My Garden Room Build - 9m x 4m

[Molynoox]

I thought I would do a thread for my garden room build in case anybody is interested in the details.
I spent a huge amount of time researching everything and have learned a lot along the way and I figured that if this knowledge is useful to me then perhaps it could be useful to other's.
I have gained some good insights from people on this forum so hope I can contribute something back to the community.
I am over halfway through the build now, but will take it from the start so you can see every stage.

Although the primary goal is to share the knowledge with others in the same boat, I am also looking to receive your constructive feedback so that I can learn from my mistakes. So please feel free to comment, even if it is a criticism.

I will start with the plan:

• 9m x 4m external dimensions
• 30 m2 internal area (to keep it exempt from building regs, although it isn't that simple)
• under 2.5m height (to keep it within permitted development and not need planning permission, although it isn't that simple)
• wooden frame (5 x 2), PIR insulation (100mm all round), cement board sheething on 3 of the external sides
• vented cold roof
• alu doors and windows
• metal header (flitch beam) above bi-fold, timber header above french door
• EPDM roof
• battens and cedar cladding
• vapour bariers inside and out
• bathroom (water and waste)
• power and internet
• groundscrew foundations
• heat pump air con for heating and cooling
• composite deck
• cedar pergola (partly for shade, partly a privacy thing, for both us and the neighbours)

its all fairly predictable stuff I'm afraid :-D

I will attach some pics of the plan and then in the next post I will start with the first stage of the build.

Martin

 

[Jameshow]

Looks simalar to Dr Bob classy recent build!

Cheers James

 

[Molynoox]

sounds interesting, I don't think I spotted that one - how recent? just so I can go and find it

 

[Fitzroy]

Looking forwards to it. ‘Although it isn’t that simple’ was a key learning when I built mine.

 

[Jameshow]

https://www.ukworkshop.co.uk/threads/bobbys-home-office-gym-build.131242/
Cheers James

 

[Molynoox]

thanks James, that build looks great. Very similar in looks to mine, but concrete base and SIPS makes it quite a different beast to mine. Very interesting thanks for pointing it out.

 

[Molynoox]

Feb 2021 - Planning

Before going any further (back in February) we considered a few key planning aspects:

• Placement and size of garden room
• Impact on neighbours / privacy concerns
• Planning permission / permitted development
• Building regulations
• Metric or imperial (sorry...)

Here are some of the things we decided

• Keep it within permitted development, the key part here being the 2.5m height rule
• It's easy enough to apply for planning permission for a build like this, but I didn't really want to go any taller as we were already concerned about height / privacy / impact on neighbours
• In terms of floor space we are easily within the '50% of your land' rule, but again we just didn't really want to go any bigger.
• Overall dimensions were set at 9m x 3.7m (2.5m tall) and 30 m2 internal area
• I do plan to apply for an LDC at some point (which will probably make it easier to sell the house at a later time), but as yet I haven't got around to that. It's better to do it before but it can be done retrospectively and I am not too concerned about it
• I have spoken to local planning officer and taken him through all aspects of the build and he is happy with everything. Based on my plans he has confirmed that I would be exempt from building control (the key part being the wall build up and compliance to fire regulations)
• If you read the building regs, it does actually state that if you sleep in the room then it needs to be controlled (i.e. the building would not be exempt), whereas permitted development is a little more specific about 'sleeping' and it goes into detail about 'self contained accommodation' and the use of it being 'incidental to the main house' etc
• Despite what the building regs say, my local planning officer does not have a problem with the odd weekend with guests sleeping in there, as long as its not a full on permanent granny anex.
• and that is kind of the plan, to have a workshop on one side (4m wide) and an office / entertainment space on the other side (5m), which can also double up as a place for guests to stay on odd occasions.
• We have planned for a small toilet in there, which is the main reason why I got into looking through these particular aspects of the regulations

I did a few basic renderings of the view of the garden room from our house and also from the neighbours house - I also tried adding a small 'topper' on top of the fence to help with privacy, which seems to make a big difference. That side of things is irrelevant now anyway, as we have now planned to have a pergola and trellis running all the way along the fence line. That essentially sorts out the privacy thing.

By the way, it was my wife that added the toilet, and hence extra complication, and it was also her that suggested the pergola and trellis stuff - because life is boring when its easy right?....

The main reason I keep going on about privacy is because the land is on a slight slope, and by the time you put in a level building it ends up about 300mm higher than the land at one end. This higher end happens to be at the end next to the neighbours fence. There is also a rule about decking being no higher than 300mm. I am just generally trying to keep everybody happy.

anyway, here are a few pictures to add some context.

oh yeah, quick word on metric versus imperial, in a nutshell, for those that don't know, building materials are supplied in a mixture of metric and imperial sizes, meaning they are essentially incompatible. It becomes a key part of your planning for this reason, mainly because you want your sheet goods to fall on your studs, but that doesn't work if you use imperial sheathing on outside and metric plasterboard on inside (for example). I won't open this box here (oops too late), but in a nutshell, my particular solution was to work in metric, which normally means you have to rip down your OSB which only comes in imperial, HOWEVER, you can get 11mm OSB in metric. Alternatively, you can get your local timber yard to rip down imperial sheets before they deliver them (they will probably do this for free). Can you do this yourself? yes. Is it fun, no anyway, I ended up using cement board, to meet fire regs, and this is supplied in metric which fits perfectly onto my studs, so it didn't matter in the end.

Martin

View from inside neighbour house:

view from inside neighbour house with proposed modified fence:

Overall view and plan for services (water, waste, power, data)

plan for wall build up to meet fire regs (although wall is now 125 x 50 C24 with 100mm PIR):

 

[Molynoox]

Feb 2021 - Groundwork and Layout

The goal with this stage was as follows:

1. remove trees / bushes
2. reduce the height of the ground on the left hand side such that the building can be sunk into the ground slightly and hence reduce its overall height above ground level on the right hand side.
3. Mark out the outside perimeter of the building such that it's ready for the installation of groundscrews (there are 28 in total, 4 rows of 7, or 7 rows of 4 if you prefer... )

Stump Removal
I was quoted £300 for tree stump removal, and I had 3 to do. Of course that meant doing it myself despite having none of the knowledge, skills or tools required. To get it done I used (primarily) a Makita cordless reciprocating saw, a Titan chainsaw (which I bought for the occasion, £50 was cheaper than £900 so it felt justified), and a 4ft wrecking bar which I found to be a mission critical bit of kit. A digging bar might be even better but what I found was that something heavy and pointy was needed to dislodge compacted mud, grit and stones from around the roots, allowing you to get your power tools in and cut the roots. I know you can hire these stump grinder things too but I looked into that and they seemed to be quite expensive and also quite spectacularly lethal looking, especially for somebody like me that has a habit of finding the most dangerous way to achieve each task. I can't believe I would have any feet left if I was to hire one of those. Stump remover, and stump creator, all in one go. I will stick to my metal stick.

Excavation
You can see from the pictures that I managed to excavate down about 150mm on the left hand side - this was quite a lot of work with just shovels but it will make a nice difference in the long term having the threshold for the main bifold door closer to the ground.
In terms of tools for this task, I bought lots of different shovelly items (not knowing anything about what was really best) and two of those became invaluable:

• Mattock: I couldn't have done it without a mattock. I bought this recently and now can't live without it. It's a really brilliant tool. There are so many different ways of using it too. It makes the ground feel like its made of cheese, even when its rock hard and full of horrible stones and roots.
• Round point, long handled shovel: also brilliant. The long handle saves your back, and the angle on the shovelly bit (the head?) means you can sort of skim across the ground and remove a top layer. Its perfect for this sort of task. But you need the mattock too. I like Mattocks, did I mention that?

Layout
I worked from a rough outline in order to excavate, but I needed something more accurate in preparation for the groundscrews being installed. For this task I made some 'batter boards' as an experiment. They worked reasonably well, but were not really strong enough for me to consider them a perfect tool for the job - they were a bit ricketty. Having said that it's nice being able to adjust each line by a few mm and get the outline perfectly square. I have since bought some fencing pins which I think I now prefer for doing marking out work.

You may also notice my home made giant square, I did consider buying a folding square but I was trying not to buy things I didn't really need. I made the hypoteneuse 2m which gave me sides of 1.414. Making one myself had the added benefit of making me feel clever, although the effect only lasted until my next mistake. Can't remember what that was but they are fairly regular.

Finally, for the overall building outline, I calculated the theoretical hypoteneuse (haven't written that word for 30 years and then along come two at once) as 9730mm and was then somewhat amazed to find both diagonals of my marked perimeter measured exactly that on the first try. I even took a picture I was so excited. I guess the home made square is pretty accurate. Probably just got lucky.

Here are the earliest pics of the garden I could find:

Mid-groundwork phase:

The blank canvas

Excavation mostly complete

Layout work

homemade batter boards

 

[Molynoox]

Layout complete and ready for groundscrews

Evidence of my flukey yet perfectly square layout - 9730mm

 

[Molynoox]

Apr 2021 - Install the groundscrews

At this point the project became very real, and also very exciting. I had been researching and designing garden buildings for a few months previous to this point, and all of a sudden it felt like we had moved out of the world of theory and into the world of construction. This was also the first real financial commitment, having spent about £2500 on the groundscrews, so there was no turning back now.

The whole family (and the neighbours) were fascinated by the process of these giant screws going into the ground. They are quite comical to look at, they really do look like giant screws, and it could only have been better if they used an actual giant screwdriver to insert them!

These are 1200mm long groundscrews manufactured by Radix, although I bought them through a company called GBGS which included brackets for the tops and installation by a professional crew. I spoke to about 5 different companies before selecting a supplier. I can't remember the exact specifications but each screw will support something crazy like 1000kg, I think even more actually. They are spaced 1.5m apart which seems to be pretty standard practice for all groundscrew companies and in fact all 'pier' based designs seem to use this rule. I will include a picture of my base design showing how the bearers sit on top of the groundscrews and take the main loading, and how the joists connect to the bearers and distribute that loading through the floor. I did do some research into joist lengths using span tables and such like, but I am well under the maximum allowable lengths for the joists so I'm hoping the floor will feel strong enough. I used doubled up 4 x 2 for bearers and single 4 x 2 for the joists, which are on 400mm centres. I hope I got that terminology correct, I am pretty new to construction having worked in automotive engineering all my life so I might sound a little bit like an silly person when it comes to this stuff.

Installation took about 3 hours start to finish, and the process is as follows.

1. They first scanned the area for electrical cables using a handheld 'CAT' tool. (you can't scan for pipework unfortunately so if you want to avoid those it's just guesswork)
2. They then made pilot holes for each screw with a large pointy metal bar.
3. Then they used a hose to spray some water into the hole and loosen the ground up, they explained that this made the screws go in much easier.
4. Then they insert the screws using a large specialist power drill (it probably has a proper name) - the big stalk you see sticking out to the side by the way is to stop the drill counter rotating, they brace this against the other groundscrews.
5. They use a post level to monitor for plumb as the screw is going in and adjust as needed, and use a laser level with bleepy device to set the height. They also agreed a 'datum' with me of 50mm above ground level which they set from the highest point of the land and then set all the other screws to that exact same height. I went round with a long level after they had left and checked every screw and they are all spot on

Incidentally I have just bought a large metal pointy stick of my own to make some pilot holes - I am going to be installing some self-install 800mm groundscrews to support the pergola and deck. I also picked up a post level for £3. I dont have the fancy big drill so I have purchased the manual self install tool for £45, this is basically just a large T-bar

Comedy screws and comedy big drill

Hand tightened first

All done

Notice that I didn't quite get the land perfectly level beforehand, so the next day I got the spade out again and levelled the land around any screws that looked too close to the ground. I ended up with about a 50mm air gap underneath each screw which is about as good as you can get on a permitted development build due to the 2.5m height restriction.

 

[DrDarren]

Really enjoying this thread!

 

[Molynoox]

Thanks drdarren, pleased to hear it. Starting to feel I was adding a little too much detail, as is often the case with me... but i suppose people can always just scroll through to the pictures.

 

[Inspector]

When I was reading and got to the first picture of the ground screw I thought it was a photoshopped joke as they are unlike any I have ever seen here.....but no! That is what you actually used. Pretty much all the ones I have seen look like these. Videos Screw Piles & Helical Piles - Ready to Build on Immediately! A round split plate pulled to a single turn helix and welded on in the factory. Doesn't matter if they are the little 3 foot ones or the 20' or bigger ones. The six I used for my decks were 10' long to get below the frost line.
Nice project and the detail is welcomed.

Pete

 

[RobinBHM]

the rendered drawings indicate cedar cladding on sides and rear -are you doing that, as I was wondering if you could clad those sides with cement board and paint or use box profile.

the other thing I wondered is the structure doesnt seem to have any plywood sheathing -only plasterboard and versapanel -are they suitable for use as sheathing.

you structure is a fair size and would be subject to a fair wind load so lateral stability will be important -plywood sheathing provides an important function in that regard

across the front there are only fairly small returns -those along with the centre section will need to provide lateral stability preventing wind load from those sides.

lovely looking project

 

[Molynoox]

haha Pete, I should have added the comedy giant screwdriver. Funny you make that comment about not seeing this type before, my research threw up 5 or 6 companies and only 1 of those used the type you are familiar with, and that was mostly for large construction projects it seems. Were yours really 10 feet long? that is massive! I thought frost line was way higher than 10 feet?

 

[Molynoox]

the rendered drawings indicate cedar cladding on sides and rear -are you doing that, as I was wondering if you could clad those sides with cement board and paint or use box profile.

the other thing I wondered is the structure doesnt seem to have any plywood sheathing -only plasterboard and versapanel -are they suitable for use as sheathing.

you structure is a fair size and would be subject to a fair wind load so lateral stability will be important -plywood sheathing provides an important function in that regard

across the front there are only fairly small returns -those along with the centre section will need to provide lateral stability preventing wind load from those sides.

lovely looking project

Hi Robin, thanks for the comments, really appreciate it.

The renderings are not accurate for the cladding, I will have Cedar on front and left side, and some cheaper alternative on right side and back because it will never be seen (box section like you say, or cement board cladding - your comment does make me wonder if I could have just painted the cement board sheets which I have already put up?? hmmm.... I like the metal box section stuff the most, but I have no idea how to fit that around the window which I have at the back, I feel like the lengths of cladding will be easier to fit around the windows.

I think the versapanel (actually I used Fermacell in the end) is just as strong, possibly stronger, than OSB. I am saying that from memory from when I researched all this stuff, so could be wrong. Well, it sounds like I might be wrong from what you are saying... The structure does 'feel' incredibly strong though, it is 5 x 2 studs, on 400 centres, which is pretty thick timber with tight spacing, with noggins, and 100mm PIR, which is also spray foamed in around all edges of all PIR boards making it massively stiffer than before the PIR went in. I also used one million screws, some of them were engineering screws. I know that is not empirical, but it does feel like it's made of concrete and not wood.

regarding your final comment, yes, I totally agree with you about that one - one of my regrets from the design phase - I put in glass without really thinking so much about lateral stability, focussing only on vertical strength. If I do another one, I would ensure I leave a sensible nominal distance (something like 500mm or a metre) between all apertures - I would need to think about that one some more to get a value to work from. But I think the fact I overengineered the materials and construction side a little gives me confidence that it will compensate for that.

Hope that didn't sound defensive, I actually really appreciate the feedback, I am hoping to learn from people like you so comments are welcomed.

 

[Molynoox]

I forgot to add the pictures of my base design with groundscrew placements, I know some of you were not able to get to sleep last night because of that, so here they are:

This is the first design, which had three rows of bearers and 21 screws in total

This spacing was in line with span tables for floor joists, but wasn't flying with the groundscrew guys who recommended 1.5m spacing minimum for their groundscrews, in both vertical and horizontal (or X and Y axis might be a better term). So here is the modified design which they were happy with; 4 rows of bearers and 28 screws in total:

Martin

 

[Inspector]

haha Pete, I should have added the comedy giant screwdriver. Funny you make that comment about not seeing this type before, my research threw up 5 or 6 companies and only 1 of those used the type you are familiar with, and that was mostly for large construction projects it seems. Were yours really 10 feet long? that is massive! I thought frost line was way higher than 10 feet?

Well when Searching from here the ones you’re familiar with don’t come up at all unless I look at the Images. Mine were actually about 11’ or 12’ long when they picked them off their truck to drive them in. They were the heavier of the two kinds they brought. The 2 3/4” were to light to drive through the rocks and boulders to get to depth. They were 3 1/2” or 3 3/4”. The alternative to the screw piles was to dig the 10’ and use rebar and forms to make concrete piles. There was no way I was going to go through that expense. The smaller helical piles would have been $275Can each and the bigger ones were $350Can or 375Can, can’t remember which. And yes the frost line can get that low, that’s why they have to be 10’ deep so they don’t heave. In about a month it will stay about zero or lower getting to -40C and we’ll still get frost some nights at the end of May.
Pete

 

[RobinBHM]

Hope that didn't sound defensive, I actually really appreciate the feedback, I am hoping to learn from people like you so comments are welcomed

not at all - I just mentioned it because lateral stability is often overlooked.

if insulation is cut a bit undersize and foamed in, it really does make the studwork rigid - I built a garden office and found the same thing - once the insulation was in, it became really rigid.

If you use box profile, it’s easy to finish around windows: cill projection at bottom, side cheeks at the sides and at the top, the box profile comes down a bit over the frame to act as a drip and a trim fits up to closes off the battens.

 

[Molynoox]

cheers Pete.
Robin thanks for the tips on the box section, I will do a drawing and get back to you to confirm I understood it. That will be in a few weeks probably
I wondered if the noggins were even needed if the PIR was foamed in like that - like you say it makes a mega difference to lateral rigidity.

Martin