Workshop roof - Steel sheets.

[oakfield]

Does anyone here have any experience using steel roofing sheets?

I am building a new workshop at home.
it is following the basics of Mikes brilliant plan with a concrete base and 220mm high brick plinth, 4x2 walls etc.

It will have a shallow lean-too roof which I plan to cover with steel box profile sheets, mainly to fit in with other buildings around it.

I have used this on another building which was being converted as a habitable space so used the construction in the picture as follows:
steel box profile sheets,
breathable membrane
OSB decking
6x2 rafters (400 centres)
100mm Celotex between rafters
100mm Celotex below rafters
plasterboard
plaster.

I was planning similar construction but with less insulation for the workshop:
steel box profile sheets,
breathable membrane
OSB decking
6x2 rafters (600 centres)
100mm Rockwool between rafters
OSB

Does that sound ok, or is it still over the top?
Could I forget about the external OSB decking and just use counter battens instead?
could I space the rafters further apart and use purlins? (this would then cause problems with insulation and internal OSB.)
Or should I look into insulated roof panels?

 

[JackMolyneux]

This is exactly what I was going to ask as well. Buiding pretty much the same structure as of Friday. I've never used this sheeting before previously however have heard of moisture issues with them.. I'm presuming if following your plan of membrane, OSB, air gap and insulation that shouldn't be an issue.

I'll watch this space!

 

[MikeG.]

Yeah, that's the way to do it.....so long as there is airflow above the uppermost layer of insulation.

As for your conversion for residential, I'd caution against the 100mm below the rafters/ joists being in one hit, because that will mean trying to fix your plasterboard in place with something like 150mm screws! I would suggest putting 50x50s across the roof timbers, with 50mm insulation between, then 25 x 50 across those (the other way), with 25mm insulation between, and finally 25mm of insulation across that. This means your plasterboard can be screwed to wood (mark all your batten positions carefully) with only 60 or 70mm screws.

 

[oakfield]

I'll watch this space!

Hopefully you'll get some useful information too!

Yeah, that's the way to do it.....so long as there is airflow above the uppermost layer of insulation.

Thanks Mike,
Would you say the OSB deck is necessary, or could I just counter batten?

With regards to the residential conversion - that sounds like a really good idea, unfortunately I did that work a couple of years ago! (with long screws!)

 

[MikeG.]

They wouldn't be counterbattens, as they'd be across the roof rather than down it. I think you could do that if the rafters were at orthodox spacings and the battens were strong enough at their spacings to enable you to work up there safely. However, my limited experience of roof sheets is that both rafters and battens are too widely spaced to take the weight of people on the roof. The OSB looks to me like A/ being for the safety of the people fitting the roofing, and B/ being a useful place for any anti-drumming membrane. I could be wrong about that, and suggest you check the manufacturer's recommendations.

 

[Jonathan S]

Metal sheet on counter batters = drum!

Metal sheet on osb will be quieter in rain.

Just done a big roove for water harvesting, put the sheet on a bed of polymer silicone before screwing down to help stop the drumming effect.

Sent from my SM-J530F using Tapatalk

 

[Hornbeam]

There is no structural need for the OSB but the counter battens will need to be sized to take all the load. A typical 32/1000 trapezoidal profile roof sheet can generally be fixed at 1800mm centres. I would recommend halving this for your construction as it will reduce the size of the cross members and stiffen up the whole sheet reducing any tendancy to drum. With noise consider rain impact, not too much of an issue with an insulated structure like yours and wind suction drumming which can actually be worse when inadequately secured onto OSB.
Make sure you have a separation layer between your timber cross members and the underside of the steel sheet. The backing coat of the sheet will only be a single coat polyester about 10 microns thick and wood contains tannins/preservatives that will cause corrosion. Tata Steel recommend a thin DPC membrane layer between. The money you will save on no OSB can be better spent on more insulation or dense acoustic insulation
Ian

 

[oakfield]

Yeah, that's the way to do it.....so long as there is airflow above the uppermost layer of insulation.

Does that airgap need to be between the insulation and membrane, or can the insulation fill the rafters and an air gap be made between the membrane and steel sheet by using cross battens?

They wouldn't be counterbattens, as they'd be across the roof rather than down it.

I thought I might get picked up on that, but after a quick google didn't find a more appropriate term to use!

I could be wrong about that, and suggest you check the manufacturer's recommendations.

I have found it very difficult to find much information from manufacturers other than referencing steel framed, non insulated buildings.

Metal sheet on counter batters = drum!Metal sheet on osb will be quieter in rain. Just done a big roove for water harvesting, put the sheet on a bed of polymer silicone before screwing down to help stop the drumming effect.

Thanks for your advice.

There is no structural need for the OSB but the counter battens will need to be sized to take all the load. A typical 32/1000 trapezoidal profile roof sheet can generally be fixed at 1800mm centres. I would recommend halving this for your construction as it will reduce the size of the cross members and stiffen up the whole sheet reducing any tendancy to drum. With noise consider rain impact, not too much of an issue with an insulated structure like yours and wind suction drumming which can actually be worse when inadequately secured onto OSB.Make sure you have a separation layer between your timber cross members and the underside of the steel sheet. The backing coat of the sheet will only be a single coat polyester about 10 microns thick and wood contains tannins/preservatives that will cause corrosion. Tata Steel recommend a thin DPC membrane layer between. The money you will save on no OSB can be better spent on more insulation or dense acoustic insulationIan

Thanks Ian, that's great information.
I was thinking of 3x2 at 950 centres across the roof.
It works out about £230 cheaper than using osb, so I'll have a think.
I can't get a delivery of materials until 17th, so have a bit of time to decide!

Thanks for all the help and advise.

 

[MikeG.]

.......I thought I might get picked up on that, but after a quick google didn't find a more appropriate term to use!.......

Battens.

Counterbattens sit along the line of the structural timbers. Battens are across them.

 

[oakfield]

Oh, simple!
Thanks.

 

[Hornbeam]

[/quote]
I have found it very difficult to find much information from manufacturers other than referencing steel framed, non insulated buildings.

I work for the largest manufacturer of steel sheeting for cladding in the UK covering technical advise for constructions and cladding system testing . Drop me a pm

 

[oakfield]

I work for the largest manufacturer of steel sheeting for cladding in the UK covering technical advise for constructions and cladding system testing . Drop me a pm

Thanks Ian,
I will take you up on that.

I am trying to decide about the roof insulation at the moment.
If I am using 150x50 rafters, can I full fill them with 150mm insulation as long as I use breathable membrane and create an air gap by using battens above the membrane for the roofing sheets? Or do I still need a ventilated air gap between the insulation and membrane? If so, I will have a maximum of 100mm insulation and then can I only use 100mm noggins as surely if I use 150mm that will stop the air gap being ventilated.

 

[Hornbeam]

The issue about air gap/no air gap/limited air gap is all about air movement to prevent a build up of moisture in the cavity which can then condense on the underside of the steel roof sheet. This then asks the question where does the excess water vapour com from.
If the interior of teh workshop is damp/humid and the underside of teh roof is poorly sealed, then it will permeate through into the cavity and if it is cold outside it will condensate on the underside of teh steel sheet. This shouldnt be an issue as you should ensure that the workshop is relatively dry and you must fully seal the underside of the roof.
The other source of moisture ingress into the cavity is through large ventilation. During the day air will be warmer and able to hold more water. It enters the ventilation cavity. At night the steel roof sheets get cold and condensation forms, which if significant can then drip. There are 2 ways of tackling this. There are commercial anticondensation backing fleeces which are attached to the back of teh steel sheet which can absorb a limited amount of condensation (at night) and then dry out during the day with warmth and ventilation ( they are commonly used in animal housing)The other approach is to have a relatively full cavity with limited ventilation any ventilation is primarily thermally driven with venting at ridge and eaves only in the crowns of teh profiles/
Most commercial steel clad buildings are constructed with a fully sealed inner sheet (or a membrane on top of teh inner sheet, a fully filled insulation cavity and a fully sealed outer sheet with vented filler blocks at the ridge and eaves
Ian

 

[MikeG.]

.....If I am using 150x50 rafters, can I full fill them with 150mm insulation as long as I use breathable membrane and create an air gap by using battens above the membrane for the roofing sheets? Or do I still need a ventilated air gap between the insulation and membrane? If so, I will have a maximum of 100mm insulation and then can I only use 100mm noggins as surely if I use 150mm that will stop the air gap being ventilated.

Noggins? They're not normally used in a roof. They're not a good idea at all.

As to the air gap question you ask.......If you fully fill the rafters, your membrane won't drape between rafters, but will lie flattish as though pulled taut. Water that condenses on the underside of the roof sheet will then drip onto a flat membrane and run down the roof until it meets a batten, which will block its path to the eaves (because there will be no gaps under the battens as the membrane is flat). That pooling of water at the top of the battens will eventually find a way through the nail holes (and any staple holes) in the membrane and into your structure.

On the other hand, if you drape the membrane in the normal way then any water that which falls onto it will roll down to the lowest point between the rafters, and have a clear route down to the eaves without going anywhere near the nail holes, which are "uphill" on the top of the rafters. Think of the membrane as a series of very shallow troughs down the roof. So the point of not filling the rafters fully isn't about airgaps, but about interference with the normal draping of the membrane. The airgap that counts is the gap above the membrane, not the one below, which is there just to allow the membrane to take its proper shape.

 

[Hornbeam]

Older steel roofs used to be built with an air gap typically 40mm and a membrane. Newer constructions do not have a breather membrane or an air gap.
The big proviso is that you must have a fully sealed inner sheet/board which can be done either through good sealing practices or a membrane on top of the inner sheet but underneath the rafters to prevent any moisture inside the building entering the cavity.
As there is very little air/moisture entering the cavity the limited ventilation through the profiles is adequate
Ian

 

[bobblezard]

Hi,
I've been reading this thread with interest.
I have a 6m X 4m garage which adjoins the gable end of my house. The long side faces the prevailing wind.
I'm planning to reroof it with metal sheet pretty much as described above with new rafters, 3x2 battens and OSB below the insulation.
I have a couple of questions if anyone is able to comment, the current pitch is around 2:12 which I would like to increase to around 5:12 is this likely to need planning permission? The low edge of the roof is around 1.5m from the boundary and well under 2.5m but against the house it would rise to around 3.6m.
My other query is how to go about covering the gap between the end of the sheets and the flashing (into the brickwork of the house). I have seen right angled steel profile, can this be bent to match the pitch? I assume there will need to be some filler under this to prevent water being driven underneath.
Sorry if this is elementary but I've not seen an obvious solution in my researching.

 

[Hornbeam]

The way you seal the ends of the sheeting depends upon what you are trying to achieve.
Normal practice would be too purchase matching bent steel flashings. At the point where the roof joins your house wall, the flashing is detailed into the brickwork . It sits on top of the crowns of the roof sheet and is secured to the roof sheet either with pop rivets or drill =drive stitcher screws with EPDM washers (always use stainless screws).The flashing should have a minimum of 75mm upstand and extend 150 mm down the sheeting Ideally the bottom edge of the flashing should be fully turned back on itself ( hemmed/welted edge)This stiffens it up considerably and removes the cut edge out of the wet area. The small gap between the flashing and the troughs of the profile can be filled with an EPDM or PE filler block but you must use vented blocks In practice the blocks keep vermin/birds out but you can leave out. If your brickwork is out of alignment with the roof you can use a 2 piece overlapping flashing to take up the misalignment.
At the eaves, it depends upon the construction of your wall and the junction details but again a bent piece of steel flashing extends upwards following the vertical of teh wall and is bent back into the cavity. Stitcher screws with EPDM washers fasten though the trough of the profile into the flashing.
Note main fixing screws should be secured through the trough of the roof profile as it is generally much more secure than crown fixing and the EPDM washers seal well, the only exception to this is for sinusoidal (wrinkly tin) profile.
From a water drainage and leak point of view there is no need to increase pitch. So long as you design for at least 6 degrees then you will be OK , particularly as you have will be having one continuous sheet from the wall to the eaves

 

[Sheptonphil]

In short, you would need planning permission for the building’s new roof because it’s less than 2m from boundary and greater than 3m for max height.

Whether anyone would either know or mind is another matter, but the lawful answer is yes.

Planning, apart from the cost of £230 or so, is not very onerous, and if you stay below 30sq metres, you avoid the need for building control.

I came up against this with my build and just put in for planning, it went through no problem and I can have what I wanted, not just the limited permitted development ( which is why there is the low pitch at present on yours to comply with permitted).

You can get most profiles to flat pieces for exactly your scenario. They are approx 600mm sections of your roofing material which end up as flat for the final 150mm or so.

This one is for box,

But they’re available for round as well, like this one.

 

[bobblezard]

Thanks for the replies, for some reason the board wouldn't let me respond yesterday so seeing if this will work.

 

[bobblezard]

It did - yes I will have to check out the moulded flashings/edgings. I had planned to a quote from a local supplier who supplied my mates reroof (but that was a standalone building).
As for raising the roof. The main Reason is to win a bit of space inside for over head storage, it will probably be worth going for planning permission.
Thanks again