Thread: Shed foudation - sleepers on scoria

What are you going to use your shed for/what machinery do you have?
Also, how long do you want this thing to last?

Getting levels on timber floors with heavy machinery is difficult due to flexing etc. But that is only likely to be an issue if you have individual items in the 100 - 200kg+ mark and that are sensitive to level. That all said, if the separations between your sleepers are minimal, there shouldn't be much issue in that regard.

Depending on where you live, if moisture/rain is an issue, you'll want to make the drainage around the base fairly decent, as the interface between the bottom sleepers and the scoria is the ideal situation for rot (moisture + air), which even though the timber is treated, can still occur. A free draining compacting aggregate would be better for this application.

I don't know what the load bearing qualities of scoria are - it can be a bit crumbly sometimes (at least, the stuff in my BBQ is), so settling may eventually be an issue. It will need to be able to be compacted or even by itself it will settle, which will cause annoying movements in your floor over time. 50mm isn't a lot for a compacted bed, so the footing of your slab will really rely on the soil below your grass, which being topsoil has a lot of organic matter that will break down over time.

It will cost a little more, but you're probably better off making concrete post footings and then building the shed up on gal supports and then a standard type bearer and joist frame. You only need about 25 - 50mm separation under the floor and ventilation will no longer be an issue. Downside of course is that this costs a little more.

All this kind of depends on how long you want the shed to be in place. With what you propose above, 5 years - probably ok, 10 years, maybe ok. Past that - highly dependent on your location conditions and luck.

I'm all up for being proven wrong if someone else has a different experience.

Keep in mind that sleepers are made from low grade timbers and can be quite happy to pretzel like mad - I'd prefer to use at least construction grade timber, and maybe two layers of yellow tongue if you have something heavy going on there.

Going by the size you stated I take it it's a garden shed? I did something similar except I put down several bearers then put a layer of yellow tongue on top of it all. The only problem was that I didn't put any flashing down, leading to water leaking inside.
Have seen on BHG where they put down some cement blocks to take place of a foundation, for a patio floor.
Personally, I think you'll have problems with the way you were thinking of.
Kryn

Originally Posted by Maddog78

I currently have a shed 2.33m x 3.84m on a concrete slab. Unfortunately this is in the way of me building a patio roof off the back of the house.

I want to move it to another part of the yard but don't want a concrete slab as the floor of the shed.

Below I have outlined what I am planning to do and some questions if anyone has any suggestions.

I was thinking of removing the grass in the proposed spot and placing 50mm of scoria in 2.4m x 3.9m to make it the same height as the soil.

On top of the scoria I am going to get 7 treated H4 sleeper parallel to each other and then join them with sleepers running perpindcular.

Once that is in place I am going to put plywood (probably marine) on top and then attach my shed to the new base.

I have worked out it will cost me around $400 all up including screws.

this doesn't seem enough. I'd expect 3 sheets of 19 or 21mm marine ply to cost more than $400. 12mm ply, would IMO be too flexible for flooring.

Originally Posted by Maddog78

What I would like to know is can anyone give me their opinion on the following
1. Does this sound like a sound floor for the shed?

I'd be happier if the scoria was 80-100mm thick, compacted with the surface 50mm above soil level. Then if the shed is basicaly just a garden shed, I'd use 450 x 450 concrete pavers for the floor.
If the shed is also a work space, and you want a moisture proof floor, I'd look to build the new floor like that for a conventional building -- using stumps, bearers and joists to support the plywood floor. In which case, you wouldn't need the scoria. But personally I think in that case, a concrete slab will be a lot less hassle.

Originally Posted by Maddog78

4. Have I missed anything?

drainage around the shed and from the shed roof. Leveling the foundation.

I've built at least 6 sheds in a similar way but they've only been as storage or garden . The oldest is coming up for its 20th birthday and was done using lilydale topping over sand as the base(all compacted) with a raft of h4 pine bearers interconnected with yellow tongue glued and nailed.

Thanks for all the replies.
Sorry if I didn't make it clear. This is only a garden shed. It will mainly hold my lawn mower and tools etc.
Obviously I like fubar's idea as it is similar to what I intended. Are you saying the yellow tongue actually acts like the joists and the sleepers are the bearers so no need for the perpendicular sleepers?
Also worried the yellow tongue would get wet on the sides and end up detioriating from there. Maybe the flashing around the sides suggested by KBs PensNmore would counteract that?
I am in Sydney and my backyard never gets flooded even after the most recent storms which were quite bad so I thought the scoria (happy to use another material if better or more secure) for drainage would be enough.
Any further help would be appreciated.

sorry I wasn't clear in my description.
excavated to 6" depth put down and compacted 2"sand then lilydale topping to ground level laid 200 x75 mm sleepers on their side put also on side 90x45 across then yellow tongue and the sheds fit over the edge and it's a step up of about 150mm into shed no water inside. oh and screwed shed into edge of 2 slepers

Originally Posted by Maddog78

Thanks for all the replies.
Sorry if I didn't make it clear. This is only a garden shed. It will mainly hold my lawn mower and tools etc.

In that case I suggest placing the shed on 450 x 450 pavers

For a garden storage shed, I second the use of large concrete pavers, just scratch back the ground, a 50mm bed of sand (compacted) and the pavers down and you're done. If you're being really particular, get some mortar or self-levelling floor mix to seal up the gaps, but otherwise, that'll be fine, especially if you didn't flood this summer just gone.

Hi Ian. Thanks. The reason for the wood is to get the shed off the ground so that I don't have to put some things on bricks etc. to get them off the ground if water does get into the shed as it does in it's current position.
Thanks Fubar. A bit confused by what you are saying.
As I understand it I am still connecting my shed to the sleepers but on the inside of the shed creating another platform from 90m x 45m H3? timber and placing yellow tongue on top to avoid the water issue and I assume actually increase airflow as there will be a slight gap around the edges on the inside?
Also I have tried to find Lillydale topping in Sydney near me and it is not available. Would road base do or is the drainage not very good with this? I am going to email the landscape place to see if they have a suggestion but if anybody else has one it would be appreciated.
Thanks.

Hi, the landscape place has suggested metal dust. It seems like the stuff compacts well enough. Any reason I shouldn't use it?

AKA crusher dust

I know of no reason not to use it.
If leveled 50mm above the existing surface, I see no reason for raising the floor further.

Don't forget the higher the shed floor above ground level, the longer the ramp you need to get the mower in and out.

20mm Crushed Rock and a half day hire of a wacker plate will do what you need if it's just a storage shed for the lawn equipment. If you really want a smooth but long lasting finish, get them to mix half 20mm crushed rock and Metal dust. If you hire a wacker, keep the rock/metal dust moist, it'll pack tighter and last longer.

This is what 40mm Crushed Rock looks like that was simply packed by driving my 5000kilo Positrack over it and one or two of my trucks that parks there everyday. Truck is about 9000 kilos empty and 16,000 kilos full

image.jpg image.jpg image.jpg

Thanks for the ideas. I am in the process of removing the grass but I have injured myself (not removing the grass) so I will update the final set up once it has been made.