Corrugated Metal Roofing

[Cpd]

i was asked me by @Visti by Pm for a bit more detail on my roof installation and a step by step account of the process of Installing velux windows in a wriggly tin roof...... these things are always hard to remember after the event...... but I will try. 

1. The positioning of the windows has to be decided at design stage as the roof design will need to take into consideration the velux openings. 

2. Confirming where the edge of the tin is to start in relation to the edge of the roof is critical so that you have a datum point to start from.

A potential problem With the roof design could be that the timbers that “box out” the opening could later infringe on the interior opening size of the window, you need to also think about cold bridges with this boxing out section as again a can see where you could end up with inadequate insulation on the internal sides of the boxing out section if it was put to close to the velux frame.  

3. Carful pre design of where your tin fixings are going to go is essential if you want it to look right........  I started at 120mm up from the base of the tin and then every 1200mm with the last fixings hidden beneath the ridge flashing. I’ve seen a lot of roofs where this has not been though about and you end up with some strange and pretty ugly fixings that can really make a good roof look bad......  I also used string lines across the roof to mark my fixing points with a permanent marker. 

4, there is some wiggle room when cut the holes out in the roof and you should take advantage of this make sure you cut the holes out with the extra allowable space around the perimeter )   as it’s the only place you have to make adjustments to the velux / flashings position if required when matching the tin up to the flashing. 

5. If installing multiple windows in a row, I did 4..... you need to remember to always go by your datum point not the last window....... otherwise the error will just follow you and the edge of the tin won’t fall into the internal gutter !  It’s really essential that you get this right as it would look like a bag of shite if the edge Of the tin doesn’t  line up within 5-10mm of where you want them to fall in the velux gutters. 

6. Avoid using cut edges anywhere that is exposed ! being on a tight budget I did use one cut edge that is at the base of the roof and I expect it will be the fit bit to start to fail once the salt water gets into it....... 

7. When cutting tin...... I used a jigsaw with the tin clamped between wood, be carful not to damage the tin with this wood, it may be wise to make up some slightly padded wood clamps for this work. After cutting there are loads of really nasty sharp filings and you need to get these off without damaging the tin as they will stick into the plastic coating on the tin and then start to rust ! Use the longest metal cutting bit you can and practice on a bit of offcut first as there is a real knack to cutting tin with a jigsaw.. .   

8. I put up a barge board at the base of the roof at the exact point where the tin was to end, this meant that when I was putting up the tin I could lay a bunch of it on the roof and CHECK to see if it was going to line up with the flashings ! Remember that if you put the barge board up at 90 degrees you will need to take into account that the tin has a 20mm profile and laid to the pitch of the roof that is 45 degrees and the top of the wriggle on the tin will hit the barge board and this is not the actual edge..... anyway you will work it out....... the barge board NEEDS to be solid as it’s going to have some good pressure on it when you get a bunch of tin laid out so go overboard when designing it.... just a piece of sarking tased In will not do......

9. The tin is very easy to handle but be carful of the sharp corners as they just live to rip your lovely new membrane! 

10. I need to stop as I am the worlds slowest one finger typist 

 

i am sure there is  more  and I am sure others will chip in with the bits I have missed or miss represented ! 

[Cpd]

Just a small detail in regards to gable flashing onto timber cladding, I have found that leaving a small gap between the roof flashing and the timber stops water running down the timber, not sure if this is standard practice but I had one which was tight fitting and water was tracking onto the timber was causing unnecessary dampness and staining. On my glorified shed I stood the flashing off by 10mm+  and the water now tracks harmlessly down the lower side of the flashing and drips away. Small detail for a very specific application but may be of help to some. I used a small strip if wood tacked into the top of the timber cladding and under the flashing as a spacer to get a consistent gap. 

 

Edited September 6, 2019 by Cpd

[Tom's Barn]

Hi

 

Does anyone have any guidance on fitting corrugated roofs? I am struggling to find a contractor to do the work. Any insights greatly appreciated. We are just getting the rafters in and OSB over the next two weeks.

 

I am going with a 25 x50 vertical batten to hold down the membrane but am struggling to find information re the horizontal batten. 65mm timbertek screws recommended by Thomas profiles. The corrugated profile is 19mm high. 

 

Any guidance is appreciated

 

Tom

[TheMick]

Can anyone advise regarding the same corrugated roofing 13/3 please. 

 

First the widths of 990mm when lapped for one side of roof at 1.5mtr width - is there any reason why I can't have two sheets with a larger overlap in order to achieve the 1.5mtr width without the need to cut? 

 

Second question is it better to use perlins as instructed with lengths of timber or full OSB board thick enough to take the fixings screw lengths?

 

Sorry if this is basic stuff but it's new to me. 

 

Thanks in advance. 

 

 

[Ferdinand]

8 hours ago, TheMick said:

Can anyone advise regarding the same corrugated roofing 13/3 please. 

 

First the widths of 990mm when lapped for one side of roof at 1.5mtr width - is there any reason why I can't have two sheets with a larger overlap in order to achieve the 1.5mtr width without the need to cut? 

 

Second question is it better to use perlins as instructed with lengths of timber or full OSB board thick enough to take the fixings screw lengths?

 

Sorry if this is basic stuff but it's new to me. 

 

Thanks in advance. 

 

 

 

I overlapped an extra section, so I see no problem at all with that.

 

Ferdinand

 

[TheMick]

On 08/04/2020 at 07:10, Ferdinand said:

 

I overlapped an extra section, so I see no problem at all with that.

 

Ferdinand

 

So you think plywood or osb sheet is OK instead of battens/perlis too? 

 

Also when you screw in this roofing I notice the screw goes through the part of the corregated roof which is not touching the roof. The top of the arch if that makes sense. 

 

What stops the screw from over torqueing or is that not an issue as of the hardness of the tin? 

[Visti]

Overlaps more than recommended are fine with the steel sheets. Just ensure they're aligned top to bottom to ensure a close fit at both ends. The more that the profiles that overlap the more any misalignment becomes obvious as gaps form between the sheets, so you may want to cut. If you do, no problem. Just have the cut edge go under the next sheet.

 

Not following you on the ply/OSB. What is your roof structural detailing?

 

And yes, you can over screw and warp the profile down. It's limited though as it's only 18mm normally, and from the ground you won't see much of that at all.

[TheMick]

1 hour ago, Visti said:

Overlaps more than recommended are fine with the steel sheets. Just ensure they're aligned top to bottom to ensure a close fit at both ends. The more that the profiles that overlap the more any misalignment becomes obvious as gaps form between the sheets, so you may want to cut. If you do, no problem. Just have the cut edge go under the next sheet.

 

Not following you on the ply/OSB. What is your roof structural detailing?

 

And yes, you can over screw and warp the profile down. It's limited though as it's only 18mm normally, and from the ground you won't see much of that at all.

I'm on about having a ply or osb full sheet then corrugated roof screwed directly onto it instead of the perlins/battens suggested in the install, video. 

[Russell griffiths]

7 hours ago, TheMick said:

I'm on about having a ply or osb full sheet then corrugated roof screwed directly onto it instead of the perlins/battens suggested in the install, video. 

This all depends on your ventilation details, what are you doing to combat condensation 

you will get lots of condensation on the back of the sheets so you need to work out how it will dry out

good ventilation is the first answer

 

[Cpd]

7 hours ago, TheMick said:

I'm on about having a ply or osb full sheet then corrugated roof screwed directly onto it instead of the perlins/battens suggested in the install, video. 

This is not a good idea..... done this with with ply and it was not a good solution..... you need explain what this roof Is for ? Unheated Shed - house with warm roof - house with cold roof - shed for cows - the list is endless and each use often demands a different solution. After we know what it’s for it will be easier to give guidance. 

[Visti]

You need to have some ventilation space between the structure and the tin. About 50mm. Battens (and crossed) or Perkins really are necessary, and drilled through to the rafters ideally so that they have the most solid purchase... Those wind speeds can be high, and you don't want to mess with negative pressure!

 

The structures skin can be ply or osp, but you'll want an external vapour control layer to let it breath whilst protecting it a bit from the elements. 

 

We also had an anti-condensation layer on the back that should help prevent moisture issues even if the ventilation fails.

[Cpd]

1 hour ago, Visti said:

The structures skin can be ply or osp,

If you live in a damp environment, like Scotland...... or the building is going to be open during winter I would steer clear of ply, osb is better as in my experience ply seems to attract black mould, even stuff I have stored has got mould spots on it and no other wood close by had suffered. 

[TheMick]

2 hours ago, Visti said:

You need to have some ventilation space between the structure and the tin. About 50mm. Battens (and crossed) or Perkins really are necessary, and drilled through to the rafters ideally so that they have the most solid purchase... Those wind speeds can be high, and you don't want to mess with negative pressure!

 

The structures skin can be ply or osp, but you'll want an external vapour control layer to let it breath whilst protecting it a bit from the elements. 

 

We also had an anti-condensation layer on the back that should help prevent moisture issues even if the ventilation fails.

Hi visti, 

I understand what you say about the ventilation between the surface and the underside of the corrugated sheet but won't the ventilation be blocked by those wavy infill pieces cladco give you to put in the bottom of each sheet? 

 

Or will the ventilation just come from the peak of the roof under the ridge flashing you put on? Plus if screwed directly onto the sheet won't there be ventilation up every corrugation arch anyway? 

 

Thanks for all your replies. Just wanting it all clear in my head before attempting. 

Edited April 10, 2020 by TheMick

[Cpd]

I left the bottom open as you need ventilation, I used these at the top to stop water blowing up under the ridge...... very windy location and water blows right over the roof..... I fitted them on top of the tin and below the ridge flashing. Until you explain what your roof is for its very hard for people to give good advice. 

[Visti]

Exactly as @Cpd did, we only used the foam filler to plug the gap at the top with the ridge cap, since that doesn't have a corrugated profile. 

 

If you screw the sheet directly to an OSB board then:

 

1. You'll have very limited ventilation, since you're restricting it per vertical profile, and allowing not very much volume of circulation at all (Cladco profiles are only 18mm thick).

 

2. Condensation will run directly onto the OSB from the tin, with no chance to evaporate first or on an intermediary medium such as a batten or purlin which don't matter as much as your structural envelope.

 

Do you have your roof buildup details?

 

Any reason you can't use battens or purlins? The former are cheap as chips. If you're in Oxfordshire I've 4 packs of 10 just lying around.

[TheMick]

It's like a detached garage with concrete floor and walls. It's always had a makeshift roof on which I've just been bodging each year or two. 

 

I want to get a better roof on then further down the line maybe board and insulate the walls, put a moisture barrier in possible board and insulate the ceiling etc etc

[TheMick]

On 10/04/2020 at 15:33, Cpd said:

I left the bottom open as you need ventilation, I used these at the top to stop water blowing up under the ridge...... very windy location and water blows right over the roof..... I fitted them on top of the tin and below the ridge flashing. Until you explain what your roof is for its very hard for people to give good advice. 

But the cladco install video shows them going in the bottom near the gutter. 

It's doesn't mention fitting then at the top at all.

 

Do you think it's better to just have the battens running lengthways or have a sheet covering the whole roof with the battens on top of it? Thx

[Russell griffiths]

This is how I would do it 

cover the tops of the rafters with a good quality breather membrane, then a 50x25 counter batten that fits on top and parallel to the rafters then a 50x25 batten fixed on top but at 90degrees to the rafters, 

fit sheets on top

use a ventilation strip at eaves and ridge you want airflow but not bugs. 

 

Those battens are 100x25 and are spaced 100mm apart 

you won’t need to do this, space them how far apart the sheet manufacturers say you need the fixings 600-750mm whatever, remember you won’t be able to walk on anywhere there isn’t a batten. 

mesh at facia level

 

[joe90]

With my garage/workshop cladco roof I enquired about “anti condensation coating” and was told it’s only any good in well ventilated places like barns. Instead I simply used roofing battens over roofing felt draped slightly, profile filler at ridge to stop driving rain and bug screen at eaves. This works very well.

[Cpd]

3 hours ago, TheMick said:

But the cladco install video shows them going in the bottom near the gutter. 

It's doesn't mention fitting then at the top at all.

 

Do you think it's better to just have the battens running lengthways or have a sheet covering the whole roof with the battens on top of it? Thx

I think @Russell griffiths design is spot on, max ventilation and no wildlife can get in, but as he said space your battens out as required. 
I really can’t see how the rubber infill  is a good thing at the bottom....... you need ventilation. I guess you could fit it on top of the first batten in conjunction with a wire mesh to stop wildlife and let the ventilation work as it would be quite hard to get wire mesh to work with the corrugations. You will need to do something at the top if you live in an exposed location as I have watched water blowing right up my 45 degree  roof and water was just pouring in under the ridge and running down the roof membrane and into the gutter at the bottom ! The rubber strip worked really well to stop this.  

[Cpd]

You would need to detail the insect mesh where it meets the roof membrane and I’m not sure how you would get the rubber in the right place as the first Fixing  batten will be at least 100mm up the roof... just needs a mock up to come up with a plan. 

[TheMick]

9 hours ago, Cpd said:

I think @Russell griffiths design is spot on, max ventilation and no wildlife can get in, but as he said space your battens out as required. 
I really can’t see how the rubber infill  is a good thing at the bottom....... you need ventilation. I guess you could fit it on top of the first batten in conjunction with a wire mesh to stop wildlife and let the ventilation work as it would be quite hard to get wire mesh to work with the corrugations. You will need to do something at the top if you live in an exposed location as I have watched water blowing right up my 45 degree  roof and water was just pouring in under the ridge and running down the roof membrane and into the gutter at the bottom ! The rubber strip worked really well to stop this.  

Thanks for that. The roof membrane if I have this right should be breathable to allow any moisture from the inside of the room to be released into the ventilated area beneath the corrugated tin. 

If the room ceiling was boarded out on the inside face of the rafters with insulation behind it wouldn't that mean there would be no warm air/moisture to release in this way? 

[TheMick]

10 hours ago, joe90 said:

With my garage/workshop cladco roof I enquired about “anti condensation coating” and was told it’s only any good in well ventilated places like barns. Instead I simply used roofing battens over roofing felt draped slightly, profile filler at ridge to stop driving rain and bug screen at eaves. This works very well.

So by draped do you mean just place off cuts of roof felt over the sky facing edge of the rafters where the screwed battens on top holding it in place? Thx

Edited April 12, 2020 by TheMick

[joe90]

7 hours ago, TheMick said:

So by draped do you mean just place off cuts of roof felt over the sky facing edge of the rafters where the screwed battens on top holding it in place? Thx

not quite, roofing felt on top of rafters but not pulled fully tight, this means any moisture (condensation) can run down the felt, under the battens and out the bottom. Battens nailed on at 600mm centres (for my 0.7mm steel roofing sheets). If the felt is very tight the moisture can build up at the battens and cause rot. No need for counter battens IMO.

Edited April 13, 2020 by joe90

[Cpd]

1 hour ago, joe90 said:

No need for counter battens IMO

Good point made there @joe90 about no need to counter batten, all my buildings have been built with sarking so you NEED to counter batten but on a roof where it’s just, rafters, membrane, batten and tin then there is no need. Living in such a windy location as I do means you need a solid roof from the get go.

 

9 hours ago, TheMick said:

Thanks for that. The roof membrane if I have this right should be breathable to allow any moisture from the inside of the room to be released into the ventilated area beneath the corrugated tin. 

If the room ceiling was boarded out on the inside face of the rafters with insulation behind it wouldn't that mean there would be no warm air/moisture to release in this way?

The roof membrane does a few things, it’s your major line of defence if you get a leek, when I had water blowing back up the roof and under the ridge it would have been a disaster if it was not for the roof membrane...... also tin is notorious for condensation and it will run down the inside of the roof sheets and drip at points where the tin sits on battens, again without the membrane this water would be into your house.... the next big thing is it’s a breathable so yes it will help to allow any moist air from The inside out whereas the old tar felt or non breathable felt would just end up with condensation on the underside with nowhere to go and cause all sorts of problems. 
 

boarding out with foil faced insulation board if foamed and taped VERY well can act as part of your VCL but the overall buildings  VCL need to be looked at as a whole and not just individual areas, but yes this is how I did mine.