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Damp in timber shed since fitting corrugated steel roof

Discussion in 'In the Garden' started by gregch, 20 Aug 2021.

  1. gregch

    gregch

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    Hoping someone can give me some clues on how to resolve a damp problem in my tool shed.

    The shed itself is timber frame clad in thick waney edge, with a concrete base. It's pretty substantial for a shed and very old, certainly it's on the plans from long before the house was built (1970s). It has a roof made of timber boards which had been felted and then corrugated plastic at some point fitted on top of the felt.

    Despite the considerable age of the building, the cracks in its concrete floor, and the terrible state of the plastic (bits of it literally falling off) and the roof felt, and that rain very occasionally made it through the plastic, felt and boards, it was dry inside and everything in it stayed in perfect condition.

    Then I decided to do something about the state of the roof. I removed the remaining corrugated plastic and fitted a corrugated steel roof, fixed in exactly the same way, over the surviving felt and fixed using the correct fixings with weatherproof washers, flashings etc.

    Unfortunately although it looks great, since the new roof went on I started to notice rust forming on some of my tools - a year or so later, the problem has got really bad with some tools now rusting badly, despite regular attention with wd40. Evidently, there is now a damp atmosphere in there.

    What I can't work out is how to fix it. Is it that condensation is forming under the steel and dripping onto and soaking into/through the roof boards then discharging that moisture into the shed? Or is it that moisture coming up from the broken concrete floor used to escape through the somewhat permeable roof but it now trapped by the new weathertight roof??

    Should I insulate (with I guess celotex boards) and/or board out the roof, to stop warm air from the shed condensing on the steel - or try and fit a raised insulated floor?

    (BTW I've tried opening the window to various degrees to try and promote air circulation, but that hasn't made any difference)

    Any pointers gratefully received.
     
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  3. OddsBodkin

    OddsBodkin

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    Why would you need to put a steel roof over felt; surely a felt roof should suffice?
    It sounds like you have no airflow through the shed.
    Maybe leave a small gap under the steel to allow a flow of air through will help.
     
  4. gregch

    gregch

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    Good questions - basically it always had the corrugated plastic over the felt, simply replaced the plastic for steel. The reasons for the "extra" roof are partly aesthetic (it's a large and prominent building) and more practically to extend the roof slightly so as to direct rainfall into the gutter, and also the felt is well past its best. There is a small gap between the steel and the felted roof boards, and of course the corrugated ends are open, which I'd hoped would provide for air circulation under the steel, ie for any condensation to drain out and/or evaporate. Sadly, still getting this problem - so I do wonder if boarding out internally, with insulation and/or tyvek or something, might be required??
     
  5. OddsBodkin

    OddsBodkin

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    How much sun gets on top of your roof?
    The heat will act on the steel like a frying pan.
    The angle of the roof should be enough to see water run off into the guttering. If you're bothered about the felt not lasting then maybe fibreglass would better suit a large, prominent building.
     
  6. gregch

    gregch

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    Well it gets a bit of sun and yes appreciate it will get hot in there. definitely not planning to replace the steel, but would like to mitigate any dampness that might be resulting from condensate sitting on the roof boards, if that's what's happening*. My gut feel is to maybe try to insulate the roof on the inside, the theory being that should reduce the warm moist air building up inside and then condensing onto the cold metal tools and rusting them - again, *if* that's what's going on. Does that sound logical to anyone? What's the best way of doing that??

    (*I can't think that it's moisture rising up from the old broken concrete floor, because that hasn't changed in years and never caused a problem until I "fixed" the roof!)
     
  7. scbk

    scbk

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    IMO the roof isn't the problem, but the shed itself needs some ventilation.
     
  8. gregch

    gregch

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    Not so sure about ventilation being the issue - have tried windows open, half open and open just a crack and it makes no difference.

    However it turns out I am an idiot because the shed is divided internally into two parts, and the part that I *don't* use to store my tools (it has garden tools etc in it) is bone dry with no rust issues. The roof is obviously the same for the whole building, but the difference is that the ceiling is boarded out with plasterboard (the previous owners used it as a sort of summerhouse)...

    So I've been an idiot, or possibly not completely an idiot because that's where I think I was headed in terms of what to do.

    Can anyone suggest best material eg rockwool or celotex or actual wool or ??? then presumably ply or plasterboard internally?? Would a breathable membrane help?

    Thanks!
     
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  10. gregch

    gregch

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    Bumping this a little bit in the hopes of getting some help re boarding/insulation!!

    What would be the most suitable insulation - celotex type, or rockwool, or actual wool? I guess this goes between the "ceiling" joists and above the finished "ceiling"??

    For boarding/lining (ie forming the "ceiling"), I'm thinking 9mm OSB or similar since plasterboard (which is what's been used in the other half of the shed) is a bit heavy and awkward for me to deal with in that space. Is OSB or ply OK??

    Finally, do I need to put a vapour barrier or breathable membrane or such between the boarding and insulation?? and/or leave an air gap??

    All suggestions as ever very welcome!!
     
  11. albaPhenom

    albaPhenom

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    What is your current roof build-up?

    Is it a timber board on rafters, with built-up felt coverings and then profiled metal sheet fixed through this into... ???

    The issue with uninsulated profile metal is that condensation will form on the underside, this can be an issue in roofs with no substrate below the metal sheets and you can get profiled sheet with a felt backing applied, sometimes referred to as no-con-drop but in reality this isn't preventing condensation, it's just mopping it up. This is generally fine on unheated industrial buildings where dampness is not an issue.

    The issue here is that any condensation should shed off the original felt roof to eaves. However, I assume that the removal of the plastic sheets may have left fixing holes through the felt roof?

    OR the new metal sheet is fixed through the felt roof to the sarking board and movement in the metal or wind uplift may be pulling the fixing out of the material and breaking the seal between the fixing washer and external metal sheet.

    If so then this might be a possible water ingress point. The ventilation thing could be right but essentially your building hasn't changed other than swapping plastic for metal top sheet.

    My suggestion here if you wish to tackle the roof would not be to insulate internally but create a small built up roofing system using your current sheets.

    Remove the existing sheets and install 1.6mm x 30mm deep galv zed bars (say 30x30x30mm, you can have a local metal fabricator make these for you) through the existing roof to the underlying roof supports running verge to verge i.e. perpendicular to the roof sheet. Avoid fixing only into original timber sheathing board where possible.
    Infill this 30mm zone with a mineral wool insulation, Knauf or similar clad-roll, you can go up to 50mm thick here and compress lightly under the roof sheet, lay a new visqueen vapour control layer over the zeds/insulation and re-fix your profiled sheets back to the galv zed supports with self-drilling tek screws with a sealing 19mm washer and stick some push caps on them.

    You might have to replace flashings at verge / ridge to accommodate the change in height, hopefully not... fit suitable profile fillers in the flashings at ridge and eaves.

    If nothing else this will prevent any condensation or water ingress through the existing roof.
     
    Last edited: 27 Aug 2021
  12. gregch

    gregch

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    Thank you for this excellent and detailed reply - and probably the right answer; just harder work than I'd hoped, so:

    The plastic sheets were fixed onto timber upstands/battens on top of the felt, themselves fixed through the felt and roof boards into the rafters. When the plastic sheets were removed, the steel sheets were fitted in their place in the same way, using the weatherproof tek fixings you describe (similarly, weatherproof tek stitching screws tie the sheets together at the specified overlaps).

    I was aware of the condensation issue when choosing to use the steel sheets but I figured any condensation would drip off the underside of the roof, onto the felt, then evaporate through air movement in the space between the roof sheets and the felt. Given the age and condition of the felt, I wouldn't have been entirely surprised if some condensation did make its way through the roof, but there's no obvious sign of that happening.

    For that reason, I was tending toward the explanation that the problem might instead be the result of the significant extra heat build up with the steel roof, drawing more moisture up from the old and broken cement floor, causing high humidity inside the building and then condensing onto the cool metal tools as the temperature drops??? Does that make sense???

    So, my plan was to try and stabilise the temperature a bit hence the boarding out and insulating.

    Also, I thought my theory was borne out by the other end of the same building. It has the same roof but is already boarded out and insulated (with 100mm rockwool) and there are no rust issues with tools stored in there. Also, I've had a peek in the roofspace and it isn't damp, implying that the condensation from the steel isn't getting through the felt and roof boards to any great extent.

    Obviously your suggestion for insulating the roof will also address heat gain and leave the interior space as is, so it's probably the better solution, but it's undeniably a bigger job.




     
  13. albaPhenom

    albaPhenom

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    Ahh so it's fixed to a batten already, that's interesting because the void between roof sheet and felt roof will be ventilated and should deal with any condensation risk as you say. Are there any signs of water ingress through the roof?

    The steel roof won't be providing much in the way of additional heat and humidity over plastic sheets as it's uninsulated. Odd.

    It shouldn't be that much more of a job tbh, laying the insulation and VCL in the zone of the zed spacers should be much easier than trying to fit insulation overhead between rafters and such like internally.
     
    Last edited: 27 Aug 2021
  14. gregch

    gregch

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    Well, I'm not sure about that. Definitely seems to get pretty hot relative to the plastic. But in any case insulating it seems to be the answer, thanks for your help!
     
  15. IT Minion

    IT Minion

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    Metal is a good conductor of heat, plastic is a poor conductor. If metal is hotter than your skin then it feels much hotter than plastic at the exact same temperature. If the metal and plastic are colder than skin temp then the metal feels colder.

    We don't actually sense temperature, we sense the relative change of temperature.
     
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