Another Garden Room - big ambitions and humble beginnings

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Hi all,
After years of hanging around this site, I finally have a big enough project of my own to share. I am building a garden room (yes... it's another garden room post) with a view to discovering if I can learn enough skills to eventually build my own timber frame house extension (which I think is the more exciting bit).

I'll need a little help along the way and am happy to hear ideas, criticism, or even the occasional "You aren't mad for trying" comment.

I have a few questions that I couldn't find the answers to in other threads, but I'll outline the details here first.

Foot print and design
The room will be a modest 2.2m by 3m, and with a low height of 2.5m from the original ground. It'll be used primarily as a place to sit with a laptop and work, so I have somewhere to go that isn't my kitchen table. This is incidental use to the house so won't require planning permission, and the footprint means I won't require building control sign off either. Result.

It will be up against a boundary facing a road and I want it to be as unobtrusive for the neighbours as possible, so I'll have the roof sloping down towards the road so the neighbours see the shortest side. This also means the highest point is at the end I will sit, by the window checking that kids aren't misbehaving in the garden, which is important because we don't have kids.

Foundations
I've opted for ground screws. The floor is heavy clay and I am looking at using 750mm ground screws. I'll dig down 200mm in the current position, and build a retaining wall around the perimeter of the room. This gives me a bit more space to play with to keep under the 2.5m height restriction as that is measured from the original ground level immediately adjacent to the building. I'll have to make sure there is good drainage otherwise the building being on ground-screw stilts and the collected water within the retaining walls will make it into a 'Little Venice' in my own garden.

Window and door
My partner and I had an argument about this - I wanted to use a second hand door and window I had found on ebay at a great price to save money. My other half wanted to get brand new ones custom made to exactly match our initial thoughts on sizes. ... Long story short, we will be going for brand new ones.

Structure
It'll be a stick build timber frame, the current plan being stud walls from 47x100mm C24 timbers, sheathed in 11m OSB3, with 80mm of PIR insualtion and plasterboard on the inside. With a breather membrane. Of course.
Floor will also be 47 by 100mm joists (which meets the requirements of the span tables as there will be ground screw supports in the middle of the structure as well), with 80mm PIR and an 18mm OSB3 deck. I'll throw in a vapour barrier for good measure - no expense spared.

Roof
I'm looking for a warm roof (accepted wisdom seems to be that they are the way to go, and it makes sense to practice this seeing as I might need this skill for the house extension roof later).
Plans here are a little more hazy.
Joists: The e-book I bought on garden rooms suggested a 100x47mm joist with noggins, but the span tables say I need a 120x47mm C24 joist for the shortest distance in this roof design.
This will be topped with an OSB3 18mm deck, 80mm PIR insulation, 11mm OSB3 and then finally an EPDM membrane.

This gives a roof thickness of 251mm, which cuts into the 2.5m allowance quite a lot and a low ceiling like that risks scraping off what's left of my hair.

Cladding
This changes each time I see something new and exciting, but the current plan is Fibre cement tiles. Lighter and cheaper than slate but a similar feel. In reality, there is a strong chance that my ADHD tendencies will kick in and I'll end up with a different type of cladding on every wall, making it look like something from Playschool. Please keep me on track if you see me wavering here in future posts.

I'll take photos and keep people updated with the build in case you want to learn from or laugh at my journey.

QUESTIONS
If any of the (brighter than me) people on this forum could help me out on a few points before I start digging that would be great.. really amazing.

Question 1 - floor
With me digging down to lower the floor level, is there anything I have missed here that means I need to do extra things to protect the building?
I will leave 100mm from the weed membrane to the base of the timber floor to allow for airflow, and then the first 100mm of the floor deck will be adjacent to the retaining wall, below original ground level. Is there anything extra I need to do here to prevent problems with damp?

Question 2 - ROOF JOIST THICKNESS
Forgive the elemental question, but can I get away with 47x100 C24 joists on a garden building at 400mm levels to save me an extra 20mm of head height? The span table for building control suggests I should be looking at 120mm joists for a span of 2.2m.
A linked question, is would regular noggins help in providing strength to the roof structure, allowing me to get away with the slightly thinner joists?

Question 3 - ROOF JOIST DIRECTION
For reasons stated above, we want the slope of the roof to be along the longest length of the building (the 3m side) which I am told is a little unusual. In my plan, I currently have the joists going across the shortest width (2.2m). Does it matter that the 6 degree slope of the roof is at right angles to the direction of the joists?

Thanks so much for any light people can shed on the above. I'm looking forward to sharing photo updates with you all.

Best,
Jack.

P.S. Picture attached. I am no artist. To the left of the garden room you see a drive way that slopes down, in front of the garden room is a little bit of grass. To the right of the garden room is a shed from the mid 90s. I got too depressed drawing it so that bit of the drawing is unfinished.
gardenoffice.png
gardenoffice.png
 
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If you're looking for a quicker option, take a look at SIPS panels.

I was going to do a similar timber framed construction for my garden room, but by the time priced out etc and factored in time and effort, the additional cost of SIPS paid for itself.

https://www.diynot.com/diy/threads/garden-office-sips-build.524880/ is my build thread, albeit it's a bit behind current state now...

Edit: Also, when it comes to cladding, I had a similar issue with needing to have non-flammable cladding due to proximity to boundary. I went with Cedral fibre cement cladding. Easy to install, lots of color options and not too expensive.
 
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I did promise to update this, and it's been a while. So here is the starting photo (we had actually already cleared most of the brambles by then!)


IMG_20200525_113213 (2).jpg



And here is the status quo. Rest of the cladding can go on this week, and then it's just final bits like roof trim and guttering.

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What wood is the cladding?

Red cedar shingles. We will stain them black when they are all on.

I bought royal size (24 inch) and am totally regretting not buying perfection size (16 inch) as royals are really hard to find in stock now just in case I find myself short! My advice to everyone is to buy the most readily available ones in case of emergencies - perfection (16 inch) seems to be the standard size.
 
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STEP ONE. Preparing the ground.

Advice: never ever underestimate how long this 'little job' will take. Possibly the most back breaking of it all. We removed all plants, dug down a foot or so to remove the roots of the most invasive plants, turfed the front half of this space and removed the top foot of the soil where the building was going to be, in order to allow us to have a higher ceiling height inside of the final build.

IMG_20190919_132718.jpg


Clay soil. Look at all those roots!

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A (for some reason tiny) photo of the prepared site. Notice by the yellow hippo bag a post with a HUGE piece of concrete at the bottom. We found a LOT of concrete, bricks, broken pottery etc while digging that. I felt like Tony Robinson from Time Team, except that I didn't have a digger and half of the archaeology students from a local uni giving free labour.
IMG_20200624_202340 (2).jpg
 
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STEP TWO. A Floor Would Be Handy.

Advice for this section:
  • buy a set of spirit levels that includes a long one as well as shorter ones. It really helps in getting foundation screws level.
  • get an expanding foam gun. Not just the cannisters with the plastic tube. The gun allows so much more control and pays for itself in allowing you to save unused bits of cans for the next job.

Here you can see the instillation of ground screws - 9 in total for this build. I used larger screws so 9 will be more than sufficient for this build. Notice the gap underneath the wood floor frame to allow air flow. I used a nail gun for this, as screws are not good for horizontal load bearing. Nail gun probably saved me days of hammering over the course of this build. And I look badass in the safety goggles and steel capped boots.


IMG_20200706_191954.jpg


Also, my shed in the background is filthy. I should be ashamed. I'm not.

Here is the floor structure now screwed into the foundation screws. And with some PIR insulation attached. I used a brad nailer to attach some battons at the bottom of the studs to prevent the PIR from falling through. Then I used a whole load of expanding foam to fill any small gaps. Notice that I have also added weed barrier to the bottom.

IMG_20200714_185443.jpg


This is a REALLY satisfying moment of the build. It's the first time you can see something tangible and it's really motivating.

Next I added a vapour membrane and (eventually) trimmed off the excess before screwing down OSB boards for the floor, making sure I left a few mm between each sheet of OSB.
IMG_20200714_191212.jpg


To the right of this photo, you can see one of the tarpaulins I used to cover everything during the rain. It rained a lot during the build. I used some saw horses on top of the floor structure to make a tent, so no water pooled on top of the tarp. I grew quite fond of my tarp tent during the next two weeks of solid rain.

Buy tarps. BEFORE you need them. They WILL be needed.

Thanks for reading!
 
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If it was me I would let them weather and become a beautiful silver grey colour.
Ah yes, I know they weather to that silver grey. And I know that is what everyone else would do in my situation. For some reason I just really don't like weathered cedar. I have never found anyone else who agrees with me on this! Everyone seems to absolutely love it.

In retrospect, I might have been able to use a cheaper type of shingle, but the price difference for the amount I am buying was about the price of a round of drinks in a pub that I haven't been able to visit for a year.
 
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