Insulated timber-framed Garden Room

Bagheera · 3 Jan 2017

Hi folks,

Happy new year to you all! It's that time of year where thoughts turn to DIY projects for the coming year so for my first post on this forum I wanted to outline my plans for a self-built insulated garden room (all the rage at the moment I gather from the number of posts on here) and get some advice along the way. I plan to update my progress regularly with plenty of photos.

At the end of the garden I have an existing garage and attached greenhouse. The greenhouse has seen much better days and the garage has subsided on one side causing a crack down the middle of the floor. Behind the garage runs an alleyway, the land of which belongs to the house but needs to be kept clear for others; though in truth nobody uses it so it gets very overgrown in summer.
I had thought about trying to save the main garage but as it's set back 3 metres (for a car to turn into and out of the garage) from the alleyway it protrudes far too much into the garden for my liking (especially if I have a rear extension to the house at some point which will make the garden very small). Also it's single brick with 3 double brick piers which isn't the strongest arrangement.
Nobody on the street uses the garages at the back to park their car in anymore and I have no plans to do so, nor would I think any future buyer would either so I'm not bothered about losing it for that function.
Here are some pics to get to grips with the layout etc:

Garage as seen looking down from house

Garage and greenhouse closer up. Garden gate to alleyway visible

Current layout and new garden room plan. See how it makes use of empty space on the alleyway side of the garage and gives back some of my garden.

The next few months will be taken up with knocking down the garage and greenhouse. Then I will embark on building the garden room. Its initial use will be a woodworking shop, but I want to keep it's options open to be used as a gym, an office or perhaps even a small flat at some point (aware of building regs/planning etc for that).
Here is my sketchup design-as you can see it's not a big windowy-bifolding door affair. Just some french doors and a couple of windows. The little window on the right will just be framed in in case I put a small toilet/showeroom there at some point in the future when I can put a window in easily.

The dimensions will be roughly 6.5m x 5m which will create an internal floorspace of 30m2 and therefore below that required for planning. The height will be 2.5m at it's max with a flat roof (no planning required).
Question 1: The front, left and back will be more than 1m from a boundary but the right hand side, I had hoped to build right up next to the boundary, as the garage is currently. As it so happens, my mum owns that house next door so logistically it wont be a problem. However, to be within 1m of a boundary the building needs to be "made of substantially non-combustible material". Obviously my design isn't. I've read that fire-proof paint or cement board can be used instead to make it non-combustible. Is this correct, does it have to be all over the building or just on the boundary side. Do you have any other suggestions or explanations?

Question 2: The foundations are confounding me the most. What would be best for this structure? Ive seen slab foundations, strip foundations along the edges and in the centre to support and I've seen block ones used like Swift Plinths. Foundations are where I'm most uncertain and inexperienced for the build so any suggestions are very welcome.

Question 3: the walls will be constructed like this (from outside to inside):

Cladding
Timber or plastic battens
Breathable membrane
OSB3
Studwork/Celoxtex or similar insulation between studs
OSB2 (can be swapped for plasterboard down the line)

Am I missing anything, do I need a vapour barrier?

Question 4: I had planned (as in the sketchup drawing) to use 2x4s for the walls thinking these would be 100mm+. I recently discovered that a 2x4 is not 2x4 but instead about 38mm x 89mm! How confusing. My 100mm celotex plan has gone out the window. Will 75mm be warm enough or should I be using 2x6s (which are also not 2x6

)? Incidentally, if I go to 2x6 can I then change to 24 inches on centre rather than 16?

Question 5: the roof rafters will span a width of nearly 5m. according to this chart http://www.home-extension.co.uk/tech2.html , I'll need the rafters to be 75mm x 220mm. I plan to have a beehive/s on top so may require being walked on! Does that sound right?

Question 6: I plan on using an EPDB rubber roof as this is best for a flat roof. Do I need a drop between the front and the back of the building or can it be truly flat? If I do need a drop, how much over 5metres?

Question 7: Nails or screws?

Question 8: Do I need noggins in the walls between studs?

I hope my queries are allowed in this Project forum. If not I'll post elsewhere.

Let me know what you think! As far as my experience goes, I've built a shed and chicken coop/run from scrap wood so this is quite a big challenge!

From my research this would cost about £40k if I paid professionals so here's hoping I can do most of it myself.

Alistair

Joff-Turbo-Nova · 9 Jan 2017

I'll be interested in the responses as I am doing something similar , albeit on a smaller scale (3m x 4m) however I can put a pitched roof on my build as I am over 1m from the boundry.

I was thinking along the same wall construction as you - (4" x 2"s) however have noted that metric sizes are now all the rage ! Will need to get my head round the conversion numbers !

Good luck !

Joff

Bagheera · 10 Jan 2017

Hi Joff, that sounds excellent. Hopefully will hear from the community soon. I have been chatting with a couple of experts and I'll make a post with more detailed plans down the line.

Don't convert 2 and 4 inches to mm. A 2x4 is in fact much more close to 1.5 x3.5 inches, the reason for which you can google!

motorbiking · 10 Jan 2017

3 x 4 is under 15m2 so no requirement on fire proofing. On the larger building, it might be easier just to get building control, you'd spend more on fire proof paint.

Bagheera · 16 Jan 2017

Just a quick update: I think I'll go for planning permission so I can put plumbing in from the start and may go higher than the 2.5m allowed under permitted development for the project i was planning.

In the meantime, a mini-project- I needed a new shed for garden equipment as I'm knocking down the existing garage so I got this done over the last month. It's made predominantly from scrap wood and sits to the side of the house. It rests on 6 flat bricks with pieces of pond liner on between to act as a DPM. The red cedar came from a neighbour. The pine cladding was £35 delivered from B and Q, pretty flimsy but did the job and it made the other door nice and light. Overall I think I spent about £70. The structure is from old roof rafters so it's a lot stronger than most commercial ones. Has two entrances, the end one for ease of getting the mower in and out. OSB roof with roofing felt. Should have brought the roof protruding over the sides but will fashion some trim to overhang.

Bagheera · 20 Feb 2017

I have been for a consultation with a planning officer who explained that I would be likely to get planning for up to 40m2 and up to 3.5m in height as well as with a shower-room so I am getting through the planning application at the moment.

The existing greenhouse and back room have been knocked down with a trusty sledgehammer and the garage emptied of 30 years of accumulated crap, albeit some very interesting crap, including plenty of rusty old tools that I intend to make good use of when this room is done. For the remaining garage, I'll remove the tiles, then the trusses and windows then knock down the walls.

Zac Spade · 22 Feb 2017

Hi Bagheera,

With your question about foundation these can typically be answered by your local planning office

Usually if you ask them what's popular/easiest to get approval with they will tell you

If that comes up nothing, then go the slab. Only if it's really damp and wet where you are then you might put some piers

- Zac

Bagheera · 8 Mar 2017

Thanks Zac.

I think I'll be going for a concrete slab with insulation in. I'll update when I know more. My concern is that it is a bit of a wet area with clay soil and trees not far away.

I also have a concern for the rafters as they need to span 5.25 metres so I'll need very thick ones, maybe 12 by 2s which adds to the height, especially if I opt for a warm roof too...

Bodge it & Scarper · 4 Apr 2017

Hi,

Ive had a crack at answering a few of your questions if its not too late! shouldn't cost anywhere near 40k but it won't be cheap cheap, worth spending money on good footings/slab, cladding and roof covering.

Question 2: The foundations are confounding me the most. What would be best for this structure? Ive seen slab foundations, strip foundations along the edges and in the centre to support and I've seen block ones used like Swift Plinths. Foundations are where I'm most uncertain and inexperienced for the build so any suggestions are very welcome. whats the ground like? assuming your on some sort of clay/muddy ground you've got a few options, trees have a huge impact of foundations so id get seek advice here, planning office might help, theres a lot of information on this subject available online. You could even consider a small raft foundation, complicated ish but sits on the ground so minimal excavation required, provides an insulated slab that deals well with unstable ground conditions. A properly specified reinforced strip foundation with a beam and block suspended floor would probably also be suitable.

Question 3: the walls will be constructed like this (from outside to inside):

Cladding

Timber or plastic battens

Breathable membrane

OSB3

Studwork/Celoxtex or similar insulation between studs

OSB2 (can be swapped for plasterboard down the line)

Am I missing anything, do I need a vapour barrier? Yea, id put one in especially if your not fully filling the stud i.e. 50mm celotex in a 94mm stud and you want to swap the OSB2 for plasterboard (be easier sticking plasterboard straight over the OSB2). Id use mineral wool myself in the walls its cheaper and you could fully fill the stud easy with it.

Question 4: I had planned (as in the sketchup drawing) to use 2x4s for the walls thinking these would be 100mm+. I recently discovered that a 2x4 is not 2x4 but instead about 38mm x 89mm! How confusing. My 100mm celotex plan has gone out the window. Will 75mm be warm enough or should I be using 2x6s (which are also not 2x6 )? Incidentally, if I go to 2x6 can I then change to 24 inches on centre rather than 16? As you're cladding it in OSB3 you can probably get away with 4x2's at 24" (610mm) centres. 38 x 89 sounds small, 4x2 CLS should be about 44x94. 75mm celotex should be plenty, you could consider rock wool or similar full fill the studs, probably cheaper. Im planning a similar shed with 4x2 at 610 centres clad in 11mm OSB (then breather, battens and cladding) full fill rock wool, vapour barrier and then 9mm OSB.

Question 5: the roof rafters will span a width of nearly 5m. according to this chart http://www.home-extension.co.uk/tech2.html , I'll need the rafters to be 75mm x 220mm. I plan to have a beehive/s on top so may require being walked on! Does that sound right? Quite a big span, could bulk it up a bit for peace of mind, 400mm centres (16in). Looking at your sketchup id also add a few rows of noggins and bind the rafters at each end with say 220 x 50mm, it wouldn't need to be 75mm thick anyway. also gives you a nice solid facia or backing for a facia. Id also insulate the floor and roof personally, might be worth thinking about.

Question 6: I plan on using an EPDB rubber roof as this is best for a flat roof. Do I need a drop between the front and the back of the building or can it be truly flat? If I do need a drop, how much over 5metres? check the minimum fall recommended by the EPDM manufacturer, normally you can get away with a very low pitch, but check.

Question 7: Nails or screws? Nails are my preference , its a lot of fixings, 4 per stud, 4 per noggin bare minimum... nail guns are good in this situation lol. Id probably be using 4" & 3" round wire nails for most of the stud work if hand nailing. the OSB sheeting will want a lot of fixings, won't need to be as big, ill be using a paslode again here. you could screw it on, id be using external grade fixings for this.

Question 8: Do I need noggins in the walls between studs? Yes. Arguably the OSB is giving you a lot of stability but id still put noggins in.

good luck, keep us updated

Bagheera · 5 Apr 2017

Bodge it & Scarper- thank you very much for your reply. Really helpful stuff! I'll post my thoughts on a separate reply. Just a quick photo update in this one. I've continued to knock down the existing garage and have some photos to share of the progress:

Tools used so far: sledge hammer, reciprocating saw, hammer and crow bar.

the wood battens that the tiles hooked on to will hopefully be re-used between my breather membrane and my cladding.

The trusses have provided plenty of wood for wood working projects when the workshop is complete.

The walls came down pretty easily, some which just a good push which shows it wasn't in good condition.

Note to anyone undertaking this: remember that guy who got crushed when demolishing a garage? Always take the roof off first rather than trying to take a shortcut of pushing the lot over in one.

I've saved a lot of tiles and bricks which I hope to sell or give away saving skip money. The broken tiles and bricks and mortar might hopefully make a good lower hardcore. I'll put proper MOT over that but it should do for the lower level, unless someone advises me against.

Bodge it & Scarper · 5 Apr 2017

Your welcome, ill be following this thread with interest, feel free to ask any questions! ill cover myself by saying my advice may not always be the best but I've got some experience.

good work on the demo job, glad to see your taking it apart sensibly and reclaiming materials is a great idea. If the bricks come down easily they may be worth more than you think, also seem some nice floors done with reclaimed bricks.

whats under the garage? I'm guessing that had a concrete slab of some form?

Bagheera · 7 Apr 2017

Okay, Bodge it & scarper (great name btw),

Foundations- I had come to the same conclusion- a raft foundation would be best for the reasons you point out. There are some trees near by - one about 3 metres away, in W London we sit on clay, not that stable, the original garage had to be knocked down as a result of the foundations slipping and lastly it means not having to dig so deep.

Here's a couple examples of what went wrong with the last slab:

This shows the foundations on my mum's side (she lives next door) with a big crack

The main floor of the garage

As you can see from that last photo, the floor is indeed concrete and could, and if I had a crusher be turned into MOT 1 sub base. It looks to me however that hiring a crusher would be more than just buying new hardcore and putting the broken up concrete in a skip. I'd welcome any thoughts on this?

As far as the construction of the new foundation goes, I have been given some drawings by a chap who is doing a similar (yet more extravagant and professional) project.

So the base from bottom to top looks like this:
150mm MOT1-compacted
50mm sand blinding
DPM
225mm raft, 450mm edge concrete reinforced with rebar.
100mm celotex or similar insulation
DPM?
60-75mm screed.

Walls- Agreed about the mineral wool and thanks for confirming I need a vapour barrier. I think I'll need to be at 400mm centres for the timbers as the roof is spanning a very large gap and need to be on 400 centres which are best if the weight is transmitted through a timber to the foundations. I could do a double top plate which would help spread the load and go on 600mm centres but I'm wary of this. If I go for a green roof down the line I'll need the extra strength. I'll have a look for those bigger 2x4s but I'm thinking 47mm x 120mm would be best. I want it to last and be very solid. I'll post more when I come to this stage.

Roof- I'll need 2 x blocking between each rafter at one-third span positions. Joists will be 47mm x 220/225mm. Thanks for reminding me about a facia board I'd forgotten about that. I'll do a new sketchup when I've got the base ready. I'm going for a warm roof construction-OSB over the joists, vapour barrier, insulation, OSB again, then EPDM.

Any recommendations for a nail gun? I don't have a compressor.

As for the bricks, I've saved the ones that haven't broken, although they have a lot of mortar attached but I'm quicky amassing a pile of broken brick, mortar and tiles that I can only assume no one would want and is not useful for sub-base?
Good bricks:

Bad bricks:

Bit more progress to report-3rd wall is now down, a lot of clear up this weekend I think:

Bodge it & Scarper · 7 Apr 2017

Sounds like you've got most of it covered, good work! Yes buying hardcore and removing the old stuffs cheaper and easier than reusing it. Yes you need that other dpm, if you build that detail then you would lay the block band course on the slab, line it with a dpm before insulating and screeding. Think that detail is using a thicker insulation or screed as a block with a bed should be 225mm high assuming it's a 215x440x150 block which I'm not sure exists it would be a 140 block. I'd also insulate the side of the screed against the block and use a thicker sole plate 20mm isn't very useful imo, don't get much of a fixing and it's usually naff timber. Definitely on the right track.

Nail gun, can't really go wrong with a paslode, new ones are expensive, a reliable second hand one would be ok or you can get cheaper ones, makita make some that are an old paslode design and a fraction of the cost. Air nailers on compressors are awesome although the lead gets annoying. could probably hire either. Nails for both are more expensive, and you could well end up doing some hand nailing anyway but it will save you a lot of work!!

Good progress, bricks could make a good BBQ pizza oven, just in case you wanted some more work lol

Bagheera · 21 Apr 2017

Hello again,

The dense blocks on the concrete will be 100mm wide x as you say 225 (with mortar) so either I get thicker insulation and screed to come level or leave it a few cm short, I can't see why the latter would be a problem. I saw from another person's build that I can get the concrete down, blocks, walls and roof joists on before coming back to finish the floor with insulation and screed, that way I can buy my insulation for both the warm roof and floor later when I need it, rather than buying in two lots (or have some sitting around for the roof).

Could you send me a link of the makita nail gun?

BBQ is a good idea, I'll think about that next summer!

Progress: all walls are down and I've made a humungous pile of rubble off of the concrete so I can now get through that. I'm buying a second hand breaker from ebay to get it up which is easier than hiring and can sell on down the line should I feel inclined. Should be about 80 quid.

I'm hoping that the concrete coming up will save me digging down so much and that there will be some useable hardcore too. Once the concrete and earth is out I'll hire a skip for the lot.

I'll post photos in due course and will hopefully sell the tiles I saved helping with the cost. The planning application is going in next week, I hand drew the plans so saved there as well, no designer required.

Oh and I'm picking up some pvcu french doors for free tomorrow so that's good!

Bodge it & Scarper · 21 Apr 2017

Only had time to skim that quickly but regarding the block and screed detail, you could leave it short but your going to need to line the floor with a dpm under the screed, up the side of the block and out under the dpc (dpc to sit between stud wall and top of block).. if you leave the screed short of the block height then this don may be on show and liable to get damaged, guess you could cover it with something like skirting or internal wall finish just DONT puncture it. Re doing the concrete floor etc after, yes you can and the added bonus is the roof will help keep it pristine but I'd consider what you're using and how your getting it in there, with no walls or ceiling a readymix truck could pull up and pour straight in for example, with doors and walls it's more likely a mixer and barrow job