# New Workshop Build - Advice and opinions welcome!



## irishthump (28 Mar 2021)

Hi guys,

Will be starting to build a new workshop in a week or two (14x12ft, all timber construction) and would love some feedback in what I have planned...

The back garden will be fully slabbed by then which should provide a solid and level base to start with. 

Subframe will be 4x2 treated timber raised on concrete blocks to minimize the chance of damp from the ground. Floor will be 18mm OSB. Toying witht he idea of insulating thre floor with Kingspan or something similar.

Walls using 3x2 stud frames. skinned with 10mm OSB then wrapped with a plastic DPM. Finished outside with Treated T&G cladding.

Roof will be flat design with 4x2 roof joists covered with 18mm OSB. Most liely uses roofing felt to keep the costs down, possibly sheet metal is affordable.

Couple of things I'm not sure off. 
Should the outer T&G cladding be fixed to battens attached to the OSB or is it ok to fix then directly to it?
Also not sure 4x2 will be strong enough for a span of 12ft!

Like I said any advice or comments welcome!

Thanks in advance...


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## Jameshow (28 Mar 2021)

4x2 Defo will not span 12ft adequately. I'd go 6x2 or even 8x2 as you would in house joists. 

Re battens yes use battens to keep the cladding from rotting. 

Cheers James


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## irishthump (28 Mar 2021)

Jameshow said:


> 4x2 Defo will not span 12ft adequately. I'd go 6x2 or even 8x2 as you would in house joists.
> 
> Re battens yes use battens to keep the cladding from rotting.
> 
> Cheers James


 Thanks James,

Yes I had a feeling 4x2 wouldnt quite cut it!


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## Fitzroy (28 Mar 2021)

Epdm for the roof. Tough, long lasting and easy to apply.
You state walls, stud work, osb, wrapped with DPM. I assume you mean a breather membrane. This is fine so long as you don’t plan to insulate the walls


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## topchippyles (28 Mar 2021)

Every time i pop on this forum someone is building a new workshop


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## Blackswanwood (28 Mar 2021)

If you have not already seen it I would recommend taking a look at MikeG’s post ”Build a shed Mike’s way”.


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## irishthump (28 Mar 2021)

Fitzroy said:


> Epdm for the roof. Tough, long lasting and easy to apply.
> You state walls, stud work, osb, wrapped with DPM. I assume you mean a breather membrane. This is fine so long as you don’t plan to insulate the walls



Hi Fitzroy,

I had'nt planned to insulate the walls just now but may in the future. How does this affect the decision on installing a membrane? 
I've also seen videos on Youtube where people have used 250 Micron DPM to sheath the OSB then placed the cladding over that.


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## jcassidy (28 Mar 2021)

Just remember the vapour barrier goes on the warm side of the insulation and you can't go wrong. If you intend to insulate in future you will have to rip off all the vapour barrier. For the cost of insulation on the walls and roof, I'd do it now. 
I used expanded polystyrene on the walls (got some for free off a demolition site and Kingspan xtherm 50mm on roof.


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## Jameshow (28 Mar 2021)

Most people think that using ply or OSB inside means you don't need need a vapour barrier. 

Cheers James


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## irishthump (28 Mar 2021)

Jameshow said:


> Most people think that using ply or OSB inside means you don't need need a vapour barrier.
> 
> Cheers James


Yes I’ve read that the high glue content of the OSB fulfils the role of a vapour barrier.


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## irishthump (28 Mar 2021)

jcassidy said:


> Just remember the vapour barrier goes on the warm side of the insulation and you can't go wrong. If you intend to insulate in future you will have to rip off all the vapour barrier. For the cost of insulation on the walls and roof, I'd do it now.
> I used expanded polystyrene on the walls (got some for free off a demolition site and Kingspan xtherm 50mm on roof.



So from what I gather I need breather membrane under the cladding whether I decide to insulate or not. Then a vapour barrier would be needed if I was to insulate the walls and ceiling and this would go over the insulation. OSB would fulfil this role.
That correct?


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## jcassidy (28 Mar 2021)

I've seen it mentioned here and elsewhere that due to the amount of glue in OSB, it can act as a vapour barrier as long as the gaps between boards isn't too big. 
Given the fractional cost and effort of a vapour barrier, vs the cost and effort of having to fix any potential condensation problems in the walls, I am not sure I'd take that risk. To each his own.


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## Fitzroy (29 Mar 2021)

Damp walls, leading to rot etc, can occur due to moisture from the outside (rain) or inside (moist air /condensation). From the outside you ideally have a rain screen (cladding), a gap to allow drying, then a secondary barrier (building wrap/membrane). You dont want dpm as it is impermeable from both directions, if any moisture gets past it it will be trapped on your osb and rot it. A building wrap is breathable and will allow the building to dry if moisture gets past it. From the inside you need to avoid warm moist air getting to a part of the building that is cold (so the moisture condenses) and unvented (so the moisture cannot evaporated and dry out). So (ideally) you either have:
- osb (or plasterboard and vapour barrier), stud work and insulation, then building wrap, gap, cladding
- stud work no insulation, osb, building wrap, gap, cladding.

if you have osb/plasterboard, stud work and insulation, osb, building wrap, cladding. You risk moist air getting past you insulation, contacting the external osb ( which is cold), condensation of water on the osb. There is no route for the damp osb to dry and you get rot.

Of course all of this is idealised and if you permanently occupied the building then you’d likely have a problem. Lots of people all the time results in lots of warm moist air. If you spend a few hours a week in the workshop then you have little moisture and lots of time between uses to dry out. However, with a non standard construction no one can tell you it will be ok. History will judge 

fitz.


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## Glitch (29 Mar 2021)

Edit: What Fitz said.

If you follow Mike G’s way you can’t go wrong.
I’d argue the theory that OSB acts as a vapour barrier (it isn’t) but the key is to get warm damp air to the outside and not condense in the building structure.

I only found his method after I’d followed YouTubers who did it a different way which could lead to condensation problems.

From the inside out Mike has OSB, insulated framework, breathable waterproof membrane, battens then cladding.

No vapour barrier and no OSB (or ply) on the outside of the framework which is what I ended up with.

With a cold roof you must make sure there is unobstructed airflow between rafters on the underside of the roof boards.


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## Skydivermel (29 Mar 2021)

Mines a 16 X 8 loglap on a 3 X 2 frame. What I did was glued 1" squares inside the frame, floor and roof to give a 1" air gap. Insulated the floor and the whole inside with 2" kingspan TP10 50mm insulation board. Sealed everything with 2" wide silver tape. Lined the internals with 12mm OSB and painted it white. No condensation problems and very warm in the winter & cool in the summer. Roof is black shingles which look very smart. As a side note I made a base from easyfit plastic grid squares layed on a base of sharp sand and a weed membrane then filled with shingle. Drains very well and keeps the floor bearers dry.


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## OldWood (29 Mar 2021)

I would avoid felting if you can. I don't know anything about epdm, but used Onduline over osb on 3 of my sheds over 20 years. 

The relative cost of doing felt properly - 3 layers - plus adhesive is little different from Onduline but the installation time is relatively zero in comparison and particularly this - clean!!
Rob


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## irishthump (29 Mar 2021)

Fitzroy said:


> Damp walls, leading to rot etc, can occur due to moisture from the outside (rain) or inside (moist air /condensation). From the outside you ideally have a rain screen (cladding), a gap to allow drying, then a secondary barrier (building wrap/membrane). You dont want dpm as it is impermeable from both directions, if any moisture gets past it it will be trapped on your osb and rot it. A building wrap is breathable and will allow the building to dry if moisture gets past it. From the inside you need to avoid warm moist air getting to a part of the building that is cold (so the moisture condenses) and unvented (so the moisture cannot evaporated and dry out). So (ideally) you either have:
> - osb (or plasterboard and vapour barrier), stud work and insulation, then building wrap, gap, cladding
> - stud work no insulation, osb, building wrap, gap, cladding.
> 
> ...



Thanks for the in depth reply, Fitz.

I reckon I'll go with - stud work no insulation, osb, building wrap, gap, cladding.
This will save me a few quid and allow for the option of adding insulation at a later date. My workshop will get only casual use and won't be heated so I think this will work fine.


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## irishthump (29 Mar 2021)

OldWood said:


> I would avoid felting if you can. I don't know anything about epdm, but used Onduline over osb on 3 of my sheds over 20 years.
> 
> The relative cost of doing felt properly - 3 layers - plus adhesive is little different from Onduline but the installation time is relatively zero in comparison and particularly this - clean!!
> Rob


. 

I've been looking into corrugated sheet for the roof, but it may be cost prohibitive. I'm also not sure of the process for installing it on a flat roof, particularly the finish on the ends and sides. Any advice here would be appreciated....


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## Pallet Fancier (29 Mar 2021)

Never mind


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## irishthump (29 Mar 2021)

Pallet Fancier said:


> Never mind


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## Pallet Fancier (31 Mar 2021)

irishthump said:


>


I asked something before I finished reading the thread, and the answer was there. So I deleted it


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## Heluvaname (31 Mar 2021)

I am planning to use coated steel box profile roofing sheets with insulation pre-bonded to them.
Ridge is a coated steel apex ridge and gable ends are 90 degree coated steel pieces.
Not cheap, but simple to install and as it is very rigid from ridge to eaves requires a lot less timber support structure.


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## MARK.B. (31 Mar 2021)

I would have liked to have found a forum like this when i built my workshop  there are a number of things i would have done in a different way  . It has stood the test of time so far almost 18 years and fingers crossed it will last a lot longer . At 36'x14' it took me around 8 months to complete minus the fitting out . Most of it i did by myself including the concrete base in two pours . All frame work was was done in sections and lifted into place and here i did need help . I did use 2x4's at 24" centers to span the gap  Mainly because i had a load of treated stuff left over and unwanted from a huge fence build and it was free if i could shift it , i did add belt n braces with 16mm threaded rods to give some extra strength every 3' or so partly as the roof is concrete tiles left over from when the house was re roofed ( I made a huge fopar when guestimating how many i would need  . If you only have 2x4's or want to save a few bob the threaded rod is an option and it is cheaper than the same length 6x2. Lengths of the rods over 3 meters and the price goes up a lot if you want it delivered ,jointing nuts are cheap and its easier to handle and install in two pieces.


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## Jameshow (31 Mar 2021)

irishthump said:


> .
> 
> I've been looking into corrugated sheet for the roof, but it may be cost prohibitive. I'm also not sure of the process for installing it on a flat roof, particularly the finish on the ends and sides. Any advice here would be appreciated....


Normally you would put a barge board front and sides. The back would overhand the rear wall and drop into a gutter. 

You would want to put some foam in the ends to stop vermin getting under it. 

Cheers James


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## Stuart Moffat (2 Apr 2021)

irishthump said:


> .
> 
> I've been looking into corrugated sheet for the roof, but it may be cost prohibitive. I'm also not sure of the process for installing it on a flat roof, particularly the finish on the ends and sides. Any advice here would be appreciated....


I have some very recent experience of this, albeit on a bigger build. My old workshop was in two adjoining stables, each just big enough for one horse I reckon. It was cramped and cold, and for processing big stuff, I would have to whell the bandsaw, Planer etc outside. At the same time there was a fairly decrepit building nearly 20M x5M that was built in the 60s as a Cattery/Kennels. For ages one end had been a home for tractor and similar and the other end was just used for storage as a barn. I had looked forward to renovating it for 10 years, but for some reason my wife wanted the house sorted first! Then in the bad storms 2 years ago a big tree fell across the middle of it. My renovation made use of the foundations, drainage and services that were fine, and I reused as much as possible. It was done to the original planning granted, including roof pitch etc, and I had intended to roof it with ply and then a 3 layer felting done by a local roofer. I should have been ready for the felting by end of summer 2019. Good weather time. Then one night I got carted to hospital with the blue lights with a heart scare. By the time that was sorted the long winter wet weather had started and felting not really a good option. A triple felt system for that size was going to cost just under £4k. I discovered that I could buy Trisomet foor cladding to do the whole thing for just under 4K delivered. A no brainer I waould say. I gought mine from Rollacald which is right next door to the TaTa steel factory where it is made. They will cut the lengths to the precise dimension you require, and that price included all the fixings, ridge capping, etc etc. The stuff is used for many warehouse constructions you see everywhere, but also sold in smaller quantities, my driver's next drop was a nearby farm where he was repairing an old barn.
I should really post some pictures of the build. I got it coloured black to match the black weatherboarding and it looks great. Becuase it is designed to be airtight if installed properly, I decided I might as well make the whole building airtight. Easy for a woodworker if you are precise in building the walls (every panel of insulation between the studs in mine had to be tapped into place with a mallet - they all fitted precisely on all 4 sides!
I had envisaged putting in a wood burner - but I really haven't had to heat the building over the winter just gone. The Camvac quickly warms the place once I start working, and the relative humidity has stayed around the 50% mark throughout.
If you are interested in installation type questions I'll be happy to elaborate.
Obviously it is good to have an insulated roof without fixing it between rafters


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