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Garage Roof - overclad or start again?


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What's the best bang for buck method to fix a single skin corrugated iron roof that both leaks and has condensation issues? I was going to simply overclad with Onduline leaving the old sheets in place, but whilst this might reduce the condensation I doubt it will stop it altogether. Would a breathable membrane help?

I could remove the corrugated iron and put OSB down, then lay onduline on top, but that complicates things a little as it would raise the roof height by 18mm and maybe give me a bunch more headaches.

It's worth getting this right so I have a usable garage space, so throwing open to the DOSBODS experts for opinions on the best/easiest solution that doesn't cost the earth.

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Bricks & Mortar

Looks like you've 10 degrees or so right enough.

That looks set up to do steel sheets again.  Looks like the timber will do another whirl...  but check, and if they're rotten it should be possible to replace them when you have the roof off.

Existing is probably fixed with large roofing nails.  Remove with a crowbar.  I use one about 2 foot long that's got a sharp claw.  I have a plate of aluminium that I use under the heel so as not to dent the metal (in case the warping makes it harder to come off).  Sometimes hammer the crowbar to get it under the nail.

You'd want to see how the edges are finished right now and do similar.   If it's cement, and the sheets have been placed on top when it was wet; you might be able to put a good silicone, like Sikaflex EBT on the existing cement to seal the new sheets if they're the exact same profile.

I think you get a choice between getting the same size of sheets, and working with that... or you could try and find long sheets, the whole length of the roof.  I guess they'd be a bit more expensive and have to be delivered by a manufacturer.  Our local one only covers Scotland with their trucks.  The advantage of full length is no joints - rain can sometimes blow up here if there's wind and rain in the right, (wrong) direction.  With the pitch at 10-ish degrees, I think this is less of an issue than on some of the lower pitches I've been to.

To refix, you might use roofing screws rather than the nails.  These have a hex head for a battery drill.  They're drill pointed, so you just push them into the steel, they cut their way through, and into the timber, they have a rubber washer to seal, and you might get wee plastic caps for the heads.  Nails are all fine until you miss with the hammer - also can be sprung up by the roof sheet in high wind easier.

Sheets look like:
https://www.ebay.co.uk/itm/323784105844

Here is a supplier offering similar, in a thicker 0.7mm version, with a dripstop membrane on the underside as an option,
https://www.cladco.co.uk/13-3-0-7-thick-galvanised-corrugated-roofing-sheets

Roofing screws
https://www.ebay.co.uk/itm/122369362670

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Where is it leaking?

My dad's garage has a similar roof, been like that before moving in and perhaps all the way back to construction some 40 odd years ago,  and discovered the screws used to fasten the sheets together hadn't been sealed so the water was getting in that way. A tradesman in to do another job mentioned it, after my dad getting an expensive quote to fix it from someone else and wanting another opinion. The guy popped up on the roof dabbed some silicon or something over each screw and no more leaks. Cost £20. xD

Inside the garage he has some insulation supposedly to stop the condensation. Not sure if it works as before the roof fix there would be water dripping down inside, but he says it's dry now so perhaps good enough.

If doing it from scratch I'd look into that spray foam insulation they use on shipping containers for storage. Used a few of those over the years and never had condensation issues with stuff getting wet.

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17 hours ago, Fully Detached said:

What's the best bang for buck method to fix a single skin corrugated iron roof that both leaks and has condensation issues? I was going to simply overclad with Onduline leaving the old sheets in place, but whilst this might reduce the condensation I doubt it will stop it altogether. Would a breathable membrane help?

I could remove the corrugated iron and put OSB down, then lay onduline on top, but that complicates things a little as it would raise the roof height by 18mm and maybe give me a bunch more headaches.

It's worth getting this right so I have a usable garage space, so throwing open to the DOSBODS experts for opinions on the best/easiest solution that doesn't cost the earth.

Is it corrugated iron? It may very well be asbestos (same shape just thicker).

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18 minutes ago, eek said:

Is it corrugated iron? It may very well be asbestos (same shape just thicker).

yeah I have a load of asbestos on my garage roof :CryBaby:

@Fully DetachedI was looking at an eco new build recently, old folks home....it's got a corrugated metal roof with metal bits down the sides about a foot deep for the 'finishing bits'.......not sure what's underneath, I think OSB is popular

for a garage you probs don't need insulation.....

I think that's a good solution....OSB on your timber supports then just screw the metal sheets straight onto the OSB

edit: never heard of that onduline before, sounds interesting...what's the price difference between that and metal? can you get 'recycled rubber' roof panels? that's be good to dampen the rain noise lol

Edited by nirvana
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1 hour ago, BoSon said:

Where is it leaking?

My dad's garage has a similar roof, been like that before moving in and perhaps all the way back to construction some 40 odd years ago,  and discovered the screws used to fasten the sheets together hadn't been sealed so the water was getting in that way. A tradesman in to do another job mentioned it, after my dad getting an expensive quote to fix it from someone else and wanting another opinion. The guy popped up on the roof dabbed some silicon or something over each screw and no more leaks. Cost £20. xD

Inside the garage he has some insulation supposedly to stop the condensation. Not sure if it works as before the roof fix there would be water dripping down inside, but he says it's dry now so perhaps good enough.

If doing it from scratch I'd look into that spray foam insulation they use on shipping containers for storage. Used a few of those over the years and never had condensation issues with stuff getting wet.

It's leaking in a lot of small holes, it's just old corrugated iron that has withstood a few hard winters. They surveyor's recommendation was to simply overclad it with more sheet material, which is nice and easy but I really want to stop the condensation. The spray foam is definitely an idea though - I'll look into that.

@eek I'm pretty sure it's not asbestos, the surveyor said corrugated iron and he pointed out the possibility of asbestos elshewhrere so hopefully that's OK.

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34 minutes ago, nirvana said:

yeah I have a load of asbestos on my garage roof :CryBaby:

@Fully DetachedI was looking at an eco new build recently, old folks home....it's got a corrugated metal roof with metal bits down the sides about a foot deep for the 'finishing bits'.......not sure what's underneath, I think OSB is popular

for a garage you probs don't need insulation.....

I think that's a good solution....OSB on your timber supports then just screw the metal sheets straight onto the OSB

edit: never heard of that onduline before, sounds interesting...what's the price difference between that and metal? can you get 'recycled rubber' roof panels? that's be good to dampen the rain noise lol

Onduline is quite good in my opinion, nothing special. It's probably a few pounds a sheet cheaper than metal. Advantage is it looks nicer and doesn't rust. Disadvantage is it's nowhere near as long lasting and you need to use it with a lot of consideration to roof pitch for example as otherwise it will sag and last only a year or so.

I've used it on top of OSB before and it worked well. Problem I have now is that the metal sheets are sitting on purlins that some to the top of the garage, so if I go screwing 18mm OSB on top of them I will raise the roof by that amount. I doubt that's a problem planning wise, but someone rendered the outside of the garage right into the contours of the metal sheeting, so it would look very odd unless I go busy with a chisel.

I was wondering about recessing the OSB by mounting baton 18mm below the top of the purlins and then seating the OSB on top of the batons (fixing appropriately, obvs). The only problem with that would be that I have a joint either side of each purlin, although I guess I could seal that easily enough.

As for recycled material, there's a place just down the road that does it and it looks brilliant - water tight, long lasting and has some insulation properties - problem is it's £55 a sheet and my budget is disappearing quite quickly :P

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Bricks & Mortar

I do these commercially.  Small builder.  Maybe 1/2 dozen a year or so.

1.  Does it have enough pitch?  Different materials require different pitches.  Standards were lower in the past.  You could measure it with a level and a tape.  Measure the length of the level, and then set the level on the pitch, and measure the vertical distance at the end.  Longer levels are more accurate.
maybe get something like 30mm in 900mm (measured) = 1 in 30 = 3.33%
https://goodcalculators.com/slope-calculator/

2.  A photo would be helpful, as the roof edges and existing rainwater provision have an influence on the best solution.  Ideally, photos of the outside, and the inside.

At this stage, I'm thinking between 
A:  replace the existing steel.  I think you'd have to pull the fixings to overclad, and if you done that, might as well just whip the sheets and put new back where they came off.  Can't see the advantage of overcladding.

B:  Maybe Onduline.  maybe could use as a direct replacement for steel.  Wouldn't be thinking about an OSB deck, unless there's some reason to.  

C:  EPDM sheet.  This would need an OSB deck.  Likely more expensive than above 2.  But a full, one piece cover over the whole roof that might outlast the above as well.  The current price of OSB might well lead us away from this option.

Condensation?  Well, in the first instance, if the leaks were stopped, there'd be less moisture inside the garage to evaporate and cause it.  Is the garage well vented?  No problems with the vents?
You can get corrugated steel roof sheets that come with a coating on the underside to prevent condensation.
Onduline should be less prone,as should OSB.
I think the spray foam needs you to buy a cannister, and is quite expensive.  you need to move everything out the garage, wear full ppe, inc respirator, disposable suit, goggles, and even shoe covers unless you have a pair you'll throw out at the end.
Might be cheaper to build a ceiling under the joists and cram it with glass wool.
I wouldn't insulate unless the new covering has condensation too, (i.e I'd wait and see).  Or, if its not just a garage but a place you use for activities and feel the need to be warmer.

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Bricks & Mortar

PS - tiny holes definitely sounds like metal, not asbestos.  Should look rusty if this is the case.  Asbestos is like a thin cement board, and is unlikely to develop tiny holes - more likely to break like glass in a window would break.
A magnet would set your mind at rest.

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56 minutes ago, Bricks & Mortar said:

I do these commercially.  Small builder.  Maybe 1/2 dozen a year or so.

1.  Does it have enough pitch?  Different materials require different pitches.  Standards were lower in the past.  You could measure it with a level and a tape.  Measure the length of the level, and then set the level on the pitch, and measure the vertical distance at the end.  Longer levels are more accurate.
maybe get something like 30mm in 900mm (measured) = 1 in 30 = 3.33%
https://goodcalculators.com/slope-calculator/

2.  A photo would be helpful, as the roof edges and existing rainwater provision have an influence on the best solution.  Ideally, photos of the outside, and the inside.

At this stage, I'm thinking between 
A:  replace the existing steel.  I think you'd have to pull the fixings to overclad, and if you done that, might as well just whip the sheets and put new back where they came off.  Can't see the advantage of overcladding.

B:  Maybe Onduline.  maybe could use as a direct replacement for steel.  Wouldn't be thinking about an OSB deck, unless there's some reason to.  

C:  EPDM sheet.  This would need an OSB deck.  Likely more expensive than above 2.  But a full, one piece cover over the whole roof that might outlast the above as well.  The current price of OSB might well lead us away from this option.

Condensation?  Well, in the first instance, if the leaks were stopped, there'd be less moisture inside the garage to evaporate and cause it.  Is the garage well vented?  No problems with the vents?
You can get corrugated steel roof sheets that come with a coating on the underside to prevent condensation.
Onduline should be less prone,as should OSB.
I think the spray foam needs you to buy a cannister, and is quite expensive.  you need to move everything out the garage, wear full ppe, inc respirator, disposable suit, goggles, and even shoe covers unless you have a pair you'll throw out at the end.
Might be cheaper to build a ceiling under the joists and cram it with glass wool.
I wouldn't insulate unless the new covering has condensation too, (i.e I'd wait and see).  Or, if its not just a garage but a place you use for activities and feel the need to be warmer.

That's excellent, thanks! Here's a photo as it is currently - pretty grim as I don't think anyone has been in there for 15 years. Unfortunately I don't have one of the outside and we won't be back there until later this week, but it's basically rendered breeze block:

20211120_103039_resized.thumb.jpg.bffa7e1ce0a7f6ec716a8a920376c8b9.jpg

The two windows are either broken or non existent so I will replace those but ensure there is decent ventilation. You can see from the spacing of the joists why there is an issue with onduline - the spacing is about 1.1 metres so no way will onduline sit nicely on that ,especially with such a low pitch (I don't have that measurement to hand, but you can see that it rises by approx 2 breeze blocks over the 6 metre length, so I estimate that to be 10% or under)

There is also a potential issue with flooding as it is at the bottom of a sloping drive, but I was planning to do something with concrete there just to stop massive water ingress during heavy rain.

Ultimately it is what it is - I won't be using it as my relaxation space but it would be good to keep tools and have a workbench in there if at all possible, so it's a case of make it as good as it can be given the circumstances.

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27 minutes ago, Bricks & Mortar said:

Looks like you've 10 degrees or so right enough.

That looks set up to do steel sheets again.  Looks like the timber will do another whirl...  but check, and if they're rotten it should be possible to replace them when you have the roof off.

Existing is probably fixed with large roofing nails.  Remove with a crowbar.  I use one about 2 foot long that's got a sharp claw.  I have a plate of aluminium that I use under the heel so as not to dent the metal (in case the warping makes it harder to come off).  Sometimes hammer the crowbar to get it under the nail.

You'd want to see how the edges are finished right now and do similar.   If it's cement, and the sheets have been placed on top when it was wet; you might be able to put a good silicone, like Sikaflex EBT on the existing cement to seal the new sheets if they're the exact same profile.

I think you get a choice between getting the same size of sheets, and working with that... or you could try and find long sheets, the whole length of the roof.  I guess they'd be a bit more expensive and have to be delivered by a manufacturer.  Our local one only covers Scotland with their trucks.  The advantage of full length is no joints - rain can sometimes blow up here if there's wind and rain in the right, (wrong) direction.  With the pitch at 10-ish degrees, I think this is less of an issue than on some of the lower pitches I've been to.

To refix, you might use roofing screws rather than the nails.  These have a hex head for a battery drill.  They're drill pointed, so you just push them into the steel, they cut their way through, and into the timber, they have a rubber washer to seal, and you might get wee plastic caps for the heads.  Nails are all fine until you miss with the hammer - also can be sprung up by the roof sheet in high wind easier.

Sheets look like:
https://www.ebay.co.uk/itm/323784105844

Here is a supplier offering similar, in a thicker 0.7mm version, with a dripstop membrane on the underside as an option,
https://www.cladco.co.uk/13-3-0-7-thick-galvanised-corrugated-roofing-sheets

Roofing screws
https://www.ebay.co.uk/itm/122369362670

Ah that is absolutely fantastic, thank you!! I'll do that but try to find the sheets with the condensation proof coating initially, then if that doesn't work look at other options. Since I won't be going down the OSB route I can afford to spend a little more on the sheets anyway.

I'l get them ordered up and hopefully fitted when we're there next weekend - cheers!

 

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16 minutes ago, Fully Detached said:

Ah that is absolutely fantastic, thank you!! I'll do that but try to find the sheets with the condensation proof coating initially, then if that doesn't work look at other options. Since I won't be going down the OSB route I can afford to spend a little more on the sheets anyway.

I'l get them ordered up and hopefully fitted when we're there next weekend - cheers!

 

It’s similar to the metal decking I used to put on rsjs in London high rise buildings.you could go ott put some rsjs down I could then stud weld it after I had decked it then fill it with concret it won’t leak but your walls will probably collapse

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24 minutes ago, Fully Detached said:

Ah that is absolutely fantastic, thank you!! I'll do that but try to find the sheets with the condensation proof coating initially, then if that doesn't work look at other options. Since I won't be going down the OSB route I can afford to spend a little more on the sheets anyway.

I'l get them ordered up and hopefully fitted when we're there next weekend - cheers!

 

If after sealing roof and fixing any water ingress from the door side - you could have gravel strip or drainage channel at front edge then condensation from airflow in the winter might be the remaining issue, sealing a standard up and over door is difficult;t ut you can get corrugated edging foam like the below. If the air is damp in the winter anything bare metal will tend to condense out water vapour - could just sling cloth sheets over any large tools, it does help a lot.

image.png

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On 23/11/2021 at 14:34, Bricks & Mortar said:

You can get corrugated steel roof sheets that come with a coating on the underside to prevent condensation.

That's what I have under box section sheeting in my workshop.  Works fine.  I've laid a few box sections in the past. Not hard as long as you use the right screws and not too tight.

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