Mike G's How to Build a Shed

[John Brown]

Ok. Does anyone have a good plan for getting the PIR in between the rafters, with those pesky joists/ties in the way? I've tried doing in in one piece, but it seems impossible to fit it around the joists. I've tried two pieces, with only marginally better results... I'm shortly going to run out of swear words. Hoping the OSB won't be quite so difficult.

 

[Mike G]

Cut it about 20mm undersize (10mm all round), wedge it in place, and spray expanding foam with a foam gun into the gaps. Once it has all set, pull the wedges out and spray foam in the holes they leave.

 

[greeno]

I've built my shed, based on Mike's design.

A friend now wants a home office / garden room.

Two questions:

Why does the slab need to be 50mm proud? For aesthetics, is there anyway to do the design flush?

They're wanting it plastered inside. Would it just be a case of swapping the OSB for a vapour barrier then plasterboard? I remember that when I added the OSB it really stiffened the structure, would there be any advantage to using say 9mm OSB then plasterboard on top?

It's going to be a slow build so photos might follow in the future.

Thanks.

 

[9fingers]

I think you will need a different slab design/DPC arrangement to take it from the shed design to more of a habitable space that will stay adequately dry with less "open to the air" ventilation. You don't want an exposed edge of the floor slab with the ability wick damp into the inner floor.
Mike might have a different solution but I'd be thinking of a strip foundation maybe with steel rebar, dwarf walls with DPC 150mm above ground. An internal insulated slab floor with DPM under tucked into the DPC all round.

Bob

 

[Mike G]

Hey Greeno.....you must have read my mind. Whilst working on my garage today I was thinking that I really should update this "how to" thread with a more orthodox alternative, more suitable for a home office or any outbuilding which must be in compliance with the regulations, for whatever reason. I 'm busy, busy, but if you can stop your friend before they commit to casting a slab, there is a more suitable alternative which won't show any concrete, and which will have a DPM continuous with the DPC. More later. I'm just resting my back after a lot of heavy lifting.

 

[greeno]

Thanks Mike, no rush. The slab will probably be in a couple of months. If there's no simple alternative then we might stick with the original design.

Out of interest, what is the thinking behind the 50mm?

Any thoughts on vapour barrier + plasterboard or osb and plasterboard?

 

[greeno]

I think you will need a different slab design/DPC arrangement to take it from the shed design to more of a habitable space that will stay adequately dry with less "open to the air" ventilation. You don't want an exposed edge of the floor slab with the ability wick damp into the inner floor.
Mike might have a different solution but I'd be thinking of a strip foundation maybe with steel rebar, dwarf walls with DPC 150mm above ground. An internal insulated slab floor with DPM under tucked into the DPC all round.

Bob

Thanks for your thoughts Bob, I was watching my neighbours builders do the ground work for their extension trying to get an idea of the sequence for a more domestic building where there obviously isn't the 50mm step.

If it was mine then I might do something a little more complex but this build will be a combination of him, me and contractors to do the heavy work as we're both short of time so Cherry picking the bits we do.

 

[Mike G]

Thanks Mike, no rush. The slab will probably be in a couple of months. If there's no simple alternative then we might stick with the original design.

Out of interest, what is the thinking behind the 50mm?

Any thoughts on vapour barrier + plasterboard or osb and plasterboard?

There is a simple alternative.

The 50mm gap serves two purposes. We have an unprotected concrete slab exposed to the weather and ground water, so having it exposed to the air helps it to dry out. Secondly, that first mortar bed, between the concrete and the brick, is very vulnerable, and if that is below ground, or if there is a "shelf" of concrete outside the brickwork on which water can sit, then water will seep through it. Actually, more than seep. It can come in at a fair old rate of knots.

Let's show you the design that will work before we mess with the ordinary design to try to adapt it for an office.

 

[meccarroll]

First published in 2010


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.

The concrete should be laid on:
-clean compacted hardcore, from 75 to max. 150 thick
-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.

Corner detail
View attachment 3566


And now a variation: with Timber Suspended Floor

View attachment 592

Mod Note: Thanks to Rod having an archive of the main drawing we have been able to re-instate this. Also thanks due to Deejay who has provided a copy of the Timber Suspended Floor sketch to reinstate Mikes original diagrams. Bob

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

That's a very nice detail Mike. Just wondered what software you used to produce the drawings?

 

[meccarroll]

Well, in a lot of respects they are very helpful. One downside of the longer overhangs with the snow on the roof of a heated building like a house is the formation of ice dams. If the attic space isn't well insulated, you can get snow melting and running down toward the eaves where it freezes because the overhang isn't heated. This forms a raised ridge of ice which allows water to back up under the shingles. There's a rubber membrane that gets glued down before the shingles go on to prevent water from seeping in.

That's interesting, sound like a heat loss problem from the house if as you say the heat from the roof melts the snow. Does not sound like the overhanging eaves cause the problem but more of a lack of insulation but very interesting to hear about the potential problems if you are not aware of them.

 

[Dave R]

That's interesting, sound like a heat loss problem from the house if as you say the heat from the roof melts the snow. Does not sound like the overhanging eaves cause the problem but more of a lack of insulation but very interesting to hear about the potential problems if you are not aware of them.

Older houses often don't have enough insulation on top of the ceilings or over time the insulation has lost its loft and thus some of its insulating capabilities. Even with excellent insulation on top of the ceilings, if there's a little exposed roofing available for the sun to beat on, the temperature in the unheated attic space can still rise to above freezing and melt snow on the higher part of the roof. Of course the water will run down and then freeze again on the lower part of the roof.

I expect more could be done in most houses to prevent heat from escaping but of course that comes at a price. Also have to consider the desired radiation of heat in the summer. Where I live the temperature can vary from around -37°C in the winter to +37°C in the summer. (I've seen lower and higher in my time.)

 

[meccarroll]

Older houses often don't have enough insulation on top of the ceilings or over time the insulation has lost its loft and thus some of its insulating capabilities. Even with excellent insulation on top of the ceilings, if there's a little exposed roofing available for the sun to beat on, the temperature in the unheated attic space can still rise to above freezing and melt snow on the higher part of the roof. Of course the water will run down and then freeze again on the lower part of the roof.

I expect more could be done in most houses to prevent heat from escaping but of course that comes at a price. Also have to consider the desired radiation of heat in the summer. Where I live the temperature can vary from around -37°C in the winter to +37°C in the summer. (I've seen lower and higher in my time.)

It sounds like you are also living in North America either in the border area between the USA and Canada or Canada itself. We don't get the temperature difference here in the UK so it's hard to appreciate what you have to put up with.

 

[Mike G]

You can be certain that if I was designing a shed, let alone a house, for the US or Canada, it would look very different to the one I drew here maybe 20 years ago.

 

[Dave R]

It sounds like you are also living in North America either in the border area between the USA and Canada or Canada itself. We don't get the temperature difference here in the UK so it's hard to appreciate what you have to put up with.

Yeah. I live in Minnesota. No real benefit from an ocean to moderate temperature here.

 

[meccarroll]

Yeah. I live in Minnesota. No real benefit from an ocean to moderate temperature here.

That is quite true the sea air can help but can also make things rather cold at times depending upon the direction. I did work inland in Germany for six months a few years ago and the temprrature was about -6 degrees but feelt a lot warmer than here on the East coast of England, the North sea wind can be very cold and harsh at times. It does sound like you have a much deeper experence of cold weather than us though. Thank you for the reply. Mark

 

[motoshed]

There is a simple alternative.

The 50mm gap serves two purposes. We have an unprotected concrete slab exposed to the weather and ground water, so having it exposed to the air helps it to dry out. Secondly, that first mortar bed, between the concrete and the brick, is very vulnerable, and if that is below ground, or if there is a "shelf" of concrete outside the brickwork on which water can sit, then water will seep through it. Actually, more than seep. It can come in at a fair old rate of knots.

Let's show you the design that will work before we mess with the ordinary design to try to adapt it for an office.

Hi, I'm new here so first off thanks for all the information. It's a great resource.

You mention here that the mortar on the first course of brick is vulnerable. Is that the case even if the slab is above ground level, say 60 or 70mm? Is penetrating damp possible? Would flashing be required? Also with the brick being a thermal bridge on an otherwise insulated framed wall would there be condensation at the bottom on the brick?

I'm new to this and just started digging for the slab and moisture is my main worry. Apologies if I'm overthinking this.

 

[Mike G]

Welcome to the forum.

The lower mortar course is potentially vulnerable to water ingress, but by having the top of the slap 50+mm above ground level, and by having the brick flush with the edge of the slab, most of that vulnerability is mitigated. Is it a cold bridge? Yes......but workshops aren't usually heated like houses, don't generally have a water source, and are left for decent periods unheated. This means in practice that there is no great issue with the detail. I haven't had any problems in 30 years with workshops of this design.

If you are planning to heat the building 5 or 7 days a week to the same sort of temperatures as a house, or if you are planning on having a sink or doing lots of, say, steam bending, or if you can't achieve a 50mm step up from ground level to slab level all around, then you'll be better off building off an orthodox strip footing, having a cavity wall below DPC, and having a ground bearing slab.

 

[motoshed]

Welcome to the forum.

The lower mortar course is potentially vulnerable to water ingress, but by having the top of the slap 50+mm above ground level, and by having the brick flush with the edge of the slab, most of that vulnerability is mitigated. Is it a cold bridge? Yes......but workshops aren't usually heated like houses, don't generally have a water source, and are left for decent periods unheated. This means in practice that there is no great issue with the detail. I haven't had any problems in 30 years with workshops of this design.

If you are planning to heat the building 5 or 7 days a week to the same sort of temperatures as a house, or if you are planning on having a sink or doing lots of, say, steam bending, or if you can't achieve a 50mm step up from ground level to slab level all around, then you'll be better off building off an orthodox strip footing, having a cavity wall below DPC, and having a ground bearing slab.

Ok, that makes sense and is fairly reassuring. I would just be heating it intermittently and main use is for a motorbike and keeping tools rust free. The plan is for a 120mm slab with most of it above ground and a course or two of brick. The slab and brick is my main worry.

 

[Mike G]

You'll need at least 2 courses of bricks , but 3 would be better.

 

[motoshed]

You'll need at least 2 courses of bricks , but 3 would be better.

Even with the top of the slab above ground level by, say, 80mm?

 

[Mike G]

Even with the top of the slab above ground level by, say, 80mm?

150 is the absolute bare minimum for timber above ground level, and so you would just squeak in with a single course. However, a single course of bricks is pretty weak, and you'd have nothing to fasten the straps to to fasten down the sole plate.

 

[motoshed]

150 is the absolute bare minimum for timber above ground level, and so you would just squeak in with a single course. However, a single course of bricks is pretty weak, and you'd have nothing to fasten the straps to to fasten down the sole plate.

OK, cheers. I was planning to concrete in the straps but not keen on that as it's a bit more fiddly. Will have to get some practice at bricklaying. Thanks again for the input.

 

[Malc2098]

Just go for it. I had no bricky experience, which shows, but my workshop is still up.

You could always buy a second hand Bricky's Mate from Facebook Marketplace and sell it on when you've finished.

 

[motoshed]

Just go for it. I had no bricky experience, which shows, but my workshop is still up.

You could always buy a second hand Bricky's Mate from Facebook Marketplace and sell it on when you've finished.

Sound. I think I will just go for it and see how I get on.

 

[paulrees1]

So I was around these parts about 10 years ago and still remember this thread. I’ve actually got to the point where I’m going to build a garden office based on this design (ish) and have a question on the insulation , does anyone know if Mike is still about ?

 

[RogerS]

So I was around these parts about 10 years ago and still remember this thread. I’ve actually got to the point where I’m going to build a garden office based on this design (ish) and have a question on the insulation , does anyone know if Mike is still about ?

He most certainly is.

 

[Mike G]

....... does anyone know if Mike is still about ?

Who sir? Me sir?