New workshop roof

[Joshjosh]

Hi
I’m planning on building a garden workshop in the new year and wanted some advice concerning the roof.
Permitted development means I’m limited to a flat roof, and finances are quite tight so im Considering a corrugated sheet roof
My main goals are for the roof to be watertight and not suffer from condensation
Ive (hopefully) attached my plan of action. Please let me know if there’s anything you’d change, my biggest concern is condensation
Also any corrugated roof sheet recommendations would be great
Thanks for any help you can give
Thanks josh

 

[Woody2Shoes]

HI,

My 2d's-worth:

I would use EPDM rather than corrugated sheeting. IMHO corrugated sheeting really only works when laid to a decent fall - looking at the datasheet on the Onduline website, it seems that it can be laid at a pitch of about 10 degrees (a sensible pitch for a "flat" roof) when the sheets are supported by OSB/ply sarking boards. IME Onduline suffers from ponding (once moss/lichen start to grow on it) and deteriorates quite quickly, if laid at such a shallow pitch. You could use steel corrugated sheeting instead, which would be better, but I'd still go for EPDM instead (all corrugated sheeting needs lots of holes for fixings - each hole reduces weather-proofing).

If I were you, I'd use Celotex-type PIR insulation (much better insulation effect, for a given thickness, for only a bit more money). I'd also install a PVC sheet as a vapour-check on the warm side of the insulation - I know you're proposing 9MM OSB on the inside, and that it acts as a vapour barrier itself, but if it were me I'd go for 'belt-and-braces'.

Cheers, W2S

https://onduline.co.uk/products/onduline/

https://www.cladco.co.uk/products/corru ... d-cladding (minimum pitch 10 degrees)

https://www.rubber4roofs.co.uk/shop/epdm-rubber-roofing

 

[Beau]

Cladco do a good range of steel roofing sheets. Can be laid to just 3 degrees and probably less if no joins in the length. Also they do a drip stop backed option or better still but pricey an insulation back version. https://www.cladco.co.uk/installation-guides/dripstop

Cant compare with many other products but way better than Onduline as it doesn't sag and longer lasting than my experiences with felt flat roofing. Fixing them is a doddle just get good quality screw fixings and there should be no issues with watertightness on them.

 

[Joshjosh]

Hi guys thanks for your input
I already knew rubber would be a better job but when I added up the rubber roof, trim and osb it worked out allot more than sheets so thought if the sheets would do a decent job I’d save a bit of money but if sheets will cause me hassle down the line I’ll pay the extra
Does the cladco have to be laid ontop of osb or would latts work?
Thanks allot
Josh

 

[Beau]

Hi guys thanks for your input
I already knew rubber would be a better job but when I added up the rubber roof, trim and osb it worked out allot more than sheets so thought if the sheets would do a decent job I’d save a bit of money but if sheets will cause me hassle down the line I’ll pay the extra
Does the cladco have to be laid ontop of osb or would latts work?
Thanks allot
Josh

Pretty sure all installation details are on their site so have look through. No need for OSB and if I remember rightly just needs supporting at 1200mm spacings. Been using their sheets for sheds and barns here on the farm for 20 plus years so I cant see you having any hassle down the line as long as you instal them well. https://www.cladco.co.uk/help-and-advice

 

[OscarG]

Having recently replaced a badly leaking onduline corrugated roof on my parent's garage, I'm strongly advise against installing it, I think it's utter crap.

I'd suggest fitting OSB boards and then lining them with EPDM or felt.

 

[Hornbeam]

Profiled steel sheeting can provide an excellent roof product but consider a number of points

Most manufacturers guarantees are business to business so as a private individual they probably wont apply to you

There are a number of different coating types and within each coating type a wide range of performance.

Trapezoidal cladding can be laid down to 4 degrees without any problems.

If going lower than that then use a standing seam profile

Fixings should always be installed in the troughs using good quality stainless fixings with EPDM washers

Remember that steel is a particularly good conductor of heat so any direct routes through are likely to create condensation risks

Insulation. Wont directly make your shed warner. It simply slows down the movement of heat, so unless you heat it will be just as cold inside as out

Regarding condensation, there are 2 issues to consider.

1. Condensation inside the building

2. Condensation within the insulation cavity

Condensation occurs when the temperature falls below the dewpoint. To reduce risk of condensation within the building reduce sources of moisture, increase heat and if applicable increase ventilation.

Within the insulation cavity, you need to apply similar principles by avoiding humid air entering the cavity but accepting that some air (and vapour ) will. Make sure that the insulation support is as air tight as possible for 2 reasons

1. Reduces heat loss through cold air infiltration

2. Reduces warm air and moisture entering the cavity



Going back to choice of roof cladding

A small standing seam profile will need to be fully supported so should be laid on 18mm OSB3 or equivalent with a membrane between

0.7mm trapezoidal sheeting can easily span 1.5m purlin spacings and does not require any additional support. Whether you will require a membrane depends upon the type of material you use for the internal liner and how that would drain in the event of condensation

Foam insulated panel. This is the easiest option as effectively you install the outer and painted inner and insulation in 1 go. The edges of the internal liner can easily be cut back to get rid if any internal condensation risk and absolutely no risk of condensation within the insulation cavity. On paper the most expensive but you are effectively buying the whole roof already pained inside and out.

Any questions regarding metal roofing/wall cladding from a small workshop to an 80,000m2 distribution shed, give me a call or ask on the forum. Its my day job and I’ve been doing it for 30 years in January

 

[nabs]

thanks for the explanation on condensation. It seems obvious now you have explained it so clearly, but before now I had never really understood why vapour barriers were needed on the inside and breather membrane on the outside. cheers!

 

[Beau]

Fixings should always be installed in the troughs using good quality stainless fixings with EPDM washers

Never seen a stainless option. Where do you get them and what problems have you seen with the standard galvanised steel fixing screws?

Got 100s of square meters fixed down here and you have me worried

 

[Hornbeam]

There are 2 reasons why you have carbon steel fasteners
Cost and Specification.
Cost. Stainless steel fasteners are about double the cost of carbon steel so if a sheeting contractor can get away with a cheaper option they will do and pocket the difference. Cost per fastener isnt much but at 6 fasteners per square metre on some cladding jobs it all adds up
Specification. If you dont specify you will get the cheapest option but also about matching fastener performance to the cladding. No reputable manufacturers of carbon fasteners offer much more than 10 to 12 years guarantee, but if you have a cheap 25 micron polyester coated steel then that also will have a maximum of 10 to 12 years. On the other hand if you have a top quality 200 micron plastisol loke HPS200 roof sheet you will get 40 years guarantee (including cut edges) so you should specify stainless fasteners. There are a lot of leathergrain plastisol coatings with all sorts of guarantee claims but beware many are just rebranded and teh guarantee claims are very questionable.
Going back to Josh's roof. How long do you want it to last. The cheapest 25 micron PE coated steel with carbon fasteners will work out about £10/m2. Th stainless fasteners between £25 to £30 (but no OSB/insulation/membrane/ paint finish etc to buy).
Ian

 

[Joshjosh]

Thanks for all your advice guys
I thinks I’ll have to crunch some numbers and see what comes out cheaper
I’ve roughly worked out for a roof 35m2 in rubber with loft insulation, 18mm osb, and timber for spars I’m looking at around £1500
So if I can get sheets and z purlins considerably cheaper that might be my best bet
From what you’ve said Hornbeam

 

[devonwoody]

I did the Cladco system three years ago and have not seen any condensation or drips in my double garage concrete block garage, no signs of rust and a very exposed site in Devon. Could be self fitted if you are a handyman and a second fit fitter.

 

[Beau]

Thanks for all your advice guys
I thinks I’ll have to crunch some numbers and see what comes out cheaper
I’ve roughly worked out for a roof 35m2 in rubber with loft insulation, 18mm osb, and timber for spars I’m looking at around £1500
So if I can get sheets and z purlins considerably cheaper that might be my best bet
From what you’ve said Hornbeam

No need for Z purlins IMO. I built all our sheds and barns with treated timber which I am happier working with. If you use Cladco sheets I wouldn't be worrying about the cheaper sheets rusting out quickly either. I have used many of their grades even seconds with no coatings on one shed and that must be 8 or 9 years old an no rust marks let alone at the end of it's life. Stainless is probably better for the fixings but again the galvanised fixings I have had from Cladco have been perfect. Have used cheaper ones from Srewstation which have started to rust and poor mastic seals. Can take some pictures if it would help. All self installs which is pretty easy IMO.

I should add I have no connection with Cladco other than they are local to me.

 

[Hornbeam]

I am only giving what is considered industry best practice
Agree no need for z purlins but if using treated timber I recommend a strip of DPC between the timber and the sheet as a barrier as some treatment chemicals can react with the steel sheeting.
Treated timber is far less likely to react with stainless fasteners. If you want advice contact SFT , Ejot or other reputable manufacturer.
Regarding your roof, dependent upon exact specification, I would expect to self install a fully insulated composite panel roof for under£1K.
Self installation is relatively straightforward but it is the detailling and flashings which will make it a quality job or otherwise.
Give me a call in work 01244 892303
Ian

 

[Hornbeam]

I have just checked prices for a foam composite roof. Works out at £1000 + VAT + delivery so around £1400 plus fasteners and a few flashings. That would be 100mm insulation which would give a U value of 0.2
What U value means is that you will lose 0.2 Watts/heat per m2 per degree C. So for your size builsing and a cold winters day you would only be losing 0.2 X 35 x 20 watts of heat through the roof are which is only 140Watts

 

[Joshjosh]

I have just checked prices for a foam composite roof. Works out at £1000 + VAT + delivery so around £1400 plus fasteners and a few flashings. That would be 100mm insulation which would give a U value of 0.2
What U value means is that you will lose 0.2 Watts/heat per m2 per degree C. So for your size builsing and a cold winters day you would only be losing 0.2 X 35 x 20 watts of heat through the roof are which is only 140Watts

Thanks for this I appreciate your time. So what would be the roof build up on this? Would a vapour barrier still be required on the inside?
Would the sheets sit straight ontop of the joists? And would the joists still need to be the same size?
Thanks for your help
Josh

 

[Hornbeam]

Hi Josh.
A foam composite panel is effectively the entire roof buildup. It consists of a painted profiled steel outer sheet, 100mm of PIR foam which is foamed during the panel making process and a white steel liner sheet. The liner sheet is impervious to water and the foam does not absorb water. The panels sit directly onto the wall plate and any joists (preferably with a separation layer of PVC DPC over the joist only to prevent timber-steel contact. Because the panels are incredibly stiff they have very good spanning capability but the governing factor may be the size of your roof purlins/joists.
Weight of this type of roof would be around 12Kg/m2. Also because PIR is highly thermally efficient you only require about half the thickness of glasswool to achieve the same level of insulation.
The panels are fixed with long through fasteners but these need very precise specification
Seriously give me a call in work and I will give you general technical advice as a fellow forum member (and I can do it in work time)
Ian

 

[Wood Monkey]

I've just replaced my 13 year old shingles with plastisol coated steel sheets. My workshop is under some trees and hence the shingles were past their best after 13 years, but their death was bought on by mice getting under them and nesting in the roof insulation. The resulting leak rotted out the OSB and eventually leaks occurred.

I cut out the rot and put a patch panel in the covered the whole roof with a moisture control membrane, some patterns for air flow and then the steel panels. I was apprehensive about starting as I was working on my own, but the roof was shot and another wouldn't have survived the winter. In the end it took me less than two days working on my own.

The steel panels when on beautifully (respect to the suppliers who were incredibly helpful) and I'm chuffed to bits with the final result. The panels on the front are 2.3m, but on the back I extended them to 3.2m to create a continuous run that covers my dry store, replacing a nasty plastic one I built years agod (3rd pic).

I have to finish off the eaves caps and a few bits'n'bobs in the spring and I ordered enough steel panels to create a new extension on the end which is why you can see the excessive overhang, but the leaks are a distant memory and it looks great.