9m x 3.7m Workshop + Office - Header Design

[Molynoox]

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.

thanks for the explanation - I have not heard of anybody doing that technique before on the walls, its a bit like a warm roof on its side, no thermal bridge... but yeah, agree it doesn't seem to be text book for damp control.
Martin

 

[Molynoox]

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

thanks for the tip on the construction, I have that side of things in my mind when designing, as a lot of the work will be done on my own I expect. Planning on building the large walls in 2 or maybe 3 sections if it doesn't compromise structure, so I can lift it myself if needed. I can always sheath it after its up to save weight, but I know this means I will need to rack it with cross braces instead of relying on the sheathing for that.

M

 

[Molynoox]

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.

that's handy knowledge, thanks
Martin

 

[Molynoox]

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.

cheers Keefy... and yes I've seen all his stuff (liam griffin), very handy source when learning. My personal approach on this is that I want to own the decisions that go into my build because this is a big and expensive project, and there are lots of things that can go wrong. Particularly in key areas, for example foundations and roof. I'm still learning so I will not pass any judgment on his particular methods, but what I will say is that there are many ways to build these things, some ways are great, some ways are OK and others are bad - I'm still on my learning curve to understand which ways are the right ways and which ways I need to avoid. Jury is still out in some key areas for me but good luck to you if you go down the purchased plans route.

I have thought about using SIPS and done a little research - two things still bother me
1. why are they not more popular if they are so great
2. don't like the faff of custom made parts, lead times, having a fixed design etc

also, I think these typically use concrete bases (I could be totally wrong there), which wouldn't work well on my slightly sloped site
ultimately my bias against those is probably due to ignorance rather than an inherent flaw with them but I decided not to pursue them further as sometimes you can have too many options and they just didn't feel right for some reason.

cheers
Martin

 

[akirk]

isn’t 9m x 3.7m above 30m?
I make it 33.3m2

go for planning as others have said...

 

[Molynoox]

The rule relates to the internal dimensions.
Even the partition wall is excluded.

 

[Fitzroy]

[QUOTE="Molynoox, post: 1435239, member: 35729"
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
[/QUOTE]

The most robust wall design to minimise the risk of interstitial condensation is, from outside to inside. Horizontal weather screen, vertical battened air gap, breather membrane, stud work with PIR insulation spray foamed into place, PIR continuous layer over studs, vapour barrier, internal board. Simplified, internal vapour barrier, insulation, external breather membrane.

A flat roof is more complicated because it needs a continuous structurally sound surface to apply your waterproof layer to. In a cold roof the solution is to provide lots of ventilation to the cavity behind insulation and the underside of the roof boards.

These designs provide maximum resistance to water vapour getting to the cold side, and then a breathable or vented space to dissipate this moisture should it get there.

Interestingly if you consider a warm roof you still have an impermeable layer that will be cold, however it is mechanically bonded to the back of the insulation, and it will not be degraded by being moist.

Moving away from these tested designs may result in condensation problems but no one can say if a hybrid design will or won’t be ok. It’ll depend on usage, moisture generation, positioning to the wind, etc etc.

When I built my shed I externally sheathed it in OSB. Now I’m left with the decision on how to insulate. It’ll be sub optimal but I use the shed for max 8 hrs per week, and it’s relative humidity runs at about 55% when in use, so I think my risk is low.

Fitz

 

[colinc]

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

Hi,

I think you are heading in the right direction in talking to the authorities, they are very likely to help you come up with a scheme that suits everyone.

 

[Molynoox]

[QUOTE="Molynoox, post: 1435239, member: 35729"
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

The most robust wall design to minimise the risk of interstitial condensation is, from outside to inside. Horizontal weather screen, vertical battened air gap, breather membrane, stud work with PIR insulation spray foamed into place, PIR continuous layer over studs, vapour barrier, internal board. Simplified, internal vapour barrier, insulation, external breather membrane.

A flat roof is more complicated because it needs a continuous structurally sound surface to apply your waterproof layer to. In a cold roof the solution is to provide lots of ventilation to the cavity behind insulation and the underside of the roof boards.

These designs provide maximum resistance to water vapour getting to the cold side, and then a breathable or vented space to dissipate this moisture should it get there.

Interestingly if you consider a warm roof you still have an impermeable layer that will be cold, however it is mechanically bonded to the back of the insulation, and it will not be degraded by being moist.

Moving away from these tested designs may result in condensation problems but no one can say if a hybrid design will or won’t be ok. It’ll depend on usage, moisture generation, positioning to the wind, etc etc.

When I built my shed I externally sheathed it in OSB. Now I’m left with the decision on how to insulate. It’ll be sub optimal but I use the shed for max 8 hrs per week, and it’s relative humidity runs at about 55% when in use, so I think my risk is low.

Fitz
[/QUOTE]
I think my plan will be to get as close to the ideal design as possible without letting the building height creep up too much, regardless of PD/PP situation. Will be speaking to planning officer about BR and PP anyway following the advice on here.
thanks,
Martin

 

[Sheptonphil]

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

You are over 15sqm and within 1m of the boundary. Not only do you need to comply with the regs as far as materials, will need building control to monitor the whole project. I’ve just completed an almost identical project.

 

[Molynoox]

You are over 15sqm and within 1m of the boundary. Not only do you need to comply with the regs as far as materials, will need building control to monitor the whole project. I’ve just completed an almost identical project.

that is good to know, I wasn't sure if you just needed to comply with BR or if you need somebody to 'sign it off'
bad news but thanks

 

[junco partner]

If I understand correctly you are using a timber frame, so this won't be substantially non combustible, and will require B Regs.
Within 1m of the boundary both sides of this wall will require 30 mins fire resistance, additionally the external wall will require a class 1 surface spread of flame and, depending on the distance to boundaries, there may still be a requirement for surface spread of flame. Note the boundary for this purpose can be taken as the distance to the centreline of a road. path etc running along the boundary, but not for the building exemption criteria.

 

[Molynoox]

thanks Junco, it seems that building control are comfortable with the stud walls being timber as long as I have cement board on either side of it and use fire rated class 0 treatment on the cladding.

 

[Sheptonphil]

Same here, fibre cement planking to exterior, but I also had to have fireline plasterboard internally. All built on a wooden frame with rockwool RW45 insulation which is also fireproof.

 

[Molynoox]

my building control guys never mentioned need for special plasterboard on inside - its strange that normal plasterboard is not fireproof with it being cement based.
is rockwool better than PIR for fireproofing then? I didn't know that

 

[Sheptonphil]

Yes, fireline was specified, twice the fire resistance of standard.
Rockwool RW45 is better at acoustic insulation. I need that acoustic dampening rather than the absolute best thermal insulation. I don’t know the best insulation as far as fire resistance.