9m x 3.7m Workshop + Office - Header Design

[Molynoox]

Hello people, this is my first post here.
Initially I'm looking for some advice on the design of my new workshop but I'm also hoping in the long term that this could be a useful community for me as I am just starting my journey into woodworking.

Information:
Type: Permitted development (restricted to 2.5m height and 30m2)
Size: 9m x 3.7m (30ft x 12ft) - split between 4m wide workshop (left side) and 5 wide office (right side)
Roof beams: Two 5 x 2 (125mm x 97mm) C24 fastened together per beam
Walls: back and sides are 4 x 2, front wall is 5 x 2 for extra strength
Bi-fold (right side): 2400 x 2000
French Doors (left side): 1600 x 2000
Windows: 1200 x 1000
Headers: doubled up 6 x 2 (145 x 97mm), other than above bi-fold which I have options for, see below question 3
Me: I'm an automotive engineer, so no real skills here to speak of, other than your usual home projects.

Questions
1. Are the windows too big? (will it look stupid, are they unecessarily oversized considering the size of the door glass?)
2. What is preferable between design A and design B for the front wall? (see picture 2 and picture 3)
3. Regarding the header above the bifold, this is 2.4m span therefore I think wood as a material will not be strong enough even when doubling up two beams side by side, therefore my options are as follows:
a) two 6 x 2 doubled up (probably not good enough)
b) two 6 x 2 doubled up with a 10mm steel flitch plate sandwiched in between (picture 4)
c) one tubular steel section (160mm x 80mm) + one 6 x 2 fastened together

I would love to simply use doubled 8 x 2 headers but I am trying to stay permitted development at moment and hence its a bit of a design challenge. Especially when you have large glass (needing stronger/taller headers) and also a 3.7m building depth because then you need stronger/taller roof beams for the span but you don't really have the height space for it. I'm finding it tricky so looking for advice and thoughts please, many thanks.

by the way apologies if this is not clear, I didn't model up all the options for the headers but I can probably do that if it isn't clear.

Martin

 

[Fitzroy]

Some shots across the bow.

I think B not A. You can split the frame between the windows and make two frames. Will be much easier to construct.

The design over the bifold depends on the expected loading. Understand it’s permitted dev but does it fall under building regs, if so you’ll likely need a structural eng to tell you you’re ok. If it’s not under building regs go take a gander at the sagulator and put some estimates in on loadings, it’ll tell you the deflections.

Other things to think about
- With a 125mm roof the max insulation you can get in is 75mm, leaving 50mm for a cold vented roof structure.
- The devil will be in the details on the bifold door threshold, worth understanding that before you design the surrounding structure.
- A 4x3.7m external footprint will translate into a 3.8x3.5m internal workshop space, if you’re thinking machine based woodwork that’s not much space.
- Your roof structure should overhang the sides of your building, which would widen the footprint by 40cm cf. the base. Not sure how permitted dev is determined, internal, external base, or external shadow. Don’t want to get pulled up once it’s built.

Fitz

 

[RobinBHM]

I built a combined shed and garden office -shed 5.4m x 2.7m and a garden cabin 4.5m x 4.2m

I struggled to keep it under 2.5m but then I included a 1.6 x 1.8 roof lantern.

The cabin I built with 120mm celetex roof, 50mm +50mm walls, 70mm floor - which was well worth doing, once its warm it takes very little heat to keep a comfortable temperature.


In regards to your design: your front elevation will struggle with lateral stability (imagine 100mph wind blowing on one end). I personally used 18mm plywood sheets each side of the corners then OSB for the rest.

You only have a small width of wall return at each end and narrow full height strips of wall in the middle.

Bear in mind you should allow for snow load on the roof (usually 0.75kn per square metre is used which equates to 1 metre depth).

Square hollow section is probably easier than a flitch plate and probably stiffer in a front to back direction, which is good for bifolds.

 

[Jameshow]

Couple of things why the doubled up 5x2 roof joists. Why not go with single 6x2??

Also if you notched the 6x2, you could use 9 X3 -4 and ditch the steel work??, double up the studs either side of an door opening and don't end the beams on 2" of stud take them 2ft to the next stud.

Also why the 5x2 in the front face? 4x2 will be adequate.

Just my observations others might have more experience.

Cheers James

 

[Molynoox]

Some shots across the bow.

I think B not A. You can split the frame between the windows and make two frames. Will be much easier to construct.

The design over the bifold depends on the expected loading. Understand it’s permitted dev but does it fall under building regs, if so you’ll likely need a structural eng to tell you you’re ok. If it’s not under building regs go take a gander at the sagulator and put some estimates in on loadings, it’ll tell you the deflections.

Other things to think about
- With a 125mm roof the max insulation you can get in is 75mm, leaving 50mm for a cold vented roof structure.
- The devil will be in the details on the bifold door threshold, worth understanding that before you design the surrounding structure.
- A 4x3.7m external footprint will translate into a 3.8x3.5m internal workshop space, if you’re thinking machine based woodwork that’s not much space.
- Your roof structure should overhang the sides of your building, which would widen the footprint by 40cm cf. the base. Not sure how permitted dev is determined, internal, external base, or external shadow. Don’t want to get pulled up once it’s built.

Fitz

Fitz, thanks, some great points made, my response as follows:

I will look at using B concept for manufacturability reasons as you say, but with better headers maybe - I am not convinced that the ones I designed in 'B' are good enough for 1200mm windows

Building regs - I don't think I fall under them, the only area where I might is that I am 15-30m2 and within 1m of boundary therefore I need to use non combustible next to fences / boundaries but I plan to use some cement based board on back and sides to get around that. does that mean I still fall under BR if I use non-combustible? or do I avoid need for BR if I am within the regs? I am bit confused about that point

hot v cold roof - I am planning on doing a separate post on that one - I was considering some sort of mixture between the two, so I can do things within 2.5 metres, and I suspect my proposal will be potentially unpopular so hoping to open up the debate on that one in another thread and gain some insight. I want 100mm PIR

size - I appreciate the comments, I agree its on the small side, but I am planning on being pretty creative in order to plan around this problem (make units multi use, flip tops, everything on castors etc)

overhang - there will be 100mm on sides too that I haven't modelled in CAD yet. The PD rules I believe consider internal floor space to determine size restrictions (e.g. 15m2), and the external dimensions inc overhang for proximity (eg 1m from boundary)

thanks again, some food for thought
Martin

 

[pils]

happened to be in sketchup using medeek wall plugin so threw this together. I can't comment any further as I have no idea what I'm talking about and other members are super experts by comparison. If I'm confusing the issue I apologise. :] note: view from "inside".

 

[Molynoox]

I built a combined shed and garden office -shed 5.4m x 2.7m and a garden cabin 4.5m x 4.2m

I struggled to keep it under 2.5m but then I included a 1.6 x 1.8 roof lantern.

The cabin I built with 120mm celetex roof, 50mm +50mm walls, 70mm floor - which was well worth doing, once its warm it takes very little heat to keep a comfortable temperature.


In regards to your design: your front elevation will struggle with lateral stability (imagine 100mph wind blowing on one end). I personally used 18mm plywood sheets each side of the corners then OSB for the rest.

You only have a small width of wall return at each end and narrow full height strips of wall in the middle.

Bear in mind you should allow for snow load on the roof (usually 0.75kn per square metre is used which equates to 1 metre depth).

Square hollow section is probably easier than a flitch plate and probably stiffer in a front to back direction, which is good for bifolds.

Robin, thanks - interesting to hear your view as it can be quite a lonely process trying to design within these constraints and it's nice to know I am not the only one, haha

50mm + 50mm on walls? was that a typo? or did you double up to get 100mm? I am aiming for 100mm on roof, 100mm on floor and 60mm on walls - I think 60mm gives enough for electrics in the front void between PIR and plasterboard?

I am planning on OSB sheathing all round, probably 9mm because I can get that in metric sizes and it works nicely with my 400mm centres. I just read some posts on this forum that has turned my thinking upside down on that actually (inside vs outside placement of OSB) - I need to think about this more. Its also very strange for me to hear about this problem with the external sheathing because every man and his dog does it this way (including pro garden room companies). But the science behind not doing it that way does make sense in terms of the placement of the dew point and avoiding condensate forming etc its just that it seems to contradict what everybody is actually doing. I probably need to read a bit more of the historical posts on this forum before asking more questions on that topic

Do you think I need to reduce the amount of glass in my design to improve the ratio of wall to glass and hence overall strength? Or are your comments based on an assumption that I was not using sheathing? (I need to put that in the CAD really)

thanks
Martin

 

[Molynoox]

Couple of things why the doubled up 5x2 roof joists. Why not go with single 6x2??

Also if you notched the 6x2, you could use 9 X3 -4 and ditch the steel work??, double up the studs either side of an door opening and don't end the beams on 2" of stud take them 2ft to the next stud.

Also why the 5x2 in the front face? 4x2 will be adequate.

Just my observations others might have more experience.

Cheers James

James, thanks. I hadn't considered the notching option! mind blown I will model that up and post back on here to check if I have understood correctly.

6 x 2 would give me sufficient strength for a 3.7 metre span, and would also allow me to get more PIR in and do a proper vented cold roof, so yes its absolutely the best design, and absolutely what I want to do... but as ridiculous as it sounds, I cannot fit the the extra 20mm into my design and still meet the 2.5m constraint. So I had to double up the 5x2 to get the strength needed. This actually increases the cost of the roof from £400 to about £700 just in CLS so quite annoying, especially when considering that it means less PIR so its actually a compromised design despite being more expensive. I can probably accommodate the 20mm if everything goes perfectly to plan, but I am really close to the limit of 2500mm in my theoretical design and I need to factor in a small buffer for manufacturing tolerances / screw ups etc.

very good comment about the way I have placed the header with only 2" overlap - now you mention it my design does look pretty stupid

5 x 2 was partly because I had seen other garden room designers doing it (not a good reason really), and partly because I thought it would help strengthening up the front wall which, with all the glass in the design, I am concerned about it not being strong enough - I am guessing you have no concerns over that?

thanks

 

[Molynoox]

happened to be in sketchup using medeek wall plugin so threw this together. I can't comment any further as I have no idea what I'm talking about and other members are super experts by comparison. If I'm confusing the issue I apologise. :] note: view from "inside".
View attachment 101741

Pils - cheers for that! medeek wall planner plugin?? another mind blown situation never heard of that, do I assume there is some automation? I guess I need to check it out.
By the way, you say that is the inside view... well from what I read so far, the general consensus on this forum seems to be about sheathing going on the inside not outside being the best, so you can control the condensate - so perhaps this is actually the view from outside

thanks
Martin

 

[Jameshow]

What's the threshold height above the base if the bi fold doors. Obviously you want a seem less transition indoors - outdoors. Perhaps a sight drop on the inside side, incase you want to carpet / laminate / tile.

Can you not sink the floor structure, slightly if the 2.5m is measured from the surrounding earth?

If money's not too tight you could put OSB front and back much like sip panels. Then you would have the best insulation properties / damp prevention.

Cheers James

 

[pils]

Pils - cheers for that! medeek wall planner plugin?? another mind blown situation never heard of that, do I assume there is some automation? I guess I need to check it out.
By the way, you say that is the inside view... well from what I read so far, the general consensus on this forum seems to be about sheathing going on the inside not outside being the best, so you can control the condensate - so perhaps this is actually the view from outside

thanks
Martin

it's the inside. :] I set it up that way for another project (which exists, I should add, solely in my mind at present): interior probably osb (with french cleats added)/natural insulation in frame/osb/battens/cladding...expensive...

 

[Boozer]

Can you not sink the floor structure, slightly if the 2.5m is measured from the surrounding earth?

2.5m is measured from the house original elevation so you should take into account if you garden is sloping and digging up may be only option if you really wan to fit under the permitted development height.

Building regs - I don't think I fall under them, the only area where I might is that I am 15-30m2 and within 1m of boundary therefore I need to use non combustible next to fences / boundaries but I plan to use some cement based board on back and sides to get around that. does that mean I still fall under BR if I use non-combustible? or do I avoid need for BR if I am within the regs? I am bit confused about that point

If you are within 1m of the boundary then you are subject to BR unless outbuilding is under 15m2 or it's made out of non combustibles.

I would recommend reaching out to your local authority for advice as it doesn't say that the whole structure needs to be made out of non combustibles but it has to be "constructed substantially out of non combustibles materials".
What that means and what is acceptable best to be clarified by people that will potentially tell you to knock it down if it goes **** up.

When choosing cladding make sure product is A1 rated to meet required standard of being non combustible.

 

[colinc]

Hi,

I faced similar issues a few years ago, I have a couple of thoughts to offer.

Building within permitted development rules seems to force a compromise in the design and usefulness. The process of getting planning permission for the building is not onerous, nor is it expensive. I would suggest you design what you want instead and talk to the planners. I also feel that getting planning permission strengthens your position if any stroppy neighbour starts complaining.

Assuming that you are in England then I do think you are going to be restricted in what you can do by building regulations. You said that you were < 1 metre from the boundary >15 sq m and <30 sq m. The important thing is floor area, not the footprint. That means the building needs to be built of ‘substantially incombustible materials’. I discussed that with building control. They will accept brick/tile conventional build or steel frame with tiled or metal roof and metal or fibre cement cladding. Timber is allowed in the roof structure. After considering many options and taking advice, I built mine using a prefabricated steel frame, steel stud and track wall panels insulated then clad externally with Marley Cedral wall panels with a Marley tile on timber rafters roof. This all sounds complex and expensive, but it wasn’t too bad and I have created a studio/workshop that has a long life and a lasting value. Having got planning permission for a pitched roof means I have a high ceiling creating a spacious feeling inside. To put it in perspective, I think the roof tiles cost about £500 and will outlive me.

Sorry about the long post, but what I am really trying to say is build what you want and what is best, don’t compromise to avoid a bit of red tape.

regards,

Colin

(the photo was taken before completion, hence the mess around it)

 

[RobinBHM]

50mm + 50mm on walls? was that a typo? or did you double up to get 100mm

I made the timber frame with 95mm x 47mm studwork, 50mm celetex cut 8mm smaller in between then foamed up the gap (makes it totally air tight and structure rigid and gets over probs of bowed studs etc)

Then I foil taped to create a complete vapour tight structure, then used 18mm OSB board, then a continuous layer of 50mm celetex. 25mm battens and featheredge. I realise it's perhaps not correct as it puts the 18mm OSB in between celetex, but it allowed me to create an insulated wall with no thermal bridging.



I built a 500mm overhang over the front which has French doors and windows - I'm really pleased I did that, almost no rain reaches that elevation, very handy when going in or out.

 

[Inspector]

When I was a pup out of high school I worked on a small framing crew of 2 or 3. We regularly built walls flat on the floor including the plywood skin on the outside with a lot more framing wood than you show and were able to stand them up with just the 2 or 3 of us. I would make that wall with a full length header (lintel for the sticklers among you) with the OSB and lift it in one go. A hint though. Use a full length floor plate to maintain the integrity of the wall as you lift it and once up and attached to the floor cut the door opening parts. Keeps everything nice and straight.

I am in agreement with Colin about getting what you want passed by whatever authority. You can make it bigger and it will add much more value to the property if it has a useful structure on it.

Pete

 

[Fitzroy]

+1 for the planning permission. I had to do it for my build as I’m in a conservation area so no permitted dev rights. Cost £200 and took a few weekends work, but got what I wanted.

 

[Molynoox]

What's the threshold height above the base if the bi fold doors. Obviously you want a seem less transition indoors - outdoors. Perhaps a sight drop on the inside side, incase you want to carpet / laminate / tile.

Can you not sink the floor structure, slightly if the 2.5m is measured from the surrounding earth?

If money's not too tight you could put OSB front and back much like sip panels. Then you would have the best insulation properties / damp prevention.

Cheers James

I don't know what the threshold is because I haven't sourced the doors yet but my plan was to create a raised decking area outside the garden room which sits slightly higher than base of building and flush with the top of the threshold and then have a drop down from threshold into the inside of the garden room which I will fill with laminate flooring and, if there is space, underfloor heating. But I haven't done any research into that stuff and don't really know what I'm talking about - does that make sense?

regarding sinking the floor lower than ground - yeah I'm thinking of going down that route, the site slopes left to right and is about 200-300mm higher on left than the right over the 9 metres, meaning that if i sunk the left hand side a tiny bit (just 50-100mm) I would still have quite a lot of ventilation under the building. Hope I am being clear there? I would still maintain about half a metre around perimeter meaning the side of the building (or any of the bottom) would not be in contact with the soil - but there would be one point in the left back corner where the base would be very close to ground. I would use shuttering to keep soil in check on the left hand side.

regarding using OSB both sides, why would that be any different to using OSB on one side and vapour barrier on the other in terms of damp control?

thanks for feedback

 

[Sachakins]

Be careful about permitted development assumptions.
I wanted convert my integral garage to a room, like others around me. This is ok under permitted development, HOWEVER, when I wanted to check availability of inspector for building regulations, (self building conversion so planning ahead) I was informed that the Permitted Development Rights for my area, about 99 houses, had been withdrawn so I would need planning permission!!.

So check with your council, also our counci have a legal permitted development document you can apply for, when granted, it stops them coming after you when the change there minds!

I have challenged the withdrawal, as i can't find it in any óf there withdrawal of permitted developments or their TPO, tree preservation order documentation.

Been emailing requests, but no joy, (reduced service due to covid!)

On back burner for now anyway.

 

[Keefy.]

Find Oakwood Garden Rooms on faceache and youtube, watch the vid's, he will sell you plans complete with a full material list.
I'm not connected but seriously thinking of either going for sips or his construction.

 

[Molynoox]

Hi,

I faced similar issues a few years ago, I have a couple of thoughts to offer.

Building within permitted development rules seems to force a compromise in the design and usefulness. The process of getting planning permission for the building is not onerous, nor is it expensive. I would suggest you design what you want instead and talk to the planners. I also feel that getting planning permission strengthens your position if any stroppy neighbour starts complaining.

Assuming that you are in England then I do think you are going to be restricted in what you can do by building regulations. You said that you were < 1 metre from the boundary >15 sq m and <30 sq m. The important thing is floor area, not the footprint. That means the building needs to be built of ‘substantially incombustible materials’. I discussed that with building control. They will accept brick/tile conventional build or steel frame with tiled or metal roof and metal or fibre cement cladding. Timber is allowed in the roof structure. After considering many options and taking advice, I built mine using a prefabricated steel frame, steel stud and track wall panels insulated then clad externally with Marley Cedral wall panels with a Marley tile on timber rafters roof. This all sounds complex and expensive, but it wasn’t too bad and I have created a studio/workshop that has a long life and a lasting value. Having got planning permission for a pitched roof means I have a high ceiling creating a spacious feeling inside. To put it in perspective, I think the roof tiles cost about £500 and will outlive me.

Sorry about the long post, but what I am really trying to say is build what you want and what is best, don’t compromise to avoid a bit of red tape.

regards,

Colin

(the photo was taken before completion, hence the mess around it)

thanks Colin, appreciate it. I'm starting to lean towards PP route but not 100% yet - I actually want to keep the height down, not just for PD/PP reasons but also to not annoy neighbours - its raised on one end by 300mm due to sloped ground so will be 2.8m at one end already which feels like it might look intrusive - its not a massive garden.
do I just need to comply with building regs or do I need somebody to sign that off? the 'only' thing I need to do different is the non combustible materials, so presumably if I meet that requirement then I don't need anybody to sign it off?
but I think this might be moot anyway because I will need to contact planning officer to make sure I am meeting their local definition of non-combustible.

Martin