Barn shed design - opinions sought - madness! Talk me out of it!

[Pallet Fancier]

Hi folks,

I'm planning a shed! I've had a mad idea! Talk me out of it!

It's 16' x 10' and about 11.5' tall (3.5m, pending planning permission).
(Metric: 4.8m x 3.0m x 3.5m)

Foundation
Concrete perimeter. I'm a fan of this because it should provide good support all around, and because living near a small water course I have to think about the occasional slight flooding (we had two inches of surface water in a couple of places after the rains, last week, but that was the first time in ten years that it's done that).

And then there's the rats, of which there are many. I don't want to make it easy for them!

Structure
Wood frame with a skin of breeze blocks.

Why wood frame?
- to keep overall cost of materials down.
- to keep the really important bits of the build comfortably within my skillset.
- to get the frame up and then a roof on it quickly, before doing the walls.
- to reduce the depth of footings needed. A self-supporting brick wall would need deeper footings. Ground is heavy clay soil with tree roots, and there is no access for machinery like a mini digger!

Question: how deep do the footings need to be for a breeze block "skin" like this, as opposed to a brick supporting wall, which would need at least 400 or 500mm?

Why breeze blocks?
- I'm within one meter of a boundary and this would provide my "substantially non-flammable" material for planning permission.
- it's good and strong, and therefore more secure.
- might get wet occasionally.
- once the roof is on, I can take my time getting the walls up as time and money allow.
- by not using sheet materials I'm more flexible in what dimensions I can build to, in terms of the floor plan (hence current working assumption of 16' x 10').
- I don't need to immediately worry about internal finishing in order to be able to use the space as soon as it's enclosed. I might even just splash some paint around and call it done! Insulation and interior cladding can all be added later, if desired.

Roof
I need good head height clear of obstructions. I thought about a standard pitched roof, and looked at ways of bracing it so as to remove the need for horizontal beams at the top of the walls (as I write, I cannot think of the correct terms! Please bear with me). Then I came across the gambrel, barn or hip roof, and this increases the internal volume even more than a double pitch.

In the image above, I've added a yellow horizontal line which more or less indicates where the top of the door would be. The eaves would extend below head height all around, which makes the brick walls shorter as well.

Roofing material? Dunno. Not thought about that, yet.

Well, there it is. Madness!

Don't let me do it!

 

[NickVanBeest]

Just do it! You know you want to...

 

[Pallet Fancier]

Just do it! You know you want to...

Don't enable me! That's incredibly irresponsible!

... how deep should my footings be, do you suppose?

 

[Jameshow]

Because if the clay and roots you really need a slab.

I'd go a ft deep at the edge and 6" in the middle.
With a layer of mesh across it.

Look at agricultural barn construction.

You want ties across the roof.

Cheers James

 

[NickVanBeest]

I'm the wrong person to ask for advice, as I tend to overengineer everything... last foundations for a shed I made were good enough to carry a 3-story steel shed

 

[Droogs]

Go on do it, and btw I can keep you going with tunnocks teacakes if you paint it purple

 

[Pallet Fancier]

But in all seriousness, I presume there is nothing that I've said about my plans which anyone regards as inherently dubious or ill-advised?

I was particularly interested in views on building the walls last, up to the roof, and on the recommended depth of footings for breeze block walls that are not structural.

 

[Jameshow]

https://www.google.com/amp/s/www.wikihow.com/Build-a-Gambrel-Roof?amp=1
Look at gambrel roof plans for inspiration

 

[glenfield2]

What are you doing about the floor and about possible flooding? Asking because our soon to be replaced shed is in a similar location.

 

[Robbo60]

Why does it need to be so high? More air to heat in winter?? Q - on the drawing you have a timber cut at 60 degrees meeting a timber cut at 70 degrees. if it is the same size timber, the faces will be different sizes. I'm not a builder but I would think if you had compacted hard core with a 200mm concrete (mesh reinforced) slab, you wouldn't need footings for the block skin? Would help with potential flooding.
Someone mentioned in a post last week to put a moat around it, but meant create a route for water to flow away from the building. Do it!

 

[julianf]

I thought that light aerated block need to be rendered etc to stop water ingress.

The issue with water ingress, I thought, was that they would be damaged by sub zero temps if wet.

Ie it's a structural consideration.

 

[OldWood]

I'll pick up on JulianF's point and will admit that when I read your post through I thought your facings were to be dense concrete blocks as 'breeze' tends to get used interchangeably. I wouldn't have thought the lightweight blocks would be advisable even with rendering if standing water is a hazard. As far as I know dense concrete blocks are waterproof and don't pass rising damp either.
Rob

 

[Woody2Shoes]

Just to clarify, all concrete blocks commonly available are water permeable - if they do get wet, they lose any insulating properties and some will be vulnerable to frost damage. You should only choose blocks specifically designed (trench blocks) to go below DPC.
The depth of your footings should be at least down to undisturbed clay all round (unlikely to be less than 3-400 minimum).
What are you planning to sit your timber on at the base of the wall?

 

[Pallet Fancier]

What are you doing about the floor and about possible flooding? Asking because our soon to be replaced shed is in a similar location.

The best floor option would be to pour it all in concrete, which is still on the table, but pushed down the list due to cost. The perimeter foundation will also have a couple of interior walls dividing it up, to provide better support and more solid floor than a few plinths would. Thus, I will make sure to add a few extra air bricks around the periphery and leave a few gaps internally. I'm also thinking about digging a French drain/soak-away inside the perimeter, but as I noted earlier, it's all an ancient river bed and is very hard going, so not sure how much difference that will make.

Other than that, if I run electric to it, I'll make sure all the outlets are at table-top height!

 

[Pallet Fancier]

Why does it need to be so high? More air to heat in winter?? Q - on the drawing you have a timber cut at 60 degrees meeting a timber cut at 70 degrees. if it is the same size timber, the faces will be different sizes. I'm not a builder but I would think if you had compacted hard core with a 200mm concrete (mesh reinforced) slab, you wouldn't need footings for the block skin? Would help with potential flooding.
Someone mentioned in a post last week to put a moat around it, but meant create a route for water to flow away from the building. Do it!

The drawing was just an example I grabbed off the internet to illustrate the roof design. I haven't drawn my own version of it, yet.

Interesting idea about the moat!

The space will be multi-purpose and have more than one user, and some of those activities need a bit of extra height.

 

[Jameshow]

If floodings an issue I wouldn't think I'd anything other than a raft foundation.

Otherwise you will have ground water coming up inside the footings.

With a dense concrete blocks the only entry for water would be the door. Even then I would coat the bottom row of blocks with bitumen to be safe.

I'd build up the walls and build the roof off on wall plates - standard construction for houses.

Cheers James

 

[Pallet Fancier]

Just to clarify, all concrete blocks commonly available are water permeable - if they do get wet, they lose any insulating properties and some will be vulnerable to frost damage. You should only choose blocks specifically designed (trench blocks) to go below DPC.
The depth of your footings should be at least down to undisturbed clay all round (unlikely to be less than 3-400 minimum).
What are you planning to sit your timber on at the base of the wall?

Trench blocks!

As for your question, it's a good question. The original design was all wood, and either used very heavily treated posts or the Japanese shou sugi ban method of charring the wood and then coating it with oil, which seems to be surprisingly effective (traditional Japanese buildings have lasted hundreds of years) and has been used in several modern buildings with good results over time.

Now that I'm thinking about a block skin, I'm not sure that original idea has changed much. I'll just have an extra "square" of poured based in the corners to sit the structural members on, so they're resting on concrete. I've seen various different approaches to this basic idea, including methods using a short concrete plinth, a membrane, even an air gap and/or a metal support (some of these come from American house building designs).

There seem to be lots of options. I am open to advice on this!

 

[mikej460]

I share your enthusiasm and subsequent headaches, so keep asking for advice on ideas and checks on progress. I'm assuming you've read MikeG's How To Build a Shed guides, although not on this forum anymore he is a very experienced architect. Because of the risk of flooding and the tree roots and the clay (it shrinks in summer and expands in winter causing heave) you really need a solid concrete re-enforced raft. I urge you to follow Mike's advice and spend a hefty amount of your budget on the concrete slab and forget the outer block skin (if security is a serious issue then thieves will just cave your door in anyway so just make sure you are adequately insured). Build a dwarf wall with DPC as shown then the timber frame, roof, insulation and outer timber cladding. The gambrel roof looks nice but, please believe me, building your own apex roof is hard enough, so I urge you to have a really hard rethink on where you need to spend your money.

My story in a nutshell: when I put a draft outline of my impending build to the forum, Mike ripped it to bits and I learned a lot from our subsequent conversations. In my case I wanted to re-use the stone effect concrete slabs that adorn the current large 1970s garage (but computer, well Mike, said nooo due to water ingress issues and weight). I have successfully built a 9m x 3.5m apex roof on the nearby outbuilding though which were built by previous owners as 3 pony stables, using a single skin of 140mm deep hollow blocks on a single concrete slab. They suffered from water ingress at the base so that I had to raise up the walls by adding one more row of blocks before building the roof then adding a thick sheet of damp proof membrane, floor insulation and concrete base inside each loose box. I also painted 3 coats of Everbuild Black Jack around the bottom row of blocks and a new set of guttering into a deep soakaway. We are also near a brook that periodically floods sending water around the outbuilding and flooding the garage. In my case I will be digging a french drain to hold this water until it can draining away back into said brook through a land drain I put in.

Anyway, sorry to be blunt but I hope it helps.

 

[DBT85]

I wouldn't other with the blocks and instead just use a cladding like Hardiplank. Will look better too and won't sponge the water up.

Lots of headroom is nice. My ties are 2.5m above the floor and there's a good 800mm above that to the actual underside of the ridge beam. With the lights I put in there people continue to think I put 8 skylights in the ceiling!