UK Exterior wall thickness and pitch roof thickness

[AlVal]

Hi,

can anyone advise what I should assume for wall thickness for traditional construction cavity wall to latest specs? all the way from external to the inside of skirting board?
Likewise for a slate pitch roof?

My project is a brick built small rectangular shed/garden room/study/summerhouse/whatever

Essentially I want to stick with traditional well known construction methods, where it's easy to find tradesmen who can work with it (hence avoiding sips and other modern methods)

At the same time I want to minimise wall and roof thickness as much as I can, while keeping to the latest building regs, i.e. the same standards as you'd build a house to, because I want the possibility at some future point to put a single bed in there if I wanted to, although I won't be bringing any plumbing of any kind to it.



I'm trying to avoid the cost of an architect for this outbuilding, but am worried that if I draw up plans that I'll get them wrong if I can't establish the correct wall and roof thickness

It's also important because I'm trying to keep the overall height of the building down, but I have to ensure I allow enough room around the door for the roof insulation - I don't want to submit plans to the planning dept stating the apex of the roof will be x and then find out that I've miscalculated the thickness the roof needs to be , and have to go back again

by my calculations a pitch roof using slate vaulted on the inside, insulation between and under rafters, no sarking board is 200mm thick all the way from outside to inside paint - please let me know if this seems wrong

2x slates 14mm
battens 25mm
breathable membrane 0.5mm
100mm rafters packed with kingspan kooltherm k7
57.5mm insulation and plasterboard (kingspan K118 which combines plasterboard with insulation)
3mm plaster skim


I think I can get the walls 337mm thick from outside brick to inside of skirting board as follows:

102.5mm brick.
10mm clear residual cavity
75mm Kingspan Kooltherm K106 Cavity Board
100mm block (aero blocks rated at 0.11W/m2K)
15mm plasterboard dabs
12.5mm plasterboard
3mm plaster skim
19mm skirting

that's all assuming for external cavity walls 0.18W/m2K is the correct current value, and 0.15W/,2K for the pitched roof

Edited by AlVal on Sunday 29th January 21:15

[Ahbefive]

At work we have 100mm brick, 150mm cavity with blown insulation, 100mm block, 25-30mm plasterboard including dot and dab, 2-3mm skim and whatever skirtings are being used.

Our roofs have 150mm celotex between the bottom of the rafters with an airgap on the outside and lots of tile vents then I believe its a vapour barrier and 65mm PL board (insulated plasterboard). This is a house roof though. Overkill and bigger rafters than your garage roof.

Edited by Ahbefive on Sunday 29th January 21:25

[RATATTAK]

How will you avoid roof spread ?

[AlVal]

RATATTAK said:

How will you avoid roof spread ?

Thanks so much for your comment. This is exactly why I should probably bite the bullet and pay an architect.

I presumed this buulding was simple enough for me to try and project manage it myself and do the plans etc, but I know little about building. Having read a bit about this roof spread, its pretty obvious in retrospect.

Assuming you mean the effect of the roof pushing the walls apart because its an open vaulted cieling:

Maybe some kind of strong steel rectangular frame that sits on top of the walls which the rafters rest on? Is there such a thing?

If you mean the roof itself sagging, its a 2.7m x 4.7m building, so not a large roof, and presumably 100mm rafters at 40 degree pitch and 600mm spacing is strikg enough to avoid sag on a slate roof?

Edited by AlVal on Sunday 29th January 23:17

[TA14]

AlVal said:

RATATTAK said:

How will you avoid roof spread ?

Thanks so much for your comment. This is exactly why I should probably bite the bullet and pay an architect.

No, but it's probably worthwhile paying an architectural technician/surveyor who does this all day long. You'll get caught out on all of the tiny details like ventilation and window area.

Roof spread is easy: birds mouth rafters and ridge beam with a strong lintol over the door.

Walls and roof thicknesses are too low due to thin cavities and no tolerance for building error, say 350 and 250? 15mm is thick for dabs.

[AlVal]

TA14 said:

No, but it's probably worthwhile paying an architectural technician/surveyor who does this all day long. You'll get caught out on all of the tiny details like ventilation and window area.

Roof spread is easy: birds mouth rafters and ridge beam with a strong lintol over the door.

Walls and roof thicknesses are too low due to thin cavities and no tolerance for building error, say 350 and 250? 15mm is thick for dabs.

good point, seems fair to allow a bit for building error/tolerance. I think in my roofing calculations I should have allowed for a space above the insulation that goes between rafters, which I didn't, and I'm not sure one layer of battens under the slates is enough or if two layers are needed (horizontal layer AND vertical layer?)

found a local architectural technician on the "chartered institute of architectural technicians" website http://www.ciat.org.uk/, so waiting to see how much his services are.

Thanks again

[Gav147]

Are you an engineer by any chance . I think you are getting a little carried away in your calculations, there really is no need to be allowing half a mm for felt etc, if you think it is going to be too high just lower the pitch of the roof slightly assuming you can or drop the height of the whole building by an inch or two also assuming you are not tied too much by ground levels.

Also why not put ceiling joists in half way up the roof if you can get the required ceiling height, will help tie the roof and also looks a lot better than a full vaulted ceiling (imo).

[AlVal]

Gav147 said:

Are you an engineer by any chance . I think you are getting a little carried away in your calculations, there really is no need to be allowing half a mm for felt etc, if you think it is going to be too high just lower the pitch of the roof slightly assuming you can or drop the height of the whole building by an inch or two also assuming you are not tied too much by ground levels.

Also why not put ceiling joists in half way up the roof if you can get the required ceiling height, will help tie the roof and also looks a lot better than a full vaulted ceiling (imo).

thanks for the input, I'm just trying to illustrate the elements that I've included in case I've missed anything

and because I want to keep the overall roof height as low as possible, the interior roof will just be barely skimming over the top corners of the door frame, so once the walls are up, there's not much flexibility to change the roof pitch..

My local CIAT member ("architectural technician") hasn't responded to my emails or calls. Phoned the local architect who pretty much laughed at me and said he wasn't interested, essentially too boring (actually he used the term "dire") a project for him.

The foundations company were supposed to show up to have a look and didn't show

electrician needed multiple chasing to come out with a view to scoping out electricity requirements

unrelated to this project, plumber been chased for the 3rd time for a quote on decent amount of work.

What is it with tradesmen not actively jumping on available work? is it my Irish accent somehow scaring them off? I find it so strange, cost or payment isn't really an issue at all, so I just don't get why it's so hard to get things done!

Edited by AlVal on Tuesday 31st January 17:43

[Jonesy23]

Isn't this going to end up absolutely tiny inside? From the drawings it looks like it will be the size of a largish garden shed but with the thickness of the walls and roof the interior is going to be really cramped.

[Gav147]

AlVal said:

What is it with tradesmen not actively jumping on available work? is it my Irish accent somehow scaring them off? I find it so strange, cost or payment isn't really an issue at all, so I just don't get why it's so hard to get things done!

I wouldn't take it personally mate, I know around here most people I know are just simply booked up for months in advance and have no interest in taking anything else on at the mo, annoying when they won't at least ring back to explain but it happens I'm afraid.

[AlVal]

Jonesy23 said:

Isn't this going to end up absolutely tiny inside? From the drawings it looks like it will be the size of a largish garden shed but with the thickness of the walls and roof the interior is going to be really cramped.

2m x 4m inside, can't get away from the wall thickness if you want it to be useable all year around and be a proper permanent structure. I have to keep the size down so it doesn't end up taking over too much of the garden, don't have a huge garden I'm afraid

Edited by AlVal on Wednesday 1st February 22:08

[QuickQuack]

I wouldn't take it personally either, even our regular tradesmen are taking ages at the moment. Although your project is a decent sized one overall, the individual trades won't have a huge amount each. Your best bet would be to bite the bullet and get a project manager or get a builder to project manage. They'll have the right relations with the right trades who would otherwise laugh at you but they know that they have a long term return with a builder or project manager.

[mikebradford]

You would be better compromising on you u values. And creating more internal space.

A nominal 300mm cavity wall is simply over kill for the intended use.

The roof won't spread if you use a sufficient ridge beam.

Depending on your external finish requirements, you'd be better considering timber frame construction.
or a simple single leaf wall internally or externally insulated.

If your not regarding it as habitable space, simply build it however you wish and accept its only as warm as a house built 10 years ago.

[Ahbefive]

mikebradford said:

If your not regarding it as habitable space, simply build it however you wish and accept its only as warm as a house built 10 years ago.

It wouls only be as warm as a house built 10 years ago if it has a 100mm block, 100mm cavity, 100mm brick and lots of insulation as thats what we were doing 10 years ago.

[TA14]

SIPS and clad it.

[mikebradford]

Ahbefive said:

It wouls only be as warm as a house built 10 years ago if it has a 100mm block, 100mm cavity, 100mm brick and lots of insulation as thats what we were doing 10 years ago.

You can achieve similar u values to ten years ago using a construction that is not as wide.

[Ahbefive]

I'm sure you can reduce the cavity width by 30mm but really thats not going to make much difference to the overall size. What are you suggesting?

[mikebradford]

Ahbefive said:

I'm sure you can reduce the cavity width by 30mm but really thats not going to make much difference to the overall size. What are you suggesting?

See my earlier comments. Timber frame construction.