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    • CommentAuthorgravelld
    • CommentTimeJan 16th 2018
     
    The latest WIP is the eaves of the lean-tos around my house. We are not retrofitting the main roof, however there are three lean-tos which, while the EWI is applied, should be converted to a warm roof so insulation on all external walls is continuous and also allow the integration of the air barrier.

    The approach I want is to insulate between and over the rafters. Here are the main ideas:

    - Use wood fibre to avoid having to have a vapour barrier
    - Air barrier achieved with OSB installed on top of the rafters. 2/3 of insulation above.
    - Air barrier is continued from the outside of the inner leaf to the OSB using an OSB upstand
    - Some sort of overhang, i.e. a soffit, is required to shade the 3G glazing installed below this. The overhang required is 410mm (as entered to PHPP)
    - Just realised I forgot the membrane... TODO

    Questions:
    - Is there a better way than the upstand?
    - How do I attach a fascia board and soffit?
    - There's currently a space where the eaves insulation should be. Is it permitted to be EPS, given the lack of VPC?

    Many thanks to everyone who has looked at my posts so far.
      lean_to-eaves.JPG
    • CommentAuthortony
    • CommentTimeJan 16th 2018
     
    Insulate the eaves box and good to go
  1.  
    It looks like the OSB upstand will need to be cut around each rafter leading to a difficult to seal junction. In addition, the existing rafters don't look long enough to provide the required overhang.

    How about cutting the existing rafters flush with the outer face of the inner leaf/wall plate? The OSB upstand could then be easily sealed to the OSB sarking board/air barrier.

    The overhang could be supported by embedding some false rafter feet in the middle of the insulation sandwich, i.e. fitted on top of OSB sarking board/air barrier, fixed through the OSB to the existing rafters and oversailing the OSB upstand and EWI to give the required overhang. The EWI could be cut around the flase rafter feet to ensure continuity of insulation.

    David
  2.  
    If you put the roofing felt membrane that should be on top of the 200mm wood fibre insulation, if that membrane is vapour permeable do you need a gap between that and the insulation?

    What supports the weight of the tiles 'cos I can't see the wood fibre not compressing over time and the whole lot drifting down the roof a bit due to shear loads.
    •  
      CommentAuthordjh
    • CommentTimeJan 19th 2018
     
    Posted By: Peter_in_HungaryWhat supports the weight of the tiles 'cos I can't see the wood fibre not compressing over time and the whole lot drifting down the roof a bit due to shear loads.

    Counterbattens fastened together at the ridge so one side supports the other is the normal way to remove shear loads, I think.
    • CommentAuthorgravelld
    • CommentTimeJan 21st 2018
     
    Thanks everyone for your ideas/points.

    Posted By: davidfreeborough
    How about cutting the existing rafters flush with the outer face of the inner leaf/wall plate? The OSB upstand could then be easily sealed to the OSB sarking board/air barrier.

    The overhang could be supported by embedding some false rafter feet in the middle of the insulation sandwich, i.e. fitted on top of OSB sarking board/air barrier, fixed through the OSB to the existing rafters and oversailing the OSB upstand and EWI to give the required overhang. The EWI could be cut around the flase rafter feet to ensure continuity of insulation.

    Is this the type of thing (you're namechecked!) http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=6908 ?

    So here a "false rafter foot" is just some more OSB or ply (should it be ply?) What I don't quite understand, given this is something of just ~20mm depth, is the strength there to support the fascia?

    Also, is this a continuous sheet or panels the same width of the rafters? I assume the second, to allow the EWI to be cut around it?

    Another option: http://www.finehomebuilding.com/2001/05/01/adding-roof-overhangs ("Method 2: Minimal overhangs")

    Here's what I've done in this detail:

    - Cut the rafters back to level with the outside of the inner leaf
    - Moved the upstand to sit on the rafters/inner leaf
    - Added OSB panel on top of OSB air barrier, fixed through to rafters

    Is that what you meant?
      lean_to-eaves-rafter-feet.JPG
  3.  
    I was thinking of false rafter feet of about 100mm thickness sitting on top of each of the original rafters. Assuming the 200mm of woodfibre is made up of two layers of 100mm, the false rafter feet could be cut into the layer closest to the OSB.

    Alternatively, you could fit them in line with the existing rafters; fixed by the OSB and counter battens above. You could fit the false rafter feet to the underside of the OSB before sliding it into place and then fix to them with the counter battens above. See thread below:

    http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15007

    You'll need to ventilate the batten space above the woodfibre board using counter battens and vents at the eaves. There a number of over-fascia ventilation solutions aimed at warm roofs which should fit the bill.

    David
    • CommentAuthortony
    • CommentTimeJan 21st 2018 edited
     
    I don’t think using osb as false rafter feet is on , use counterbattens may be if you can give them additional support from the outside skin
    • CommentAuthorgravelld
    • CommentTimeJan 22nd 2018
     
    Thanks, I'll look at that this evening.
    • CommentAuthorgravelld
    • CommentTimeJan 22nd 2018 edited
     
    Ok, I wanted to just try this method out on you all, because it looks quite neat.

    Here the "rafter tail legs" are fastened to a "rafter ledger" which is fastened to the OSB upstand (air barrier).

    Would need AT treatment around the fastenings between ledger and upstand.

    Could this work?
      lean_to-eaves-ledger.JPG
    • CommentAuthorgravelld
    • CommentTimeJan 22nd 2018 edited
     
    The inspiration for that is this design, as described here: http://www.finehomebuilding.com/2001/05/01/adding-roof-overhangs
      hb138QA01-02_lg.jpg
    • CommentAuthorgravelld
    • CommentTimeJan 22nd 2018 edited
     
    Posted By: davidfreeboroughI was thinking of false rafter feet of about 100mm thickness sitting on top of each of the original rafters. Assuming the 200mm of woodfibre is made up of two layers of 100mm, the false rafter feet could be cut into the layer closest to the OSB.

    See the attachment, is that what you meant?

    (BTW the wood fibre sailing off at the top of the drawing is just because I've lifted the insulation and the covering, membrane etc out of the way while working on this - this would be lowered back down, stepping over the rafter feet as required).

    Posted By: davidfreeborough
    Alternatively, you could fit them in line with the existing rafters; fixed by the OSB and counter battens above. You could fit the false rafter feet to the underside of the OSB before sliding it into place and then fix to them with the counter battens above. See thread below:

    http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15007
    Does this refer to @jamesimgram's drawing? Issue with that is that the rafters penetrate the air barrier making it much more complicated (maybe I misunderstand).
      lean_to-eaves-rafter-feet2.JPG
    •  
      CommentAuthordjh
    • CommentTimeJan 22nd 2018
     
    I believe your last drawing is what David meant and is both easier (since the airtightness is easily dealt with) and stronger (since the rafter feet/sprockets are fastened along their length to the existing rafters) than other techniques. The possibility is a bonus of using a warm roof design.
    • CommentAuthorgravelld
    • CommentTimeJan 23rd 2018 edited
     
    Ok. About strength, maybe this is a question for the contractor, but how small can the rafter feet be and how much length is required above to securely fasten? If possible I'd like to minimise both. David said 100mm for the rafter feet.

    Ah yes, I realise getting the air barrier treated between ledger and rafter tails will be tricky... might be a better design there to improve that.
    • CommentAuthoradam_w
    • CommentTimeJan 23rd 2018
     
    Could the top face of these rafter feet not end up being a water trap? Personally I would have thought your previous design would be better if you thicken up the ledger board and screw the OSB sarking in to each 'tail leg', just my opinion though :)
  4.  
    Yes, you've captured both options well.

    As djh says, the second one will be stronger and easier to make airtight. Its also more likely to be accepted by the roofer as a good solution. However, it requires that you cut the woodfibre around each false rafter foot.

    The first one relies on good workmanship and fixing. I would implement the OSB upstand/ledger as two layers of 22mm timber with staggered joints. The OSB air barrier should be 18mm with a whole sheet at the eaves end and a cut sheet at the ridge. It needs to be well fixed to the original rafters, the false rafter feet and, if possible, the OSB upstand/ledger.

    To avoid temporary proping/clamps, you can fix the false rafter feet to the underside of the OSB air barrier before lifting it into place. If you do this then you'll need to fix the air barrier tape to the underside of the OSB air barrier beforehand.

    David
  5.  
    I built a number of externally Insulated Timber Frame Passive Houses about 10 years ago using this detail.
      Warm Roof.jpg
    • CommentAuthorgoodevans
    • CommentTimeJan 23rd 2018 edited
     
    Is it possible to have a break in the sarking at the osb upstand (see dwg below). This allows the upstand and air tight sarking to be joined together. The 'sarking tail' is kind of optional - it can be used to support roof insulation - or it can be left off to allow the wall insulation to reach the underside of the roof felt - easy access to the top of the wall insulation. Or it can be shorter to cover just the bit of the tail rafters beyond the outside of the wall.

    Lots of variants - but flexibility in constuction and order of construction.
      rafter tails.jpg
    • CommentAuthorgravelld
    • CommentTimeJan 23rd 2018
     
    Posted By: Viking HouseI built a number of externally Insulated Timber Frame Passive Houses about 10 years ago using this detail.
    Looks similar, assuming the 100x50 timbers are sitting on the air barrier (that sheet doesn't appear to be labelled).
    • CommentAuthorgravelld
    • CommentTimeJan 23rd 2018
     
    Posted By: goodevansIs it possible to have a break in the sarking at the osb upstand (see dwg below). This allows the upstand and air tight sarking to be joined together.
    I don't understand - why is the break required? I was already assuming the upstand and OSB would form a satisfactory air barrier if taped (maybe on both sides).
    • CommentAuthorgravelld
    • CommentTimeJan 23rd 2018 edited
     
    Posted By: davidfreeboroughThe first one relies on good workmanship and fixing. I would implement the OSB upstand/ledger as two layers of 22mm timber with staggered joints. The OSB air barrier should be 18mm with a whole sheet at the eaves end and a cut sheet at the ridge. It needs to be well fixed to the original rafters, the false rafter feet and, if possible, the OSB upstand/ledger.

    To avoid temporary proping/clamps, you can fix the false rafter feet to the underside of the OSB air barrier before lifting it into place. If you do this then you'll need to fix the air barrier tape to the underside of the OSB air barrier beforehand.
    Thanks David. Truth is, I don't really understand the "proper" building, structural, stuff. I think the benefits you have outlined for the "second way" make it the best way to go forward.

    I assume it is generally good practice to have the whole OSB sheet at the eaves level for both methods?

    Would 100x50mm timber as VH suggested be ok or should I be consulting a SE?
    • CommentAuthorgravelld
    • CommentTimeJan 23rd 2018
     
    Finally... and more of an open question... should I just be using EPS instead of wood fibre? Wouldn't it be cheaper? Do I need some sort of VPC if I do that (but then, why don't I need one for the walls - because there's no timber in the walls?).

    This feels like a noddy question I should already know the answer to.
    •  
      CommentAuthordjh
    • CommentTimeJan 23rd 2018
     
    Posted By: gravelldWould 100x50mm timber as VH suggested be ok or should I be consulting a SE?

    Basically you need to design it so you can stand on the ends of the rafter feet, or on the roof in between, without breaking anything.
    • CommentAuthortony
    • CommentTimeJan 23rd 2018
     
    +1 that was my earlier point
    • CommentAuthorgravelld
    • CommentTimeJan 24th 2018
     
    Ok, thanks both.

    Sounds like a rule of thumb... but kinda crazy? How does that deal with wind loads or differences in tile weight etc? Does it work because it is a massive over estimate how much strength is needed?

    Sorry, don't want to derail the thread.
    • CommentAuthorgoodevans
    • CommentTimeJan 24th 2018
     
    Posted By: gravelldI don't understand - why is the break required? I was already assuming the upstand and OSB would form a satisfactory air barrier if taped (maybe on both sides).
    As i said - its optional - but it means you can delay putting on the tail rafters until all the air barriers and insulation in the eaves triangle have been installed.
    •  
      CommentAuthordjh
    • CommentTimeJan 24th 2018
     
    Posted By: gravelldSounds like a rule of thumb

    Its what the design rule translates to in practical terms.
  6.  
    Posted By: gravelldSounds like a rule of thumb... Does it work because it is a massive over estimate how much strength is needed?

    Essentially yes.

    The suggestion above of 50 X 100 would be OK. What ever goes up needs to be strong enough for someone to stand on it - because sooner or later someone will. It also has to be strong enough to withstand any uplift from wind, this is dependent on the location.

    Given the strength of the 50 x 100, imagine your self standing on the end of a piece of wood with say with a 1M over hang, would you be concerned about the wood breaking - I would not. (assuming that the wood is vertical not flat) If it were to break it would be where the supporting beam ends, however that is not the whole story because you then come on to the fixings. If you had an overlap of 20cm this could be enough - but you could not get reasonable fixing to hold the inboard end in place whilst you were stood on the end. If you were nailing the inboard end to the supporting beam you would need quite a long overlap as nails are relatively easy to pull out (even if 2 are put in dovetailed) screws however have a much greater holding strength so a shorter overlap can be used.

    I would probably go for 2/3 of the overhang as the length of the overlap onto the supporting beam and fix the wood with 2 screws at the top (inboard) and 1 screw at the end of the supporting beam. Screws to use - about 160mm long but make sure that there is a long unthreaded shank so that the wood is pulled together. You could get a SE (which I am not) to calculate it but over-engineering is cheaper and easier.
  7.  
    Posted By: gravelldWould 100x50mm timber as VH suggested be ok or should I be consulting a SE?
    I think 100x50mm will be fine to cover a 600mm overhang (400mm overhang plus 200mm EWI). How this translates to length of false rafter feet depends upon the roof pitch, but the length of unsupported false rafter feet will be around 900mm.

    Builders tend to work to a rule of thumb of 1/3 unsupported : 2/3 supported, mainly to avoid excessive fixing pull-out forces. So if 900mm is unsupported then the false rafter feet will need to be 2.7m long.

    Posted By: goodevansIs it possible to have a break in the sarking at the osb upstand (see dwg below). This allows the upstand and air tight sarking to be joined together.
    Yes, this is the simplest way to make it airtight. The "sarking board tail" could be made of plywood or T&G boards to form a sloping soffit.

    David
  8.  
    <blockquote><cite>Posted By: davidfreeborough</cite>Builders tend to work to a rule of thumb of 1/3 unsupported : 2/3 supported, mainly to avoid excessive fixing pull-out forces. So if 900mm is unsupported then the false rafter feet will need to be 2.7m long.</blockquote>

    Its all about the fixings.

    I found a table on the internet that showed a pull out load for spruce pine for a 6mm screw embedded 60 mm as 7.7Kn
    Another table showed 7.7kN as 785 kg force
    So I presume that a 6mm screw if placed 50cm inboard would require 785/2 = 392,5 kg load at the end of 1m to effect a pull out
    Did I get this right - its a long time since school !!

    The builders rule of thumb I suspect is about using nails as fixings rather than screws

    And another table where the screws concerned in the table below were 4x50mm
    https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2635684/table/t4-ijms-9-4-626/
   
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