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Shed base on uneven ground questions

MarcusMaximus

I am in the process of building a base for a new metal shed. The problem is our small garden is on a slope and the site for the shed (the only place it can go) is uneven in both directions. So I have built a sub-base out of blocks, bricks and gravel to bring the lowest part of the site up to the level of the highest. The actual base will be made of paving slabs. The site is in a corner of the garden.

Although it is now pretty much level and quite sturdy, due to the gradient one corner of one side is slightly higher than the surrounding ground while the opposite corner on the same side is about 7 inches below the surrounding ground and is also near a wall. This was unavoidable and seemed the best compromise in the situation. My concern is that even with the paving slabs in place, the lower corner will still be a bit below the surrounding ground and this might allow water to gather there when it rains, which would obviously lead to problems with the shed walls. So I need to build in some kind of drainage from the ground above it in order to help prevent this, if possible. I wonder if anyone might have any suggestions on how to achieve this?

The other problem is to do with the sides that will be raised above the surrounding ground. I am wondering how to make sure the outer slabs and the sand and cement mix under them do not move. Maybe the weight of the shed sitting on them will keep them in place but I’d like to be more sure. Of course ideally the bottom of the slabs would be a bit below ground level so as to contain them but that can’t be the case here.

Thanks in advance.

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10-10-20

Hi, good project and it sounds like you've done some good work so far. I think a few photos of the site and works would help us as we can then see the conditions and the slopes as well as understand the risks with slippage or similar. Would that be possible?

MarcusMaximus

Hi and thanks for your post and comments about the job. Yes, good idea. I'll take some pics now and get them up.

MarcusMaximus

Here are some photos. I hope they give a good idea of the issues involved. I might take some more soil off the top side which has both high and low corners in order to give more room for the shed but that won't change the conditions I've described. I still have a bit more work to do in terms of levelling but I am awaiting a delivery of sand to help with that as well as for the base.

blackbox

If the base is level there should be no build up of standing water on the slabs. The shed should be able to cope with getting wet as long as it's not left submerged.

bfclancy2

tasty blockwork 😀

MarcusMaximus

https://www.boards.ie/discussion/comment/120653860#Comment_120653860

That's good to hear. I was concerned about water gathering in that low corner but as you say once I get it level it shouldn't sit. 🤞

That's not my blockwork, that was how the gardens were built many moons ago! All my work is underneath the gravel surface..

10-10-20

The one thing that I'll say is that the plastic deck boards which you are using aren't going to work to retain that sub-layer over time, they are going to bow out and cause a slump as the lose material moves. I'd recommend that you either build-up the soil in front of them or otherwise go with a concrete 'lip' (like a curb) or even an old-style railway sleeper which has been secured to the ground using steel pins. If you can get a sleeper and drill 8mm hole into it and then find what's called rebar, cut the rebar into 33cm segments and hammer the rebar into the sleepers and subsoil. Then build-up that base again and level it with blinding sand and away you go.

https://www.goodwins.ie/products/id-30564.html

MarcusMaximus

Ok I see what you mean. In fact the top one is already bowing out a bit. The only thing is that most of what’s immediately behind them is concrete blocks. There’s only a small amount of gravel there, which I think is causing that slight bow. I can probably fairly easily remove that. I don’t think that much loose material is going to move because it’s mostly behind those blocks, apart from a small section at the very end against the wall and the ivy trunk which I did have to build up with gravel.

Does it still sound like I should replace them? The problem is the shed doors are going to be right there at that corner as it’s a cottage style shed. Here’s how it looks in a similar setting to ours, apart from the fact that it will be raised at the door end:

So of course I will need a solution that allows access to the doors, if I do still need to do something about this. I’d want to keep it as simple as possible and from that point of view I like the sound of building up the soil in front of the boards, however that would probably get in the way of access. A concrete lip or a sleeper sounds like a lot of work but if that’s what it takes.. Maybe it would be simplest to build a small wall there which could also act as a step up to the doors. Hmm, I’ll have to think about this.

Thanks for alerting me to this issue anyway.

P.S. I have been using the soil from digging out the uneven side to level the area, rather than sand. I have compacted it using a hand tamper. Does that sound alright?

MarcusMaximus

How about some lengths of rebar hammered into the ground at intervals immediately in front of those decking boards? Would that hold, especially if there isn’t much loose gravel behind them?

Lumen

You can't build a retaining wall with decking boards, rebar or no.

Sleepers will rot.

Build a proper retaining wall with blocks and mortar.

The shed will need gutters and downpipes so think about where you're going to drain those in to.

blackbox

https://www.boards.ie/discussion/comment/120656333#Comment_120656333

I hadn't noticed the deck boards. I agree with Lumen, though if you can get genuine oak railway sleepers they will probably outlast the shed. Very hard to find these days and not cheap. A little block wall would do nicely.

If your garden has decent drainage you could make a soakpit for the rainwater.

awec

I would have thought it would be very difficult to lay paving slabs on top of that soil, getting them completely level and sturdy enough that they stay completely level?

MarcusMaximus

Thanks for all the suggestions and remarks. Sounds like a retaining wall might be best, though I’ll look into the sleeper idea some more.

I will be putting down a couple of inches of a dry sand and cement mix over the soil/gravel, as well as a damp-proof membrane. The slabs will sit on the sand so I should be able to level them from there. I am following some instructions from the supplier on building a shed base so hopefully that will work out ok. 🤞

I am reasonably handy and have done a good few diy projects over the years but this is the first time I’ve had to build a base and sub-base like this so I am learning as I go along!

As you can see from the photo the shed comes with gutters and outlets on each corner that you can fit downpipes to. Yes I will have to think about where to drain them to. I will look into the soakpit idea.

GreeBo

https://www.boards.ie/discussion/comment/120656093#Comment_120656093

Soil will continue to compress, especially backfill. You really want to dig down to undisturbed solid and then backfill with whacked 804.

A filled rim would also work well

https://www.youtube.com/watch?v=9vmqJq8Rl0I

MarcusMaximus

Thanks for that. I have used a combination of blocks, bricks and gravel to build up the sub-base which now feels pretty solid. There are a lot of solid items in there so I don't think it's going to shift or sink. I stopped using the soil to level it because it didn't feel firm enough. This will all have to do because I won't be excavating it again!

Incidentally I decided to use the sleeper and rebar method for that raised section (thanks again for that 10-10-20), seemed the most straight-forward though I accept a wall would have been more robust. It also feels very solid, long may that last..

I should have mentioned before that the site is where our old wooden shed used to be. There were loads of blocks supporting it as the ground was very uneven. However it was also very firm and it never shifted over many years with that heavy shed on it. I dismantled the shed and used the blocks that were already there as ballast, albeit in a different pattern. Unorthodox I know but I think it will be ok.

MarcusMaximus

I have just finished laying the paving slabs for this base and have a question about drainage. As I mentioned before the base is raised off the ground somewhat and is sitting on a bed of about 40mm of dry sand and cement. There is a damp proof membrane under that and then a sub-base consisting of mixed hard material, as described above.

I was intending to go around the whole edge of the base and seal it with mortar so as to support all the outer slabs as well as to prevent the sand/cement mix from washing out during rainfall but it occurred to me that this might create another problem in that with the DPM underneath the bed, water might have nowhere to go except up. The slabs are tightly up against each other but still I presume some water will get down between them.

So I wonder if anyone could advise which would be best, to go ahead and put down the mortar or to leave it as it is. Thanks.

10-10-20

I think a photo of the DPM and slabs would be best. How much does the shed overlap the slabs, if at all, anyway?

GreeBo

If you don't want to build a wall you could just slope the land down/up to the correct levels.

MarcusMaximus

I won’t need to build a wall at this stage as I have put mortar in the worse gaps along the raised side and it feels pretty solid.

The shed should be smaller than the base by a few inches all round so the walls will be sitting in a bit on the slabs. The DPM is not visible as it is underneath the bed of sand and cement that is under the slabs.

The shed supplier has advised that I shouldn’t surround the whole thing with mortar due to possible drainage issues, as I suspected they might. I am not sure though as I would prefer to stabilise everything by using the mortar. Maybe just on the corners or on the most raised sections?

Some pics as requested, though obviously not of the DPM:

IMG_20230613_151839.jpg

IMG_20230616_114617.jpg

IMG_20230616_114637.jpg

IMG_20230616_114510.jpg

The jagged edge that’s visible in one of the photos is because I had to cut those slabs to fit and to leave some space to get around the shed for maintenance. All that will be hidden once the shed is up.

10-10-20

Ooooh, very tasty!

I'd put haunching between the pavers and the soil, angled away from the shed, of course.

Concrete mIx is mentioned in section 3 here:

https://www.pavingexpert.com/edging3#laying

MarcusMaximus

Thanks!

Yes if I understand you correctly, haunching is the equivalent of what I had intended to do, i.e. put mortar around the edges to seal the base to the surrounding ground. My question is whether this might cause drainage problems given there is a DPM under the sand/cement bed the slabs are sitting on, so where would any water that gets between them go?

Here is a picture of a similar base someone did which the shed supplier approves of. It appears to have mortar all around it and apparently it also has a DPM installed:

Good bases for Steel sheds (It's all about the base) - Sheds Direct Ireland.pdf

MarcusMaximus

Ah that didn’t work.

I’ll try again:

10-10-20

That's amusing! I found the original page.

https://www.shedsdirectireland.com/bases-for-sheds-2/

Anyway, back to the question on the DPM and haunching.

If I understand you correctly then yes, you'll have a problem with moisture there as you have sand and stone on top of the DPM, so the sand and stone can take on a large load of moisture and release it back into the shed as it has no other path to travel. Ideally the DPM should be above the sand and stone and under the pavers. If my understanding is incorrect then let us know.

What I was going to suggest was to lay a new DPM under the existing pavers now (annoying, I know,) with the DPM folded down about 2cms over the edge of the sand base and then haunch as I mentioned above, but that's going to create a swimming-pool during a rain shower. So what I think might be the better plan is to retain the base as it is but install the shed on the base and then install a DPM in the shed with the DPM turned up at the edges of the shed wall, then cover the DPM with a 1cm layer of blinding sand (soft sand) and then install the pavers internally in the shed. That way you avoid messing with the existing DPM and you also stabilise the base with added weight.

I do think that those pavers on the edge still need some support though, even if that was some type of partial haunching just to support their edges.

MarcusMaximus

Thanks for all that. I don’t have the heart to take up those pavers, very heavy they are too. I already took a load of them up to adjust the level the other day and now they’re staying put!

I did ask the supplier where to put the DPM and they said to put it under the dry sand and cement mix rather than over it. Also I couldn’t see how the pavers could be placed directly on the DPM and still allow for any adjustments to the level, also surely it could tear easily when placing the slabs?

There are no stones between the DPM and the pavers, just the dry sand and cement mix. Then perhaps by stone you meant the pavers?

Your suggestion makes sense however I can’t afford to buy another load of pavers. There will be a wooden floor going into the shed, probably OSB, which will sit on a frame. I could always just put a layer of DPM under that and directly on top of the existing pavers? What I might do is live with it as is and if I do get damp problems, then I could install that layer.

I agree about the edge and corner pavers needing extra support, which goes back to the original question. Maybe I’ll have to make do with a trade off of extra support vs. some potential drainage issues. Hmm.. if only things were simple! I’m keen to finish with the base and get on with erecting the shed - it’s taken me weeks to get to this stage having to fit the job in around work etc!

10-10-20

https://www.boards.ie/discussion/comment/120722877#Comment_120722877

There will be a wooden floor going into the shed, probably OSB, which will sit on a frame. I could always just put a layer of DPM under that and directly on top of the existing pavers?

I think that's your answer there! DPM under the wood - preserves the wood and keeps the damp down. Just don't forget to tuck the DPM upwards against the inside of the wall.

MarcusMaximus

Excellent, thank you! I’ll do that so.

Now, to mortar the edges or not to mortar, that is the question! 😉

MarcusMaximus

“I do think that those pavers on the edge still need some support though, even if that was some type of partial haunching just to support their edges.”

(How do you do partial quotes rather than the whole post?)

I’ve decided to go ahead and mortar the edges but leave frequent gaps for drainage. Seems like the best compromise. 🤞

MarcusMaximus

https://www.boards.ie/discussion/comment/120722899#Comment_120722899

Just coming back to this as I have finished putting up the shed and am about to lay the metal frame for the floor. I’m wondering if there might be a problem having two layers of DPM with the pavers and the bed they’re sitting on sandwiched in between? Would this not seal in any moisture leading to a build-up of damp?

standardg60

https://www.boards.ie/discussion/comment/120765321#Comment_120765321

It's still best to put in a DPM internally and lapped up the sides of the shed.

For me the original DPM is absolutely pointless and I've no idea why the shed company would recommend it. Moisture is free to collect on it from outside the shed.

10-10-20

https://www.boards.ie/discussion/comment/120765321#Comment_120765321

https://www.boards.ie/discussion/comment/120765548#Comment_120765548

As standardg60 said, we're proposing what we believe to be the best solution for the circumstance.

Anyway having the sides of the 'DPM sandwich' open means that it should dry out, but what you said is a problem of sorts.