solid concrete blocks glued with landscape adhesive, backfilled with gravel over a 4”perforated drainage pipe. Beyond the 5 or 6 inches required for this us it really necessary to have a wide backfilled area within my boundary (local codes don’t require this). Or is this just a code thing in some countries ie the USA where most of the videos I’ve watched of people building retaining walls this is mentioned. I feel like it should be fine to build close to the fence pier, but maybe it’s different for the wall due to weight?

Also what is a minimum gap between the retaining wall and the shed wall? Is it ok for that to just be wide enough to be able to get a hand in to clear out leaves etc, as long as I ensure it’s kept clear?

 

[Spectric]

I’m currently planning to use standard solid concrete blocks glued with landscape adhesive,

You say it is a retaining wall but that looks like a wooden fence in your picture, retaining walls normally retain a mass of ground or are you saying the wall to the right is the retaining wall ? For a retaining wall you need footings and something like 8 inch hollow blocks that are then filled and often rebar used as well. Who owns that bottom fence ?

 

[artie]

am I right in assuming this is the orientation of your proposed shed?
Is the wall on the right yours?

 

[Mandrake]

You say it is a retaining wall but that looks like a wooden fence in your picture, retaining walls normally retain a mass of ground or are you saying the wall to the right is the retaining wall ? For a retaining wall you need footings and something like 8 inch hollow blocks that are then filled and often rebar used as well. Who owns that bottom fence ?

The wall and the fence in the picture are staying as is, they are both boundaries co-owned by me and the adjoining properties. The muddy earth in the pic will be dug out to lower the ground level by 90cm, the retaining wall will then be built to hold back the higher level ground the fence and wall stand on. So in the final version the top of the retaining wall is level with the pier and the bottom is 90cm liower. Does that make sense?

 

[Mandrake]

am I right in assuming this is the orientation of your proposed shed?View attachment 174312
Is the wall on the right yours?

No other way round. The two green upright stakes are roughly the front corners. Wall/fence are shared ownership with neighbours

 

[artie]

In that case a 900mm wall to hold back undisturbed clay/soil won't need much reinforcing.

If it was me I wouldn't build any closer than 3ft to the boundary, that way, in the future you or your heirs can maintain the building without recourse to the neighbours.

On the other hand if it's a flimsy wooden shed easily moved you could go closer, but damp can become a problem if it's too close to something else.

 

[Mandrake]

In that case a 900mm wall to hold back undisturbed clay/soil won't need much reinforcing.

If it was me I wouldn't build any closer than 3ft to the boundary, that way, in the future you or your heirs can maintain the building without recourse to the neighbours.

On the other hand if it's a flimsy wooden shed easily moved you could go closer, but damp can become a problem if it's too close to something else.

Yeah I was hoping to get away with 2 feet. Just enough to squeeze down to clear and paint when needed I thought. Not that it would be much fun but that extra foot equates to nearly 40 sq foot more lawn which will get used all the time so seems a fair trade.

Shed will not be movable once constructed in looking at something fairly substantial and insulated etc

 

[Fitzroy]

The back of my workshop is 50cm from the boundary wall, I chose to build in larch as it is semi-durable so should last 10yrs before I have to take off the shed cladding and replace. The cladding was installed with the building in-situ so I know it is possible. I'd not want to be in the gap painting every other year.

 

[ChaiLatte]

If you are worried about the wall overturning, consider horizontal or inclined-downwards ground anchors.

Then, in principle, you could install something like trench sheets (Trench Sheets - Standard Lap) for minimal thickness and maximum space.

Anchors to look something like these, but perhaps longer and sturdier:

https://www.fourteenacre.co.uk/shop/securing-ground-anchors/
Another example:

https://www.anchorsystems.co.uk/products/vulcan-earth-anchor/
If you are smart, install the shed in a way it can be dragged forward a foot once every five years for maintenance and then pushed back into position once done. That smartness could be as simple as screwing a piece of stainless steel strip to the underside of the shed bearers and having another piece of stainless sheet on top of the foundation pad.

 

[Mandrake]

If you are worried about the wall overturning, consider horizontal or inclined-downwards ground anchors.

Then, in principle, you could install something like trench sheets (Trench Sheets - Standard Lap) for minimal thickness and maximum space.

Anchors to look something like these, but perhaps longer and sturdier:

https://www.fourteenacre.co.uk/shop/securing-ground-anchors/
Another example:

https://www.anchorsystems.co.uk/products/vulcan-earth-anchor/
If you are smart, install the shed in a way it can be dragged forward a foot once every five years for maintenance and then pushed back into position once done. That smartness could be as simple as screwing a piece of stainless steel strip to the underside of the shed bearers and having another piece of stainless sheet on top of the foundation pad.

Thanks some interesting ideas there I had a look at those products, the trench sheets might be a good shout although there’s no access for anything but the smallest micro digger unfortunately so not sure how I’d drive them in. I suppose they could be dug in manually.

The sliding shed idea is a non starter for me as immediately in front of the shed will be another retaining wall dropping the ground level another 90cm so nowhere for it to go. I’m happy with a 2 foot gap around the shed for maintenance though. Bit fiddly but doable I think.

 

[Mandrake]

The back of my workshop is 50cm from the boundary wall, I chose to build in larch as it is semi-durable so should last 10yrs before I have to take off the shed cladding and replace. The cladding was installed with the building in-situ so I know it is possible. I'd not want to be in the gap painting every other year.

Good to know it can be done in that sort gap cheers!

 

[Cabinetman]

My post evaporated lol.
You will have great difficulty stopping the Ivy worming it’s way into the shed, seal everywhere! Including around the gutters and soffits.
Went to look at a renovation semi a few years ago and the attic couldn’t be entered as it was completely, and I do mean completely full of Ivy, I didn’t buy it!

 

[Mandrake]

My post evaporated lol.
You will have great difficulty stopping the Ivy worming it’s way into the shed, seal everywhere! Including around the gutters and soffits.
Went to look at a renovation semi a few years ago and the attic couldn’t be entered as it was completely, and I do mean completely full of Ivy, I didn’t buy it!

haha yeah I'm expecting to be clearing it regularly. Looks nice though!

 

[Mandrake]

Here’s my literal back of an envelope idea. Looking along the back wall and Very much not to scale, in case it’s not clear what I’m hoping to do here.

 

[Gordon Tarling]

Before you go too far with your plans for 600mm between boundary and shed wall, have a look at building regs as applied to sheds. IIRC, if the shed wall is made of flammable material, it must be at least 1m from the boundary.

 

[Dave T]

Hi, in my opinion, and I am not an expert (!), there are a number of issues with your cross sectional diagram. If I were afraid of pouring concrete, putting in steel rebar, and building a shallow wall, I would be looking at stone or rubble filled gabions. These will resist the sideways movement of the base of the fence posts (inevitable with your gravel infill) and will self drain. If well installed, They will support the static soil load which will add up to quite a few tonnes (4 x .0.9 x 0.9 x 0.5 m^3 is about 2.5t over a 4m length) - this will not do your shed any good at all of it all moves in heavy rain.
There are various websites which go into the technical calculations for a properly built retaining wall, but the principles are that the cross section resembles a human lower leg / foot / ankle - it needs a very strong 90degree connection (gravity alone will not be sufficient) and the longer the foundation at the “toe end” to resist toppling, the better.


Dave

 

[Dave T]

Structural foam instead of backfill is another option to reduce the weight of soil that the wall has to hold back…but this will not hold the fence posts in your situation.

 

[Mandrake]

thanks for the reply, couple of responses to your points based on my research, bearing in mind that I'm no doubt even further from being an expert myself.

If I were afraid of pouring concrete, putting in steel rebar, and building a shallow wall, I would be looking at stone or rubble filled gabions. These will resist the sideways movement of the base of the fence posts (inevitable with your gravel infill) and will self drain

I have considered concrete, but the skill necessary to construct the mould and reinforcement etc pushes this into 'just pay someone who knows what they're doing' territory for me. Which is fine if that's the best/only option, but I want to explore whether I can make something work myself first.

The gabions idea is interesting, the self draining aspect would eliminate the need for gravel backfill and drainage pipe making the whole job simpler and more secure, my only concern would be the lifespan of the steel cages when partially buried in the ground. Seems like this can be between 10-120 years depending on the exact materials and environmental conditions. I definitely want this construction to outlive me (optimistically 40 or so years) so if I can satisfy myself this is realistic then gabions copuld be the way to go.

the cross section resembles a human lower leg / foot / ankle - it needs a very strong 90degree connection (gravity alone will not be sufficient) and the longer the foundation at the “toe end” to resist toppling, the better

My understanding is gravity can be sufficient for walls up to 900mm tall, there are a number of products advertising this as a feature e.g. Secura Lite Concrete Retaining Wall System For Landscaping Projects - these are basically just faced concrete blocks. I assume they are specified to maximise friction to resist side loads. I was planning to just use standard blocks but to add some waterproof concrete adhesive which I feel should get me in the same ballpark if not be more secure than this kind of product.

 

[MikeJhn]

Concrete H fence post and concrete gravel boards, been there over twenty years now, never moved.

 

[Mandrake]

Concrete H fence post and concrete gravel boards.

Are you suggesting replacing the timber fence with these? It seems like a pretty good idea to be honest (assuming the neighbour doesn't object). Only issue would be (I probably should have mentioned this in my initial post) that thew access is really poor - narrow side path with 2 90deg corners, to even get to the path is up several steps and round a corner, so my concern is how to get these presumbly very heavy boards to the site, and whether the posts could be driven in with a micro digger. If I can figure out a way to overcome this could be the answer!