Home  5  Books  5  GBEzine  5  News  5  HelpDesk  5  Register  5  GreenBuilding.co.uk
Not signed in (Sign In)

Categories



Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

Buy individually or both books together. Delivery is free!


widget @ surfing-waves.com




Vanilla 1.0.3 is a product of Lussumo. More Information: Documentation, Community Support.

Welcome to new Forum Visitors
Join the forum now and benefit from discussions with thousands of other green building fans and discounts on Green Building Press publications: Apply now.




    • CommentAuthorMarkyP
    • CommentTimeAug 2nd 2016 edited
     
    I'm managing my extension and renovation, we're having the roof re-covered and at the same time will change the roof build up to:

    80mm PIR under rafter
    170mm 0.032 rafter roll wool full depth between rafter
    OSB3 15mm sarking over rafter
    counter batten
    breather
    batten
    slate

    Now, we have not got a tin roof over this so I want to avoid getting the OSB wet and I am planning for the worst case which will be a period of time where the joiners are done but the roofers not ready to start. I'd be happy to DIY fit the breather membrane but to do that I'd need to fix it under the counter battens. My drawings indicate it over counter battens and draped. But I cant see any reason not to put it under, allowing me to get water tight and not worry too much if I have to wait a few weeks for my roofers. Any thoughts?

    edit: just to add the OSB is both for an air tightness layer and to provide structural/racking strength in allowing me to remove some large under rafter internal cross braces as we convert the loft space. There is no ventiliation gap under the OSB in this design.
    • CommentAuthorJC48
    • CommentTimeAug 2nd 2016
     
    • CommentAuthorMarkyP
    • CommentTimeAug 2nd 2016
     
    thanks, hadnt seen that document before. I've got tyvek supro on the drawings and as you say they have an illustration showing an under counter batten detail which is good news for me.
    • CommentAuthorCWatters
    • CommentTimeAug 9th 2016 edited
     
    Different manufacturers of membrane say theirs can or can't be used "in contact with insulation". I assume same applies to OSB. Some say there must be a gap even if it doesn't need to be a ventilated gap.
    • CommentAuthorSprocket
    • CommentTimeJun 6th 2018
     
    This has just happened here too. Curvey clay tiles on battens over membrane over counter battens on OSB3 sarking board on top of rafters.

    Roofer has put breather membrane (TIL-R) over the counter battens.
    He has also pulled it tight so it is pulled up against the battens.
    So we have an air gap above the membrane and below it. He says this is better because it's all ventilated.

    Manufacturer installation instructions say "must not be pulled tight against the underside of the battens" and "When installed as a fully supported system, the Underlay is laid over the support and secured using counter battens".

    So, it's clearly not installed how the manufacturer says, but now that it's done what should I do?
    If I make roofer pull it up re-do it then I make myself unpopular and potentially mess up the underlay and counter battens by removing and replacing it all.

    Is this a significant issue? Should I just let it go?
    • CommentAuthorgyrogear
    • CommentTimeJun 6th 2018 edited
     
    Posted By: SprocketHe says this is better because it's all ventilated.


    Well, I fail to see how the gap between the sarking and the counter-battens is "ventilated" ?

    Geometrically, it should consist of a series of partitioned enclosures...
    In any event, per literature,
    http://www.til-r.co.uk/products/pitched-roof-underlay/breathable-pitched-roof-underlay/
    TIL-R is precisely made for use in "non-ventilated cold pitched roof systems"

    therefore the "ventilated" argument looks spurious.

    Add spuriousness to non-compliance with mfr's instructions, and in my view you could be heading for trouble in the short or long term...

    Sounds like the membrane should be over the OSB, nailed down by the counter-battens.
    Vapour permeating from within should then condense on the membrane, and run down to eaves, gutter etc.
    Idem, precipitation getting in to the roof through a broken tile etc. will run down the topside of the membrane.

    However, if the membrane is drum-tight across the tops of the counter-battens, said precipitation, and vapour permeating, condensing then running down the topside of the membrane will, in places, be intercepted by the tile battens, which will effectively be "sitting in wet".
    The tile battens, being horizontal, will prevent or impede water runoff - that is presumably why the mfr says it must not be pulled tight under the battens - latter could ROT...

    FWIW, I am not a roofer

    gg
    •  
      CommentAuthordjh
    • CommentTimeJun 6th 2018
     
    Posted By: gyrogearHowever, if the membrane is drum-tight across the tops of the counter-battens, said precipitation, and vapour permeating, condensing then running down the topside of the membrane will, in places, be intercepted by the tile battens, which will effectively be "sitting in wet".
    The tile battens, being horizontal, will prevent or impede water runoff - that is presumably why the mfr says it must not be pulled tight under the battens - latter could ROT...

    Plus the standing water will eventually find its way through the membrane, initially at nail holes. Thus potentially rotting whatever's below.

    I'd say the issue is significant and the roofer needs to make it good, which will be a pain. It's a pity it couldn't have been spotted and discussed when the first piece went up.
    • CommentAuthormark_s
    • CommentTimeJun 6th 2018
     
    But then again if the membrane is sitting in contact with the osb will some moisture pass through the membrane ?
    •  
      CommentAuthordjh
    • CommentTimeJun 6th 2018
     
    Posted By: mark_sBut then again if the membrane is sitting in contact with the osb will some moisture pass through the membrane ?

    In that position there's no standing water, and thus no static head of pressure. So the membrane is more likely to remain waterproof. And if it's draped, as opposed to laid flat on the OSB, the nail holes won't see much water at all.
    • CommentAuthorgyrogear
    • CommentTimeJun 6th 2018 edited
     
    Posted By: mark_swill some moisture pass through the membrane


    well, per mfr's info, *NO*...
    (assuming that nobody tore, ripped or cut it during installation etc.).

    The purpose of the membrane is to let moisture *out*, and not let water *through*. Period.

    gg
    • CommentAuthorgyrogear
    • CommentTimeJun 6th 2018
     
    Posted By: SprocketIs this a significant issue? Should I just let it go?



    I'd contact the membrane mfr's help line...

    http://www.til-r.co.uk/keeping-in-touch/

    gg
    • CommentAuthorSprocket
    • CommentTimeJun 7th 2018 edited
     
    OK, thanks all. TIL-R seem to agree... it's definitely not right here.
    On closer inspection (partly because of this) it seems there are other "issues" so I have added "roof underlay incorrectly installed" to the list :-/
    I did not expect to have to get this closely involved in the details of our roofing but I have no choice now.
    :-/
      IMG_1678.jpg
    • CommentAuthorSprocket
    • CommentTimeJun 7th 2018
     
    And I'm told I don't need to worry about this gap because the plastic eaves protectors will cover it?
      IMG_1694.jpg
    • CommentAuthorgyrogear
    • CommentTimeJun 7th 2018
     
    er... as far as I understand, the Eaves Protector should be *already* on...
    :shamed:
    and in any case there is quite a large gap in the sarking board along there... Why ?
    (The eaves protector is not intended to replace the roof structure...)

    Don't know how much ice-damming is likely in your part of the world but I'd try reading this:

    https://www.aviva.co.uk/risksolutions/help/faq/answer/1769/

    "Snow and ice on a roof exerts vertical loads that can cause a roof to deflect or bow downwards. Also these extremes in weight load can transfer horizontal forces through the structure that can cause walls to deflect outward at either the top of or base of the wall. Where snow loads imposed on a roof are within the design limits for the roof then generally any minor sagging or deflection that may occur will usually be temporary and simply disappear after the load is removed".


    best of luck,

    gg
    •  
      CommentAuthorfostertom
    • CommentTimeJun 7th 2018 edited
     
    I wd put ordinary breather felt flat and tight on the OSB, held down by the counterbattens (I prefer 'downslope battens'). But if looking for that extra bit of insulation offered by shiny/reflective breather felt, then it has to go over the downslope battens, shiny side down, draped so it doesn't quite touch the OSB.

    Certainly not tightened under the tiling battens, for reasons given by gyrogear and djh above - must be draped.
    Reflective insulation doesn't work unless it can 'see' free air i.e. between its shiny under-face and top of OSB - so mustn't touch the OSB. Shiny side down because less liable to coating with dust and muck - half a chance of staying shiny.

    Some say the space between tiles and breather should be through-ventilated, to allow evaporation, whether condensation from below or water from above. But any space between breather and OSB should not be ventilated.
    • CommentAuthorborpin
    • CommentTimeJun 8th 2018
     
    Posted By: SprocketI did not expect to have to get this closely involved in the details of our roofing but I have no choice now.
    LOL If you don't the buggers will just do what they want, how they want - one of the most frustrating things about the building industry. It is almost a pathological desire to bodge things.

    I'm glad that isn't my roof.

    Posted By: fostertomI wd put ordinary breather felt flat and tight on the OSB, held down by the counterbattens
    +1
    • CommentAuthorSprocket
    • CommentTimeJun 8th 2018
     
    Membrane is now replaced with new, on top of the OSB.
    They have nailed it to the OSB with clout nails. Is that OK?
    I was expecting the only nails to penetrate the membrane to be at the counter batterns?
    (Counter battens are not fitted yet so I presume they mean to add those after the weekend).
    • CommentAuthorgyrogear
    • CommentTimeJun 8th 2018
     
    They have nailed it to the OSB with clout nails. Is that OK?.


    Not really - the mfr's procedure states:

    "When installed as a fully supported system, the Underlay is laid over the support and secured
    using counter battens of at least 12mm thickness fixed through to the support or rafters with
    appropriate corrosion resistant staples or galvanised clout nails".

    The membrane is not nailed to the OSB.

    They seem to be making it up as they go along, and it is *your* roof (TG it is not mine...).

    Your 15-year warranty is probably not going to work...

    gg
    • CommentAuthorBeau
    • CommentTimeJun 9th 2018
     
    Looking at those pictures and thinking you need a new roofer!
    Only a self builder so dont take my word for it but it looks a complete mess to these eyes.

    Sorry Mark but I would want to cut my losses sooner rather than later.
  1.  
    Not much to add here, other than to highlight the gutter detail in this arrangement.

    We've got the same thing, membrane onto OSB with counterbattens as fixing points. When we got to the eaves the gutter guy wanted to fix onto the horizontal battens directly below the slates to be as close as possible to surface. However, this would leave our secondary drainage plane (i.e. membrane) with nowhere to drain to, so we've insisted on mounting the gutter lower, below the vertical counterbattens instead, so that the membrane can lap straight down into the gutter.

    What this means is we'll have a ~75mm gap (50mm counterbattens + 25mm tiling battens) between the slate surface and the gutter below...I'm thinking this is fine (and necessary to allow ventilation) but feels a bit like a gap in the weathering surface which is odd.

    Lots of details online suggesting I'm on the right track, but the gutterer was equally uncertain. Worried that a flash of rain would just sheet straight off the roof and over the top of the gutter!
  2.  
    Posted By: Doubting_Thomasbut the gutterer was equally uncertain. Worried that a flash of rain would just sheet straight off the roof and over the top of the gutter!

    It shouldn't if the gutter is wide enough and stands out from the wall enough, positioned to catch the vert. drips and anything sheeting off the roof. It's about positioning the gutter properly.
    •  
      CommentAuthordjh
    • CommentTimeJun 9th 2018
     
    Posted By: Doubting_ThomasWorried that a flash of rain would just sheet straight off the roof and over the top of the gutter!

    As PiH says, it's about getting the gutter in the right position. But even if it isn't, it's not a disaster if rain blows over the top of the gutter. It will also be blown away from the house wall.
    •  
      CommentAuthorfostertom
    • CommentTime7 days ago
     
    Posted By: Doubting_Thomaswe'll have a ~75mm gap (50mm counterbattens + 25mm tiling battens) between the slate surface and the gutter below
    You're eave-ventilating the airspace between breather and tiles underside? That's good - are you using something like http://www.glidevale.com/fascia-ventilators/p/3 RV250 (equiv to continuous 25mm vent opening) which is 35mm high, fixed to top edge of fascia board to support the tiles underside? Then bring the breather out under the RV250, supported on an eave tray ramped up through the counterbatten height (making sure it still falls outward).
    • CommentAuthorSprocket
    • CommentTime6 days ago edited
     
    Thanks everyone.
    I have now stopped these roofers and will try to get someone else to take on the job. Stopping the job with rooflights still out is a bit of a last resort but I didn't feel I could trust them not to keep screwing up more and more.

    The lastest messes are the hacking about of my nice straight-edged gable end (which was a lime-mortared limestone rubble wall) trying to fit timbers down to it (for their attempt to fill the 2" gap where sarking falls short of the wall);

    and today they fitted new counter battens (now on top of membrane over OSB as it should have been) with the bottom end nailed over the eaves protectors which are propped up against a spare batten nailed to the top of my old facia (to kick out the bottom tile row). Of course this means the counter batten (now curved as nails try to hold it down) cannot sit flat because the end is lifted... leaving a gap under the batten to expose nail through membrane.

    This was the last straw :-(
      IMG_1922.jpg
    • CommentAuthorSprocket
    • CommentTime6 days ago
     
    I presume they were expecting to fill these new holes with cement...
      IMG_1913.jpg
    • CommentAuthorgyrogear
    • CommentTime6 days ago
     
    Posted By: SprocketI presume they were expecting to fill these new holes with cement...


    an extremely concrete possibility...

    Good riddance to them :shamed:

    gg
    • CommentAuthorborpin
    • CommentTime5 days ago
     
    Posted By: Doubting_ThomasLots of details online suggesting I'm on the right track, but the gutterer was equally uncertain. Worried that a flash of rain would just sheet straight off the roof and over the top of the gutter!
    The advice I was given was to place a straight edge on the top of the final finish and position the gutter such that the outer edge touches the underside of the straight edge.
    • CommentAuthorborpin
    • CommentTime5 days ago
     
    I knew I had a photo somewhere.

    Plastic should be nailed to the facia (that is what the small flat profile is for). The batten shown is just temporary. the membrane then stops on top of the plastic, no need for it to be in the gutter (that is what the plastic does) - put a small piece of DPC where the plastic lengths joined. I actually did not kick my slates - they just follow the same angle all the way. If you do have a tilting fillet, put it under the membrane and let the tile weight hold it in place.
      IMG_0620.JPG
    • CommentAuthorSprocket
    • CommentTime5 days ago
     
    I just discovered that they nailed counterbattens down at 450 (on the visible side facing the house), but at 600, 700, 800mm spacings, pretty much at random on the other side... and all nowhere near rafters. The result is lots of exposed long nails poking down in my loft all over the place... strikes me as rather dangerous!
    • CommentAuthorborpin
    • CommentTime5 days ago
     
    I think you are well rid of them.
   
The Ecobuilding Buzz
Site Map    |   Home    |   View Cart    |   Pressroom   |   Business   |   Links   
Logout    

© Green Building Press