Build a shed Mike's way

[MikeG.]

I've put it off long enough!

Hopefully, this will save me hours of repetition:

Note that this only applies if your building does not require Building Regulations approval. If it needs to meet the regs, the base will need to be properly designed according to the local soil conditions and tree locations etc.

[img]http://i637.photobucket.com/albums/uu96/mikegarnham/Shedscan1700x2338.jpg[/img]

The concrete should be laid on:
-clean compacted hardcore, say 100 to 150 thick
-50mm sand blinding (for the protection of the DPM)
-1200 gauge DPM

Note the golden rule of walls: The Vapour Barrier goes on the warm side of the insulation!!!!. The vapour barrier in this drawing is the OSB, which is full of glue and therefore highly resistant to the passage of moisture.

Key features of this design are the brick plinth and the 25mm air gap between the frame and the back of the cladding. In the roof, it is essential that there is a 50mm clear ventilated void above the insulation, and that there is a continuous 25mm gap at the eaves (with insect mesh) to provide air movement. These features are essential to prolong the life of the building, and to keep everything inside dry.

You can omit the floating floor if you wish. My own workshop simply has the concrete slab as the floor.

I imply no structural calculations for the roof! Each roof should be designed individually, and I am always happy to help with that. I would suggest min. 150mm rafters so that you can fit 100mm of insulation in without restricting the airflow, but with some roofs the rafters will need to be much deeper for structural reasons.

The boarding can easily be replaced with render, so long as the airflow behind is maintained, and this is made easier by using a backing of building paper behind the EML (mesh) so that the render doesn't get pushed through too far into the cavity. Note that with boarding there should only be one nail per board per stud position, and that that nail should be situated about 30mm up from the bottom edge of the board.

And now a variation: with Timber Suspended Floor

[img]http://i637.photobucket.com/albums/uu96/mikegarnham/Shedscan21600x1886.jpg[/img]

I like this less, particularly because of the large step up and the resulting taller building (or reduced headroom). This isn't the only way to do this, but the principle is to have the insulation hard up under the flooring, with a continuous ventilated void below the joist.

The surface of the reduced ground level below the floor should either have a geotextile membrane or a layer of lime laid on it, or it should be treated with a weed-killer. Avoid using this design in wet/ boggy areas because of the reduced ground level below the floor.

I hope this helps.

Mike

Mod Edit:- Thanks to Myfordman for retrieving images.

 

[mickthetree]

Thanks for taking the time to post this Mike. Now watch the questions ensue...

What are the L shaped metal straps for?

 

[MikeG.]

To hold the shed down onto the bricks. Otherwise a good wind could start you thinking of the Wizard of Oz.

Mike

 

[Krysstel]

With the exception of the insulation thicknesses; exactly the way we build wooden houses in Norway, and here it's COLD !
As you say, the really important features are the air gap around the entire construction and that the vapour barrier is on the inside of the insulation.

Mark

 

[dedee]

My vote is for this to be set as a sticky as the information is clear and concise and probably one of the most discussed topics on the forum.

Andy

 

[MikeG.]

I've already asked Philly to sort that Andy..........and I expect to insert the link into a few posts in the future!

Mike

 

[Krysstel]

I posted a similar explanation in an old thread here :-
https://www.ukworkshop.co.uk/forums/timber-house-extension-t25054-30.html?highlight=insulation

Mark

 

[MikeG.]

Mark,

there is a nice breathable external sheathing board now available called Panelvent, which would do a much better job than the plasterboard from your original posting. I use it all the time, but wouldn't bother for a shed because it isn't easily obtainable from local Builder's Merchants.

The pricipal difference between a house and a shed will be in the single skin masonry plinth, which wouldn't do for a house, and in the insulation thicknesses, which also aren't enough. I even specify OSB behind all plasterboard in houses I design......it makes a much nicer wall.

Mike

 

[Krysstel]

The advantage of the waterproof coated plasterboard is it's cheap, quick and easy to use and readily available. But I agree, there's various other, probably better options.

Mark

 

[Paul Chapman]

Thanks for taking the trouble to post this, Mike.

Cheers :wink:

Paul

 

[bodgermatic]

Thanks Mike!

 

[MikeG.]

No problem guys! It was a simple calculation of self interest, though. How much time will it take me to type the same thing out every week for the next x months, measured against how long it would take me to do a drawing and a posting.

I hope I've saved myself a lot of typing!

Mike

PS Its been stickified now, too, so I will be able to find it again.

 

[dh7892]

Nice to see this "stickied". I'm sure many of us will be refering to this post in the coming months.

As a small suggestion; on some of the other forums that I visit, they have a "tutorials" section where people can place useful info. Perhaps this would be a nice this to have here.

Few questions about the actual design:

Concrete base direct on top of ground or do you recommend a bit of rubble below it?

Brick plinth butted up to the edge of the slab? I guess this makes it easier to layout the galv. straps when casting the slab. I've seen a few builds where the slab is a bit wider than the shed. Is there any reason for this?

Should the 1200 guage poly, come up the side of the slab too?

When you say that your prefered roof sheeting is Onduline, do you mean the corrugated sheets attached straight on to the rafters? Or would there be some sheet material between?

 

[MikeG.]

I have now added a variation to the original post, with a timber suspended floor in place of the concrete floor.

Few questions about the actual design:

Concrete base direct on top of ground or do you recommend a bit of rubble below it?

Brick plinth butted up to the edge of the slab? I guess this makes it easier to layout the galv. straps when casting the slab. I've seen a few builds where the slab is a bit wider than the shed. Is there any reason for this?

Should the 1200 guage poly, come up the side of the slab too?

When you say that your prefered roof sheeting is Onduline, do you mean the corrugated sheets attached straight on to the rafters? Or would there be some sheet material between?

You are quite right. The concrete should be laid on:
-clean compacted hardcore, say 100 to 150 thick
-50mm sand blinding (for the protection of the DPM)
-1200 gauge DPM

I reckon that having the slab over-size is simply creating a ledge for moisture to sit on, and is to be avoided if possible. It isn't easy casting concrete exactly accurately, so there will inevitably be a point or two where the concrete is outside the line of the bricks, but in my view this should be kept to a minimum.

I don't turn the polythene up the edge of the slab in these circumstances, but it is a bit of an academic exercise. It would be fine to turn it up if you had somewhere to turn it over and trap it, like under the bricks, but by having it loose and turned up, all it can do is trap moisture that runs down the concrete. Ideally, instead of having soil around the edge of the base you would have a trench filled with shingle.

In this design, the edge of the slab can get wet. The point is, being exposed by 50mm min all round, it also dries off. Remember, this is a shed........I wouldn't do this for a house!

As for the roofing, if I were using Onduline then I would probably using a breathable sarking membrane (Tyvek or similar) and then 50x50 battens.....and omit the ply/ OSB. However, the difficulty with this is keeping insects out of the void. The roof sheet manufacturers make a profiled infil to keep insects and vermin out, but this prevents ventilation. You would still, therefore, need a ventilated void below the Tyvek and above the insulation, open at the eaves.

My own shed roof is a hybrid, having originally been OSB with felt, which I then covered in an Onduline-like product. It was particularly simple to over-clad, so, whilst it may be more expensive, it is perfectly possible to use a sheet material such as Onduline above an OSB sub-roof.

Mike

 

[wizer]

Thanks very much for doing this Mike. I know I'll need it one day and as you say, it'll save lots of repeating yourself. Why don't you stick it in your sig?

Can I also suggest that every time you answer something that's not covered in the original post, that you edit it into that first post? That way the whole thread does not have to bee read to glean all the info we need. Otherwise you;ll continue to get repeat questions.


Hopefully Charley really is doing some work on the rest of this site and it can be added to a specific area. Pigs might fly tho...

 

[MikeG.]

Tom,

I agree with you re adding to the original post...........and I did that this morning with the suspended timber floor variation. As for adding a link in my signature......good idea, but it might take some working out! I'll have a look.

Mike

 

[yogibe4r]

Mike,

Thanks for putting this together.
Below is a design for my workshop roof I plan to build this spring. I would appreciate if you could check my sizings, especially the rafter size ( 6x2). Where the joists stop in the middle I plan to leave the ceiling open. The slightly irregularly rafter spacing in the centre is to ensure they sit above studs.
Regards, JK

[/img]

 

[MikeG.]

[/img]

Right, let's have a look.......

Seems like you need to re-post your picture, JK. Firstly, you haven't enough postings to get through our spam filter (which I can overcome), and secondly you appear to have 2 [/img] instructions at the end of the image address.

Mike

 

[yogibe4r]

2nd try's a charm...

 

[MikeG.]

OK, that's better.

Before I check the timber sizes, let's look at the principles. Are you hoping to rely on the joists to prevent the walls from spreading? There is a huge amount of good sense in having a hybrid roof like this, because it does enable access into the loft areas from inside the building, and with easy access for longer pieces of stuff than if you try and use a loft hatch. It is exactly what I am planning to do in my new workshop.

If you aren't relying on the joists to prevent spread, then your ridgebeam needs to be structural, and will be huge. If you are relying on the joists then we have to deal with the trusses which don't have ties.

Mike