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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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    • CommentAuthorJamster
    • CommentTimeApr 1st 2014
    Hi everyone,

    We always intended our build to have an oak-framed porch and matching fascia. Also, we hope to leave the timber untreated so it 'silvers' over time. We therefore need to use green oak as kiln dried would just readjust to atmospheric moisture levels (aka wet) - correct? I'm relatively confident about the porch anyway as we'll likely have this at least machined by a professional...

    My main query is around what to use for the fascias - if we use totally green oak, will it shrink, and if so, is there a method we can use to stop it twisting my rafters as it shrinks? Finish is stone skin walls. This is also complicated by the need for a ventilated soffit! Should we use another kind of timber? The plan is to have the fascia and porches matching...

    Also, does anyone a sawmill in the Durham area with a good reputation for supplying very durable timber?
    Do you need to ventilate the soffit? Could you use an over-fascia vent instead? They are more discreet & work for warm roofs (where ventilation is above the sarking membrane) & cold roofs (where ventilation is below the sarking membrane).

    • CommentAuthorJamster
    • CommentTimeApr 1st 2014
    Never seen one of those before David - it could make things a lot easier. We've a semi warm roof with insulation between and underneath the rafters and a gap between the insulation and the membrane. Are they ok in an exposed environment or might we see runoff being blown back through the ventilation? Apologies if I haven't quite grasped how they're fitted.
    • CommentAuthorBeau
    • CommentTimeApr 1st 2014 edited
    You will get far more shrinkage and movement using green oak than expansion using kiln dried. Most kiln dried I get is around 10%-12% moisture content. Depends where you are but this would go up to around 15%-18% when outside. When you are talking about green oak this could be as much as 60% moisture content so has to lose a lot of water to reach equilibrium outside resulting in movement and shrinkage.
    Posted By: JamsterWe've a semi warm roof with insulation between and underneath the rafters and a gap between the insulation and the membrane.
    I would always recommend using a breather membrane & putting the ventilated gap between the membrane & the tiles/roof covering. You can put the ventilated gap between the insulation & the membrane, but this doesn't take advantage of the membrane's breathability & encourages thermal bypass, i.e. where wind bypasses the insulation leading to increased heat loss. I imagine it would also increase the risk of run-off being blown back under the membrane, but I've not heard of this happening in practice.

    If the ventilated gap is above the membrane then the over-fascia vent fits on top of the membrane. If the ventilated gap is below the membrane then the over-fascia vent fits below the membrane. In either case it is mechanically fixed to the top of the fascia.

    If you're using breather membrane then this shouldn't be left exposed to sunlight. So the first run of membrane is normally replaced with something more substantial, e.g. an eaves protector, an eaves carrier or a type 5U eaves sheet. This would limit the extent to which run-off could blow back under the membrane.

    • CommentAuthorTimber
    • CommentTimeApr 1st 2014
    I would suggest using kiln dried oak.

    Kiln dried could mean anywhere between circa 10% and 20%. In service, I would expect soffits to be at around 16% moisture content. Therefore it is likely that the kiln dried oak will be near enough the target equilibrium moisture content.

    Green oak could be anywhere from 60% to well over 100% moisture content, but that is not relevant as timber only shrinks once it drops below the fibre saturation point which is around 30% for oak.

    So, green oak will always shrink quite a lot after fitting (in this application), but kiln dried oak is likely not to move much at all.
    • CommentAuthorBean
    • CommentTimeApr 1st 2014 edited
    Second the Kiln dried option, although finding some air dried may no be too difficult in 1 inch or so thick (1 year to dry).
    Problems you will get though by leaving the timber to silver as you put it, would be in a timber only an inch thick and say 7 inches wide with no protection would be subject to "dimension instability".
    The rate in which the timber takes in and sheds moisture, have solar gain/drying on outside face and not inside will result in cupping, twisting and splitting.
    There are obviously coatings you can use to make the Oak more stable but if it was me I would use a more stable Hardwood and coat it in a contrasting solid opaque colour of a F and B Gray or Black shade for instance.
    A water based product like Jotun Denidekk with their Visar primer for instance could give you 8 - 10 years protection. And a nice contrast against the more rustic chunky sections of Green Oak?
    • CommentAuthorJamster
    • CommentTimeApr 4th 2014
    Thank you all for your comments / opinions - very valuable as ever. Will have to think about this - I'll try and find an old-school sawmill somewhere who might have something suitable for us.
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