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    • CommentAuthorMHicks
    • CommentTimeJan 4th 2017
     
    I'm trying to get my head round how the roof - essentially - attaches to my building. The house is a simple rectangular structure, solid wall, EWI, warm roof, but the roof is built on one masonry gable and three oak king post trusses (one of which is the other gable). The three trusses sit on the wall, I get that, but the common rafters sit on the purlins and then end within the insulation envelope - I'll use the 'hit and miss' rafter method I found here (thanks!) for the overhangs.

    What I don't quite get is how - given the rafters don't birdsmouth onto an outer block skin as per usual - the roof will attach to the building... Should the three oak trusses be somehow strapped to the wall?

    Tricky one this, I'm not finding much info out there...
    •  
      CommentAuthordjh
    • CommentTimeJan 5th 2017
     
    Posted By: MHicksShould the three oak trusses be somehow strapped to the wall?

    Absolutely - it's a building regs requirement to stop the roof blowing away. The architect and/or structural engineer (not sure which) should specify this detail.

    It's normal to have a wall plate on top of the wall (and fastened down to it) I believe. I don't see why the rafters wouldn't end on the wall plate since that is inside the insulation. Again it's an issue that should be detailed in the design.
    • CommentAuthortony
    • CommentTimeJan 5th 2017
     
    I would expect to see common rafters sitting on purlins and a ridge beam, I have encountered purlins at the same place as a wall plate. The alternative is to have little rising pieces of wood on the wall plate that are nailed under the rafters with the rafter angle at the top, these look well nice if left exposed.
    • CommentAuthorMHicks
    • CommentTimeJan 5th 2017
     
    Posted By: djh
    The architect and/or structural engineer (not sure which) should specify this detail.
    It's normal to have a wall plate on top of the wall (and fastened down to it) I believe. I don't see why the rafters wouldn't end on the wall plate since that is inside the insulation. Again it's an issue that should be detailed in the design.


    Ah. Yes... We're 'self' building in a pretty literal sense in that we're actually physically doing the work mostly and, whilst we started with an architect designed structure, we've changed it (cavity to solid wall with EWI) significantly and certainly as far as the roof goes what we're doing bears little resemblance to the original; we've building a green oak cut roof where before there were supporting walls and a flitch-plate truss. The correct way to go about things would be to get the Architect to re-draw the house but that's £££ gone we don't have, plus this way is more interesting. We did get a structural engineer to spec the roof, he didn't draw or mention anything about truss-wall connection. The trusses sit on the wall, the (common) rafters don't.


    Posted By: tonyI would expect to see common rafters sitting on purlins and a ridge beam, I have encountered purlins at the same place as a wall plate. The alternative is to have little rising pieces of wood on the wall plate that are nailed under the rafters with the rafter angle at the top, these look well nice if left exposed.


    Tony - thanks, that's confirmation something is doable. We do have a purlin directly above the wall plate. Maybe we can do something with that.
      Screen Shot 2017-01-05 at 18.16.58.png
    • CommentAuthorMikC
    • CommentTimeJan 6th 2017
     
    As a side issue, but of interest to me, is why did you go for solid wall & EWI rather than cavity? How thick are you going with the EWI? I'm asking as I'm trying to decide what type of wall construction to go for in my own PH build, so any theories from others are most welcome. ☺
    • CommentAuthortony
    • CommentTimeJan 7th 2017
     
    Wei is great, simply join it up to the roof insulation.

    Roof sits on the walls and hangs over to form eaves and carry gutter, soffits touch onto the EWI

    I see no need for the roof to touch the walls, you have purlins, I would space the more evenly if they were mine.
    • CommentAuthorDarylP
    • CommentTimeJan 7th 2017
     
    @tony, Can you illustrate how the EWI meets up with the roof insulation? in the above.
    We are scratching our heads for a similar junction detail at the moment...:confused:
    • CommentAuthortony
    • CommentTimeJan 7th 2017
     
    I can try, wall insulation MUST join roof insulation, so it carries on up to under roof insulation that I assume runs between and over the common rafters.
    •  
      CommentAuthordjh
    • CommentTimeJan 7th 2017
     
    The OP said it's a warm roof, so the roof insulation sits on top of the common rafters. The rafters are inside the insulation blanket and definitely shouldn't poke through it. The OP described how he would address this.

    If it was me, I'd be think about putting a vertical board upwards from the outside top of the wall up to the top of the rafters, and then extending the wall insulation up abovethe wall until it meets the roof insulation. Presumably there is sarking on top of the rafters to carry the insulation. Exactly what type of board I would use depends on what other requirements it needs to meet.
    • CommentAuthorMHicks
    • CommentTimeJan 7th 2017
     
    Our plan is to have the insulation between and over the common rafters, as Tony suggests. The sarking will sit on top of the insulation - the wall EWI is EPS but I'm thinking of phenolic on the roof.

    I have considered boarding upwards from the wall exactly as djh suggests above. We're still gathering details.

    Posted By: DarylP
    We are scratching our heads for a similar junction detail at the moment...http:///newforum/extensions/Vanillacons/smilies/standard/confused.gif" alt=":confused:" title=":confused:" >


    If you find one please let me know!
    • CommentAuthorgyrogear
    • CommentTimeMar 16th 2017 edited
     
    .
    • CommentAuthortony
    • CommentTimeMar 16th 2017
     
    Wall plate if you have on sits on wall, with or without it strap every fourth rafter or ceiling tie to the wall.
    • CommentAuthorMHicks
    • CommentTimeMar 17th 2017 edited
     
    Posted By: MikCAs a side issue, but of interest to me, is why did you go for solid wall & EWI rather than cavity? How thick are you going with the EWI? I'm asking as I'm trying to decide what type of wall construction to go for in my own PH build, so any theories from others are most welcome. ☺


    We inherited the plans when we bought our plot and they included the typical part-filled cavity. I wanted a much more efficient building so started by increasing the cavity width, moving to full-fill, increasing the cavity more... Eventually I came up against the structural difficulties of large cavities, the requirement for fancy ties and the need for aperture detailing and it hit me that all that faff was brought about by the external leaf and THAT was really just a raincoat, a 100mm concrete raincoat. Further reading led me to conclude that cavity construction is in some respects an evolutionary dead end that's used in the UK as much out of habit and familiarity as anything else.

    There are undoubtedly well-built and efficient cavity construction buildings where the shortcomings and difficulties of the cavity have been cleverly overcome. It just seemed to make more sense to avoid them entirely.

    We went for solid wall as it was partly familiar (I have some limited conventional building experience) and I liked it's simplicity and the concept of the tea-cosy. We have 250mm of EPS under the slab and plan on 200 on the walls although 250mm is a possibility.
    • CommentAuthorMHicks
    • CommentTimeMar 17th 2017 edited
     
    Posted By: tonyWall plate if you have on sits on wall, with or without it strap every fourth rafter or ceiling tie to the wall.


    We'll have a wallplate, it's just that the common rafters sit on top of a 200mm purlin that in turn sits on top of the kingpost trusses (of which there are three approx. 4 meters apart). I need to work out how to attach the trusses (very long bolts resin anchored into the wall???) and then decide whether the common rafters have to be attached to the wall as well as the lowest purlin.

    Your suggestion of "little rising pieces of wood on the wall plate that are nailed under the rafters with the rafter angle at the top" seems like a solution Tony.

    (see drawing above)
    • CommentAuthortony
    • CommentTimeMar 17th 2017
     
    You may find that the roof does not need to held down as gravity is greater than wind uplift but engineer calculates that.
    • CommentAuthorMHicks
    • CommentTimeMar 17th 2017
     
    Posted By: tonyYou may find that the roof does not need to held down as gravity is greater than wind uplift but engineer calculates that.


    It was in the calcs, I'll have to ask him. But by my non-empirical reckoning, having spent the last month shifting the oak beams about - I'd say it'd have to be blowing a hoon to lift it off...
  1.  
    Posted By: tonyYou may find that the roof does not need to held down as gravity is greater than wind uplift but engineer calculates that.

    Posted By: MHicksI'd say it'd have to be blowing a hoon to lift it off...

    When I lived near Portsmouth the wind that Michael Fish said would not be a hurricane to bother the south of England :cry::confused: shifted my roof on the downwind side by the suction vortex coming over the ridge.
    Fix the roof down !!
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