Shed on a steep slope advice part 2

[Mandrake]

A few weeks ago I started this thread about building a retaining wall so I can build a shed. I settled on using gravel boards and h-posts for the wall, not conventional I know but it seemed like a good solution in order to maximise space and the wall is only 90cm so they should be strong enough.

I had a builder friend come and do an silly person check on my plans, and he thought it should work fine, but recommended pouring the shed base right up to the retaining wall, to resist any lateral movement of the posts. Here's a rough mockup I did in sketchup:

Shed is 3.6m x 3m on a 100mm concrete base that extends past the shed at the top and right to meet the walls

So couple of questions I had on some of the details at the marked points

A - what would you recommend as a distance here between the base and the block retaining wall (yet to be built, planning to do it after the shed is finished). My instinct is you should be able to have this quite tight, maybe just a few inches (backfilled with gravel and with a draininage pipe of course), as the concrete base should be stable, but if that's wrong please let me know.

B - with the shed sealed to the base, and the concrete poured up to the gravel boards, this area becomes a water trap, whats the best way to solve that? I was thinking maybe pour the the whole thing so the concrete under the shed is an inch higher than the bit that runs to the gravel boards, or maybe just forming a small channel where the red line is - but could that need a thicker base as I'd be weakening the concrete?

C - this is just going to be a soil slope, I want to avoid any more wall here if at all possible, so how steep can this part be without needing a retaining wall? I was hoping to have it maybe 30 degrees.

Thanks!

 

[Jameshow]

Just a couple of ideas.

You could make your slab 4x3.5m so a minimum of 400mm all round?

You could put a piece of 2x2 against the back wall temporarily say a inch or two higher than the front level this would mean your slab would drain back to front. You could then lift this 2x2 and you would have a drain to remove water from the retaining wall.

 

[ChaiLatte]

recommended pouring the shed base right up to the retaining wall, to resist any lateral movement of the posts.

(B) Do what exactly he says and locally to the posts pour the slab up to them to stop them moving (maybe 150mm each side of the post centreline). For the rest of the 3m span of the gravel board, line the earth with membrane and pea shingle it. If you are really worried, add another pier at mid span of the gravel board.

A clever person would make the 300 strip 10mm higher at its midpoint so it sheds water into the gravel trap adjacent.

In what direction does the shed roof fall? Will you be installing a rainwater gutter on the shed? If not, the narrow strip of slab has to cope with a lot of water falling from the shed roof, which will bounce up and wet the shed sides (hence the suggestion above of a porous gravel surface).

(C) Have a look at embankment reinforcement (https://www.tensar.co.uk/applicatio...lls-reinforced-slopes/steep-reinforced-slopes). They claim up to 70 degrees. With a more wallet-friendly application of the principles, you could comfortably exceed the 30 degrees you state.

 

[Mandrake]

A clever person would

I've been called many things in my time...

make the 300 strip 10mm higher at its midpoint so it sheds water into the gravel trap adjacent.

The slab is 50mm above grade (as it's currently designed anyway) so unless I raise the bottom gravel board, the slab would be in contact with the board preventing proper runoff to that side, same situation at the back.

Only a rough sketch but you get the idea. I could lay some extra gravel to raise it up a bit, is that what you'd suggest? So then the concrete would be in contact with the posts and some gravel inbetween, allowing some runoff. However I'd wonder about what happens to the water then. I had planned to put a 4" perforated pipe behind all the gravel boards, but that wouldnt be a great position to help with the runoff, so wouldn't this just end up dumping all the runoff around my retaining wall piers?

I feel like ideally I would be taking all the water from that concrete strip away to join the drainage system at the front of the shed, here's a diagram of what I mean:

In what direction does the shed roof fall? Will you be installing a rainwater gutter on the shed? If not, the narrow strip of slab has to cope with a lot of water falling from the shed roof, which will bounce up and wet the shed sides (hence the suggestion above of a porous gravel surface).

Was planning a pent roof high side at the front, mostly for aesthetics, with a guttering at the back, hadnt thought through what to do with the water from there though to be honest, probably I'd need more guttering at the side and a downpipe to bring it to the drain.

Having it guttered at least means runoff issue is not as big I guess, hence me considering small fixes like a channel formed into the slab.

(C) Have a look at embankment reinforcement (https://www.tensar.co.uk/applicatio...lls-reinforced-slopes/steep-reinforced-slopes). They claim up to 70 degrees. With a more wallet-friendly application of the principles, you could comfortably exceed the 30 degrees you state.

Very interesting thanks

 

[Mandrake]

(B) Do what exactly he says and locally to the posts pour the slab up to them to stop them moving (maybe 150mm each side of the post centreline). For the rest of the 3m span of the gravel board, line the earth with membrane and pea shingle it. If you are really worried, add another pier at mid span of the gravel board.

Sorry just reread this and understand what you're suggesting now, about the gravel.

 

[BillHoughton]

Dirt buildup against a structure on a slope is inevitable. I'd consider leaving a trench wide enough to stand in, with a retaining wall built on the opposite side of the trench from the shed, with drain tile behind the retaining wall, the drain tile then brought to daylight (exiting) somewhere below the shed. You can then clear out the trench from time to time. I had to do this on the old tankhouse (used to have a water tank on top) that's got my shop in it, because the grade has, over 125 years, built up to be a good foot/300 mm above the slab.

 

[Sachakins]

I understand you don't want another concrete panel wall, but adding a third retaining wall, then French drains with gravel on both sides and back would solve all your problems.
Yes it's added expense, but the result is far better and far less prone to future problems.
Guttering at least across the back is a must IMHO, with down pipes at each end and then further down pipes run at angles from back right into your front drain.
This reduces water load on the back and side French drains and reduces back splashing up onto shed walls.

 

[Mandrake]

I understand you don't want another concrete panel wall, but adding a third retaining wall, then French drains with gravel on both sides and back would solve all your problems.

The issue with french drains is they wouldn't allow me to pour the slab up to the retaining wall to strengthen it against lateral movement.

Guttering at least across the back is a must IMHO, with down pipes at each end and then further down pipes run at angles from back right into your front drain.

Is it not possible to direct the water from the guttering at the back to all come around the right side to the drain?

 

[Sachakins]

Looking at the post distance, and using full length 9 foot posts, the depth of the posts should be sufficient to hold the earth back. But if that's a worry, then cast the base up to the panels and set guttering into slab surface to make a drainage channel, across the back and down each side to drain to the front and either leave them in place or remove them. You could also do a couple of channels from back to front under the shed too, just ensure they don't land in line of shed base joists, assuming the floor joists run front to back, not side to side, as side to side joists will act as a water trap.

Also concrete up to base boards will add strength to base of retaining panels, true, but it's the posts doing most of the work for the panels above the slab.

Yes, guttering to one side would work, but I suggested one each side, so in the event of a blockage in the rear gutter then water can safely run off the other end, whereas with only one downpipe, a blockage near its entrance would cause an instant over flow onto back wall of shed.

 

[Mandrake]

set guttering into slab surface to make a drainage channel, across the back and down each side to drain to the front

Great yeah that sounds good. I was planning a 4 inch slab so I guess with this approach I’d need to make that thicker? Since I’m using the slab to reinforce the fence laterally

You could also do a couple of channels from back to front under the shed too

I was planning to seal the perimeter of the shed base with gasket tape for air tightness (it’s more of a workshop/garden room really, insulated etc) so I’ll have to forego these.

Yes, guttering to one side would work, but I suggested one each side, so in the event of a blockage in the rear gutter then water can safely run off the other end, whereas with only one downpipe, a blockage near its entrance would cause an instant over flow onto back wall of shed.

Ok that makes sense thanks

 

[Sachakins]

You really want air movement under the base, it helps to prevent damp on the underside.

4 inch is plenty, just use the mini guttering, not the full size house stuff.

 

[pauljhaigh58]

My thoughts are
1 cast shed slab first
2 leave 600 mm gap between shed-to walls (access to clear away debris)/maintainance
3 set plastic drainage grate as in font of garages 25mm lower than slab and concrete up to
4 the banking will need an upstanding to prevent storm water flooding shed
5 as said before set shed up off floor also put land drain behind wall as belt and braces

 

[MikeJhn]

This is being way over thought, return the retaining wall along side C, over dig the base and put in a suspended timber floor, damp proof membrane at the back of the walls lapped onto a membrane under the suspended floor sloped towards the front, drainage through the front retaining wall, as others have said gutter across the back downpipe along the sides to the front, my constructed base has been there for over twenty years and the wall and posts have not moved, unless there is excessive loading behind the wall it will not move.

 

[Mandrake]

This is being way over thought, return the retaining wall along side C, over dig the base and put in a suspended timber floor, damp proof membrane at the back of the walls lapped onto a membrane under the suspended floor sloped towards the front, drainage through the front retaining wall, as others have said gutter across the back downpipe along the sides to the front, my constructed base has been there for over twenty years and the wall and posts have not moved, unless there is excessive loading behind the wall it will not move.

There's a 7ft garden wall 2 feet away from the wall on the right, hence the plan to use the base to resist lateral movement in the posts

 

[MikeJhn]

What lateral load would than 7ft garden wall put onto the retaining wall, if any? what is on the other side of that wall.

 

[Mandrake]

What lateral load would than 7ft garden wall put onto the retaining wall, if any? what is on the other side of that wall.

the ground on the other side of the wall is roughly 700mm higher than on my side

 

[MikeJhn]

The angle of repose is 45degs so any thrust from that ground will be at its maximum 350mm above the base of the retaining wall, as long as vehicles are not driving down that side of your wall it should not be a problem.

 

[Mandrake]

The angle of repose is 45degs so any thrust from that ground will be at its maximum 350mm above the base of the retaining wall, as long as vehicles are not driving down that side of your wall it should not be a problem.

I've had advice from a builder on the general design of what I'm doing, and I'm going to stick with that. I'm just looking for advice on the best way to approach drainage within that design.

 

[MikeJhn]

@Mandrake

You have a private message.