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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.

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    • CommentAuthormarsaday
    • CommentTimeSep 18th 2020
     
    I am doing a single pitch slate roof next week. Going to remove all the old welsh slate and use new spanish. Just easier to get these tiles than trying to match up etc.

    Any way i thought i would add some insulation to this roof as it is a room in an attic below and currently only has 50mm ridgid insulation screwed into the old sloping ceiling and then new PB.

    The rafter depth is 75mm and i am intending on fitting 50mm between the rafters with rigid. Then use tyvek breathing membrane over the top. Will my 25mm air gap be ok or should i also add in some ventilation tiles. If so what would i use ?
    •  
      CommentAuthorfostertom
    • CommentTimeSep 18th 2020 edited
     
    With breather felt, no need for through-ventilation above the insulation/below the felt - so fill your rafter space 75 deep.

    However, unlike tiles, slates fit so close together that they don't 'breathe' well through the slating, so your batten space needs to be through-ventilated, between the felt and the slates, in at both eaves, clear ventilation across under the ridge, mainly to cross-ventilate from one eave to the other, but with some supplementary ventilation slates at the ridge.

    So the battens don't block the flow, the battening either raised up on downslope counterbattens, or the felt heavily 'draped' to hang down into the rafter space below the battens - but that means back to 50 rather than 75 insulation fill.
    •  
      CommentAuthordjh
    • CommentTimeSep 18th 2020
     
    I thought if significant work was done on a roof it had to meet current building regs afterwards? Is 50 or even 75 mm insulation enough to do that?
    • CommentAuthormarsaday
    • CommentTimeSep 18th 2020
     
    No its not and i have spoken to BC. They will accept 50mm which i am fitting. It has 50mm inside the room already. So still short, but they will accept it. I wouldnt inform them if not.

    My roof is 4.6m wide, so do you know how many vents i need to put in ?
  1.  
    djh, it's a strange one. 50mm *is* enough on its own - i.e. it is deemed to satisfy Part L - if you have 75 rafters and are not removing the ceiling. If you remove the ceiling you have to achieve 0.18W/m2K (typically 125mm of PIR).

    So you can have a roof which is compliant because it has been re-roofed and had a '50mm job', but take the ceiling down and it's not compliant!
    • CommentAuthorCliff Pope
    • CommentTimeSep 19th 2020
     
    <blockquote><cite>Posted By: fostertom</cite>

    However, unlike tiles, slates fit so close together that they don't 'breathe' well through the slating, </blockquote>

    I've lived in a house that had an attic with an old exposed slate roof with no membrane, and it was not so much breathing as blowing a gale through the undersides of the slates. Most of the old lime "filler" had fallen out, they used to try to limit the ventilation.
    • CommentAuthorgyrogear
    • CommentTimeSep 19th 2020 edited
     
    Idem, I have a French (Anjou) slate-hung facade, 100 sft, and I manage to drag air through the cavity with no problem.

    gg
    • CommentAuthorrevor
    • CommentTimeSep 19th 2020
     
    Depend on the slate as to how much air movement you get. Spanish and Brazilian slate are very flat and generally more even in thickness between slate so form a more of a seal than Welsh slate. These are more "hewn" vary a lot in thickness and are generally tapered and require more skill to lay as they need sorting into groups of thicknesses before holing and laying. Welsh slate is the most superior slate they tend to be thicker and stronger less waste as less breakages and roofer can walk on them without a tendency for them to crack if they have been properly fitted.
    • CommentAuthormarsaday
    • CommentTimeSep 19th 2020
     
    I have worked out what i need and it will be some breather slates fitted top and bottom. Cost about £7 for a basic one.
    • CommentAuthortony
    • CommentTimeSep 19th 2020
     
    Without sarking or sarking layer there will be plenty of lovely ventilation between the slates, Victorian slate roofs stay in very good condition under the slates, battens often 15 to 19mm thick and thinner joists and purlins than engineer would require now all in perfect Nick.

    My particular favourite innovation that is no longer utilised art ‘tilters’ - ask me more if you like - I once published on them in building for tomorrow the forerunner GBM nor GBEZine
    • CommentAuthormarsaday
    • CommentTimeSep 20th 2020
     
    Tony are you saying i dont need breather slates ?

    I was chatting to a roofer today and he said the permeable membrane will be fine and i wont need the breathers.

    I am a bit confused. Why do some people say you need more ventilation and other don't ?
    • CommentAuthortony
    • CommentTimeSep 20th 2020
     
    If no breather membrane or sarking no vents needed

    If breathable sarking vents are required by most but not all building inspectors - this is confusing

    If counter battened generally vents not needed, if normal battened then draped/not draped comes into it

    Confusing yes
    •  
      CommentAuthorfostertom
    • CommentTimeSep 21st 2020 edited
     
    Posted By: marsadayWhy do some people say you need more ventilation and other don't ?
    What revor says above - what kind are your slates?

    Some years ago Glidevale published papers with evidence - well, they would, being manufs of popular roof-vent products - but still, it takes little to act on double-lap smooth slates (let alone triple-lap shingles) being far less reliably leaky than single-lap lumpy tiles.
    • CommentAuthormarsaday
    • CommentTimeSep 23rd 2020
     
    I have put in a breather eaves support tray the full length of the eaves. I will also put in 3 breather tiles high up.

    Got the roof stripped off on monday and almost finished sheeting and latting it yesterday.

    Its raining today plus i'm well ill with a sore throat / cold (back to school time so i always get the bugs).

    I have draped the membrane so water can run through. It's going ok, but much harder than i expected. Getting old and having a bug doesnt help. Luckily i got some help from a teenager. Back on it on friday according to the weather.
    • CommentAuthortony
    • CommentTimeSep 23rd 2020
     
    Little tip for you, stick the underlaps with strong tape or glue - they can make a lot of noise, vibrations or whistling in strong wind, difficult to do afterwards
    • CommentAuthormarsaday
    • CommentTimeSep 27th 2020
     
    Thanks Tony
    • CommentAuthormarsaday
    • CommentTimeSep 28th 2020
     
    Update and question:

    I got the starter slates in today and added a bit of no nails where they contact the eaves breathing vent. Then got first full slate row in and the second row.

    When i added my breathing eaves tray i raised the roof pitch by a lat height (25mm) plus the eaves tray height (10mm approx). So when i came to add the slates they wouldn't contact with the lats set in place. So i added another lat on to of my first lat and thought i was good to go (this was yesterday). So started the slates today and after i cleared the first full row i realised the next 3/4 rows of slate will not contact with the lats put in place.

    So tomorrow i will get some new lat ripped down to about 12mm as this seems to be the gap i have got to fill. I think the roof will balance out as i move upwards and i should only need to baton out the next 4 rows.

    What is the normal think to do in this situation ? The roofer who advised me to fit the eaves tray with another baton never mentioned this problem and when i called him to ask what i do he said to use longer nails and just fix the tiles in place.

    Apparently thats what they would do. But if i need to service the roof in future (and i do need to use the roof ladder to fit my ridges in place) i will need a strong roof so it can be accessed again. If i dont fix some packing lat in place those tiles will just snap in two (10mm gap between the lat and the tile).
    • CommentAuthormarsaday
    • CommentTimeOct 1st 2020
     
    I ve sorted it. Had to grade the batons on the next 4 up. Wasnt to difficult as used some new batons and planed them to fit.

    Ive got to the top working in a staggered pattern. Got to infill the left side now. Cant work out how to fit my ub17 breathing slates. Hopefully the technical help will be open tomorrow.
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