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Green Building Bible, Fourth Edition
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    • CommentAuthorphil303
    • CommentTimeMar 25th 2019
    I find moisture on underside of roof sheets and membrane is completely saturated all over.
      moisture on sheet underside.jpeg
    • CommentAuthorphil303
    • CommentTimeMar 25th 2019
    Mushrooms growing underneath the roofing sheets!!! WTF!!
      mushrooms under sheet.jpeg
    • CommentAuthorphil303
    • CommentTimeMar 25th 2019
    under the roofing sheet the 18mm OSB is completely saturated all over the whole area, there is white mould on the OSB, surely this can't be from a single leak in a fix?

    I take a walk over the whole roof and it seems the whole roof deck is soft all over.
      osb and mould.jpeg
    • CommentAuthorphil303
    • CommentTimeMar 25th 2019
    So, In Summary.

    It makes me think that this is some sort of condensation issue as opposed to a leak?

    the resultant leak into the ceiling below seems to be from where a sheet fixing has pulled out of the soggy OSB and its got in that way.

    I will have to remove the sheeting, the 18mm OSB top layer and relay it.
    I don't want to relay the same system, or any metal roof for that matter.

    roof is 11m x 11m

    What would people suggest? EPDM? Fibreglass?

    I'm gutted it has come to this, but am willing to admit i made a mistake. And hopefully i can find a cost effective way forward.
    • CommentAuthorMike1
    • CommentTimeMar 25th 2019
    Personally I'd go for a single ply membrane, but fix it to an high-density roofing insulation board instead of a timber-based sheet.
    Posted By: phil303It makes me think that this is some sort of condensation issue as opposed to a leak?

    As an armchair observer I would agree that it seems like a condensation issue because it covered the whole roof. I am left wondering if, had you used the standing seam roofing sheet specified by the architect would the result have been any different ?

    Was any ventilation specified between the top membrane and the roofing sheet - and should there have been some ?

    Is there a flat roof lay-up using metal sheet that is known to work ? I have an interest because I am (was !) considering a similar lay-up using standing seam metal but I have had concerns about condensation on the underside of the sheeting - which from your experience my concerns seem to have some foundation.
    • CommentAuthorjfb
    • CommentTimeMar 26th 2019
    i have a tata roof - the spec was for :
    metal roof
    18mm OSB
    counter battens for ventilation

    I am surprised no ventilation gap was specified originally.

    Makes detailing trickier at any abutments though.
    • CommentTimeMar 26th 2019 edited
    Posted By: jfbI am surprised no ventilation gap was specified originally.

    Me too. To be fair the BBA cert for the tata roof specifically mentions Protect Zytec membrane and has some illustrations without a gap as well as some with a gap. I think it's a clear case where the architect should have got written confirmation of the design from Tata. But having said that, the roof wasn't built with the Tata product so it's all a bit moot.

    I think it's condensation as well. I'd try to include some ventilation in the repaired roof design. I'd probably go for an EPDM membrane on top myself, but maybe even corrugated metal would work with ventilation.

    With my unventilated aluminium standing seam roof, I got Ecological Building Systems to run a condensation analysis using Pro Clima Solitex UM Connect which has a sort of plastic brillo pad on top to provide some ventilation. They reckoned it will work, so I hope they're right; it's a bit difficult to check!

    I also have a couple of flattish bits of roof with EPDM membrane on top of ply, but the ply is ventilated underneath in those cases (battens in one area and exposed underside in the other).
    In the mushroom photo it looks like some of the metal sheet fixings are located through the 'valleys' of the profile, not the 'ridges', was this as intended? Could this allow a seep of rainwater at these fixing locations all over the roof, coinciding with where the fixing punctures the membrane? (Edit: is this correct, I'm getting the Escher effect from the photos?!)

    The zytec membrane is breathable which would obviously require ventilation above it to work, is there airflow through the 'ridges' of the profile from eaves to ridge, or is it blocked at either edge? Is there any kind of vapour barrier under the OSB layer?
    (Edit - you did show the vapour barrier, so the source of the condensation shouldn't be vapour rising from inside the house. Could be rainwater that has got under the sheeting, evaporating each morning and condensing under the sheeting each night, so distributing itself all over the roof, but not escaping due to insufficient slope or ventilation).

    When we replaced a felt roof with EDPM, the installer insisted on adding a layer of PIR insulation on top of the plywood deck and putting the EDPM on top of that, despite the copious ventilation underneath the plywood which had sufficed for the previous 30 years. He thought the EDPM manufacturer insisted on the insulation for the guarantee. The idea was to avoid the plywood changing shape slightly in the sunshine and so disturbing the EDPM, was apparently nothing to do with condensation. His price was good so we didn't argue for long.
    • CommentAuthorphil303
    • CommentTimeMar 26th 2019
    Thanks folks.

    I went back to my architect, first thing he said was I changed the roof sheets so that’s the problem. Tata sheet is flat and has no air underneath to condensate, where as the profile I choose allows air, but not enough to ventilate properly
    I do wonder if it would still have done it had I used the Tata roof spec he gave. I will check with Tata for my own piece of mind. Even though it makes no difference now.

    I’ve had 2 roofers over so far to look for further opinions. The first said it’s obviously sweating, hence the white mould, and he would never have laid those roof sheets, he recommended epdm or type of felt that he said would be great. Not sure felt looks as good but it’s all about keeping water out I suppose!
    The second roofer said he was surprised it had done it at all. He said he couldn’t find fault with the way it was all laid, and that he too thought it’s condensation issue.
    One of them spook me a little saying he’d had to pull off an epdm roof for the same reason before, sweating under the membrane and the board basically disintegrated.
    I’ve got one more chap lined up for a third opinion. Then it’s decision time on what to do?

    As for fixing. I was advised to fix in the flat section of the valley so it pulls flat and tight against the membrane. So it’s not leaking from every fixing. Therefore again points to condensation.

    • CommentAuthorJeff B
    • CommentTimeMar 27th 2019
    I would go for fibreglass. It is firmly bonded to the substrate so no possibility of interstitial condensation. We had a large garage roof and an extension roof done that way in our previous place and no problems after 20+ years. (Just discovered that I contributed to this thread back in November 2015 - scary how fast time goes by when you get to my age!).

    Sorry for your problems Phil303. As you can see in my previous posts, we had a big problem with profile sheets on a low pitch. Not with rain/condensation but with snow which caused us to go down the fibreglass route. Although your OSB has had it, presumably the insulation can be reused? The reason I went for profile sheets in the first place rather than fibreglass was that it was the cheaper DIY option - I would not have attempted fibreglassing myself as I do not have any experience.
    Phil, you've been very unlucky with this. It's hard to see how condensation forming below the steel managed to get under the membrane which is supposed to resist liquid water. Thank you for sharing the details for others to learn from.
    • CommentAuthorphil303
    • CommentTimeMar 27th 2019
    Tata's diagram for typical installation for warm roof shows a 50mm ventilation cavity?

    the SIP panel system shows no ventilation.

    Our build up is effectively the same as a SIP system, although all elements are not glued together..

    they do recommend the Protect Zytec layer
    • breather membrane — for example, Protect ZYTEC (resistance 0.15 MN·s·g–1) to BS EN ISO 12572 : 2001

    along with the following General guidance:
    4 General
    4.1 The Colorcoat Urban Roof Panel is suitable on roofs with a slope of between 5° and 60° as:
    • a protective/decorative covering over cold and warm roofs supported on a continuous layer of minimum 15 mm thick OSB/3 or plywood decking for use in residential buildings.
    • weather covering to the outer skin of a structural insulated panel system (provided that they have a minimum thickness of 15 mm OSB3/ply board).
    4.2 The design of the roof must include:
    • a ventilated cavity system incorporating an insect guard to all ventilation openings at the eaves
    • an effective vapour control roof underlay between the OSB/plywood board and the steel sheets to ensure that the system is protected
    • the design thickness of OSB/plywood board and the rafter spacing to cope with wind pressure and greater than 15 mm board must be used if required.

    4.3 The panels are dimensionally stable. The fixing arrangement and the recommended construction tolerances will adequately accommodate thermal movements.
    4.4 It is important for designers, planners, contractors and/or installers to ensure that the installation of the product is in accordance with the Certificate holder’s instructions and the information given in this Certificate.

    It looks like, to me at least, that there should have been a ventilation cavity in there regardless of which cover sheet we used?

    Have i been sold duff information by my architect.... or am i the duff one for switching the sheets?
      Screen Shot 2019-03-27 at 08.11.02.png
    • CommentAuthorBeau
    • CommentTimeMar 27th 2019
    Is it salvageable? It seems to be a condensation issue from vapor coming from underneath. As others have said it's surprising that no air gap was spec'd with the original Tata roof as how else is the moisture that comes through a breathable membrane supposed to escape? You say the OSB has gone soft. We had this on an outside shed that lost it's felt. I left it years before coming to fix it. The OSB was soft and had swelled but I fixed some corrugated over the top to keep it going for a few more years. To my surprise, the OSB has regained much of its structural integrity as it dried out. Are the fixings through the profile sheets in nice straight lines? If so could you simply remove the sheets noting where they all came from? Then lay counter battens fixed through the OSB to the rafters below lining up with the fixing holes in the membrane and then re-cover with the same sheets. For ventilation don't use eave fillers but you may need to improvise a vermin proof mesh to fit in place of them. It would be a low-cost solution that may well be a permanent fix.
    • CommentAuthorgyrogear
    • CommentTimeMar 27th 2019
    Posted By: phil303As for fixing. I was advised to fix in the flat section of the valley so it pulls flat and tight against the membrane.


    Sorry to hear of your troubles, I also contributed to this thread and have more grey hairs since...

    If the fixing is done through the *tops* of the profile, given the absence of any support below (except air...), then there is a risk of the fixing pressure causing a depression or dimple, which in turn could hold water, and eventually bust the seal (due to freezing etc.) and create a leak into the roof...


    • CommentAuthorMike1
    • CommentTimeMar 27th 2019
    Posted By: phil303Tata sheet is flat and has no air underneath to condensate

    However the timber-based boarding beneath will have air in the interstices between wood particles - air which will have moisture in it which can condense within the board. And no 'flat' metal sheet is going to lie so flat to the board that there is no air below, unless fully bonded to it. So personally I'd not use a timber board without adequate ventilation on one side or the other on the cold side of any insulation.

    As per my earlier brief suggestion, I'd use a high-density roofing insulation board (closed-cell and very vapour resistant) with a single ply membrane bonded to it - a standard warm roof construction. I presume, incidentally, that you do have confidence in the integrity of the VCL?
    • CommentTimeMar 27th 2019
    I'm not clear whethger or not there's generous through ventilation above the breather and below the corrugated. If not then the symptoms described seem completely as-expected.

    I'm a great fan of corrugated iron incl at v low pitch in cases where it can be done in one length without end-laps.

    But really generous and effective through-ventilation is indispensable - there's bound to be masses of condensation on the underside of the corrugated, it will drip onto the breather, and the ventilation will be much needed, to evaporate water soon enough off the breather.

    On top of the breather, I go for 38h downslope battens with 38h cross battens over that - so a 76h ventilation space that's able to cross-ventilate in all directions - handy when the roof doesn't have two opposite-exposed edges.

    The upper crossbattens act as purlins to the corrugated so I make them of nom 3"x2" (actual 63x38 - not much bigger than nom 50x38 battens) CLS and space them at whatever purlin spacing the corrugated requires. Then the downslope battens don't need to be continuous - just offcuts of CLS as blocks under the crossbatten/purlins.

    The moderately substantial battens-and-crossbattens are also chunky enough to support a perimeter overhang of the corrugated.

    I use screened plastic ventilators at the open edges with 25mm-equiv continuous open area e.g. Glidevale RV250.
    The second tata detail isn't how I'd understood a warm roof works, it has an air cavity above the insulation but below the deck. This must be ventilated outside air as specified at 4.2 above. So the deck is at cold outside air temperature (not warm). That being the case, I don't understand why the membrane needs to be breathable, unless it's to let the underside of the steel dry downwards through the deck on sunny days. Does anyone understand how it works?

    Interesting info on the fixings, on trad corrugated steel the fixings go through the ridges to keep them away from the draining rain. The photos show some in the ridges and some in the valleys (i think), are they all tight and waterproof or have some gone loose with the soft OSB?
    • CommentAuthorgyrogear
    • CommentTimeMar 27th 2019 edited
    Posted By: WillInAberdeenThe second tata detail isn't how I'd understood a warm roof works, it has an air cavity above the insulation but below the deck.


    I'd tend to agree with you: it looks like they have bastardized the definition of ''Warm Roof''.

    By definition, a ''warm roof'' has the insulation above the deck (''roof structure'').


    Therefore the assembly consisting of the steel and its substrate, is not , for my money, "roof structure" as mentioned in the definition.

    Which begs the question, "what then is the roof structure in their drawing?"

    Per the definition, ''roof structure'' is what the insulation is laid upon.

    In this case, this looks like :

    -- a foam pad, with a (noptional) VCL beneath it (and upstand of VCL around the perimeter...);
    -- a series of battens;
    -- a boarded ceiling.

    Not Much of a Roof Structure... more of a "ceiling structure"...

    In the referenced dwg, the steel etc. might be the ''deck'' as commonly referred to, but IMO it is actually the ''roof covering'.

    Drawing an analogy to my own warm roof, but replacing the steel with slates:

    I then have an airspace idem in their drawing (materialized by lathes across tops of rafters), giving approx. 40 mm;
    -- below the airspace, the insulation (70 mm PU foam);
    -- below the PU, an (assumed) VCL;
    -- below the (assumed) VCL, the roof boarding (23mm chipboard);
    -- below the chipboard: purlins and principal rafters;
    -- below the purlins: my lounge;
    -- in between the purlins: my cathedral ceiling (tongue & groove pine stripping, fixed to underside of roof deck).

    Therefore I have an ''unvented warm roof'' because there is no crossdrafting of the airspace under the slates. No vents.

    N.B. This is *not* the same as saying that the roof is ''unventilated'' - on the contrary: it *IS* ventilated -- by random ingress of air via the slates, which are not hermetic. But it is not VENTED - there are no vents...

    (when my neighbor had the roofers in, one of them tried taking me to task because he did not see any vents on my roof - I guess they were scouting for work...).

    I state that the VCL is 'assumed' because I have not actually seen it, buried as it is (may be...) below the PU. However, US documents state that one must be used with in-situ PU foam.

    I do not have an interior airspace neither, the idea being that the ceiling structure is airtight, thus no air vapour is allowed to contact the cold side of the roof, which is in any event WARM, and Far Off, on the other side of the insulation, thus a ''warm roof''.

    Whence the importance of correctly ventilating the house, to keep moist air off the ceiling in the first place...

    For ref., the French tend to call it, ''an upside-down roof''.

    Mine has survived 36 years so far, I intend to strip the slates and beef up the insulation in a couple of years, given the assumed 40-yr life of my slates... (and giving my son time to save up as it's HE that'll be paying for it, LOL).

    Apologies for digressing, but I thought this contribution might be useful given the context of the current thread...

    To recap, IMO, the referenced drawing should show (substantive) ''roof structure'' BELOW the insulation, as per the WIK definition.

    Also, since not all warm roofs are vented, the drawing title should also mention same: their simple ''Warm Roof Construction'' is seriously insufficient and even misleading...

    thus: "Tata Steel's Version of a Vented Warm Roof" etc.

    • CommentAuthorphil303
    • CommentTimeApr 8th 2019
    Thanks folks,

    i've taken a little while to digest what i can of the above, and trying to work out my route forward..

    I can't raise / vent the current sheeting and relay it as it will lift the whole roof by approx 50mm and mess up the finish at 3 edges, creating a whole lot more work / cost. in addition to the roofing costs.

    I think the thing i need to do is get the roof sheeting off to see if there is someway the 18mm osb roof deck is salvageable.
    Perhaps some form of well vented temporary cover might then allow it to dry up rapidly? although if it turns out to be saturated then it could be forever drying from the one side.

    - Any tips on drying out OSB and temporary covers much appreciated.

    IF it drys out ok (more hopeful than expected) i could then belt sand off any lumps and bumps and lay EPDM on it.
    I have concerns of it drying and the warm roof in skew fixings pushing up through it and potentially puncturing the membrane as we walk on it if there's any flex/compression in it.

    IF it doesn't dry or will take too long, i'll have to remove it, and the warm roof fixings with it. Fingers crossed the insulation is still in good condition.
    Then i would need to re-lay new 18mm board (approx 50 sheets of 8'x4') = £1000 in board
    and fix with about 1600 of 240mm stainless in skew fixings = £750 in fixings
    + labour etc..

    So, here's hoping it's salvageable!!
    • CommentAuthorgyrogear
    • CommentTimeApr 8th 2019 edited
    Posted By: phil303o, here's hoping it's salvageable!!


    In the meantime, here is some interesting reading if you are not already "saturated" :shamed:


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