Writer's Den - Shed Conversion

[Gatehouse]

Hi Guys,

First time posting here, but as per my introduction this is the project that's brought me here!
I'm a writer/journalist and I'm currently trying to build myself a nice little hideaway to escape my family...

Essentially, I've purchased a shed kit to build a 7' x 5' Shiplap Tongue & Groove Apex shed on our current driveway. (Odd choice of location, but it's the most convenient space we have at the moment)

Anyway, I'm essentially aiming to convert the shed into as much of an office-space as I can.
This includes insulation, electrics - all the usual. But I wanted a bit of advice.

First of all - the driveway dips towards the middle from both sides with a trench of crushed stone which runs along the entire length. Obviously, this isn't an ideal flat surface. So I wanted to run my solution by you guys.

My idea is to build a large rectangular frame (2.8m x 2.2m) on top of the driveway to create my own level ground. This will be lined with weed control fabric and filled with gravel, leaving a gap at the top. The fabric is mostly just to keep the gravel in due to the dip in the middle. Once tamped down, I will lay patio slabs on top to essentially create a raised platform. The slabs will be laid with several centimetres inbetween for drainage.

Will this be a sufficient platform on which to base the shed?

On top of that assembly will be the shed - the original idea was to put 3 bearers down lengthways to create airflow underneath, but my first question - would this be necessary?

Here's a diagram to help illustrate what I mean:

(Obviously, not quite to scale... hah)

So back to my first question: Would this benefit from the 75mm fence posts as a base, or should I build the bottom of the shed directly onto the paving slabs? I believe the floor of the shed has supports along the width (which in this diagram, I intend to reinforce across the length with extra timber and insulate with 25mm Kingspan/Celotex - the width of the frame is just 35mm so I don't have much internal room to play with).

(Note: I'm actually thinking of doing away with the paving slabs and laying the 75mm fence posts directly onto the gravel).

My second question is about the insulated walls - would the insulation in my diagram be suitable for home office use? I'm very mindful of condensation as I will be using the shed for extended periods every single day. There is a fixed window (which I will be double glazing) but that doesn't really help the condensation issue. My diagram shows insulation as follows: 12mm T+G (exterior), 10mm air gap, Breathable Membrane, 25mm Kingspan, Vapour Barrier, approx 12mm MDF/OSB (internal wall).

Would the air gap be beneficial in this instance? And due to the nature of Kingspan being foil-backed, would I need the Breathable/Vapour Barrier layers? I've been trying to compile as much info as possible and make decisions based on others work, but as you can see, the limited internal space between the frame gives me very little to work with.

Any other improvements/suggestions would be hugely appreciated.
And as a bit of background, I intend to use the building daily and am attempting to kit it out to be as much of an office space as I can. The internal walls will be painted, and I'm probably going to go with carpet/laminate flooring. A heater will be used for the winter months.

Also, I wondered about electrics - I was originally going to call in a sparky to fit a standard light installation and double socket... but I was thinking of running a solar-powered light and a Diall Weatherproof Outdoor electric kit to supply electricity. Not 100% ideal, but the only electrical supply is going to be for a small heater and laptop while I'm inside. Not sure it really needs a 'proper' electrical setup. But again, I wanted your opinions on that!

Cheers guys!

 

[Mike G]

Sorry not to get to look at this before..........I've been a bit busy.

Firstly, welcome to the forum.

Your base could work, but I think with some small variations it could be made a little better. You are proposing a raised base larger than the area of the floor of the shed, but with a little care it could be possible to raise a good base only of the size of the shed. I would suggest that instead of using timber in contact with the ground, you use pre-cast concrete lintels. These are strong, cheap, and don't rot or wick water. Lay a square of these the same external size as the outside dimensions of the shed frame (they can be cut), using a little mortar if necessary to fill gaps and get level, then put down your geo-textile. As you are going to insulate between your floor joists, you now fill up to the top of the lintels with shingle, then lay a DPM and build your floor with no air gap. It is critical that your shed wall boards come low down to cover the junction between the timber shed and the concrete lintels, such that any water running down the walls drips off at a point below the DPM (ie it can't run onto the top of the DPM).

I can't tell what the structure of your wall is. From your description it sounds as though it may only be 25mm thick, but that can't be right. Your wall build-up doesn't need a vapour barrier if you use OSB, because OSB is vapour resistant (it's mainly glue). So, working from inside out, I would suggest using 10mm OSB, then fully filling your wall structure with Celotex, then your breather membrane, then batten out to create a 25mm void (using insect mesh top and bottom to allow air flow but to keep insects and vermin out), and finally your boarding (starting just below DPM as described above). You won't get condensation within the structure with such an arrangement. Ventilation within the shed is another matter, though, and you will either need trickle vents, or a small extractor with a humidistat.

As for your electrics..........a 12V system with photovoltaics will be fine for lighting, and with an inverter it can run your PC..........but it will never be enough to heat the place. I would just run an armoured cable out to the shed from the house, and have a proper 240 volt supply. You will need an electrician to sign this off.

Good luck with this! Photos of work in progress are of course compulsory!

Mike

 

[Gatehouse]

Ah brilliant - that's excellent Mike, thanks so much!

I hadn't thought to use concrete but it seems like the obvious solution hah.
Don't suppose you can tell I'm new to this...

Called a local concrete firm and they're even able to cut them to size for me and it works out slightly cheaper than the timber option - sounds like a much sturdier build too. Will let you know how I get on and post some WIP pics!

With the insulation, it's working within the already framed panels, so it's only a space of 35mm in there.
I'm still waiting for it to be delivered but I have the schematics and it's been put together in ready-made panels that you then build together - nice and easy for a newbie like me but has its limitations!

Would I need a vapour barrier if using MDF? A friend has some spare so I may snag some free boards if it works. Otherwise, I'll grab some OSB - will it need treating on the inside to get a good finish when painting?

Think I'll bite the bullet and get a sparky in for the electrics - I'm not confident enough to tackle anything like that myself and I hadn't thought about the power issues. Still, I doubt it'll prove too costly for a small-ish job.

Thanks for the help with this one - ready to go with the base now I think... just need to get it done before the shed arrives then get cracking on the building! Exciting stuff for me as it's my first shed build. We'll see how it turns out hah!

 

[Mike G]

I wouldn't use MDF, personally, unless it was moisture resistant. I'm afraid I simply don't know how vapour permeable it is, so if you use it I suggest you use a vapour barrier. You could use old-fashioned hardboard instead, although it is a pain to nail through.

The 35mm frame (how do they get away with that?!!) is quite limiting. If I were you, and going to spend long periods in that shed, I would put 20mm of Celotex between the frames and at least 50 on the inside face of the frame, sandwiched by your OSB lining. You may have to use some extra battens to provide something to screw your lining board into in the corners if you do this. I'd also put a ceiling in, and fully filly the gap to the pitched roof with insulation. Your space will get a little smaller, but it would be worth it to stay a little warmer in the winter. You'll freeze otherwise, I'm afraid. The rule of thumb with Celotex is that it is twice as efficient as mineral wool (fibreglass), so 20mm is the same as only 40mm of fibreglass. Even 70mm (equiv to 140 of fibreglass) isn't great.

Painting OSB is easy enough if you get a roller for rough surfaces, and if you don't expect miracles! It helps throw the light around, but you won't be entertaining the queen in there. You can paint emulsion straight on to it without any preparation, but it's always worth watering the first coat down.