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Doubling up rafters

Discussion in 'Building' started by TatskyPT, 22 Nov 2010.

  1. TatskyPT

    TatskyPT

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    Hi Guys

    Quick question.

    I am carrying out a loft conversion. No Dormer, just roof windows.

    I need to double up the rafters, and trim for the roof windows. I also need to double up the rafters in order to install the insulation, as at 90mm the current rafters aren't deep enough.

    My plans state to install new 6"x2" rafters along side the existing.

    My question is, I cannot install rafters full length wall plate to ridge due to the purlin position. So is it ok to fit these in 2 parts, ie wall plate to purlin, then purlin to ridge?

    If 1 piece were possible, I would have to notch out at the purlin anyway to fit around the purlin as the new rafter is deeper than the available gap.

    For insulation purposes the rafters aren't structural and just to allow the depth for insulation.

    For roof window they are structural, as they are added to take the weight of the window and compensate for the removal of rafters and addition of trimmers.

    Cheers

    Tatsky
     
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  3. freddiemercurystwin

    freddiemercurystwin

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    Ask your designer.
     
  4. TatskyPT

    TatskyPT

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    Cheers Mate

    I have asked the question, just waiting for a reply.

    I asked on here because I always like to get a couple of opinions. My view is the only way to do it is in 2 bits, joined at the purlin.

    If anyone has done this before I appreciate any pointers.
     
  5. Deluks

    Deluks

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    What he said.
    But seems a bit weird buying in a load of 6x2 just to bump out insulation depth. Would be cheaper to just add 2x2 to the face of the existing rafters, even then you'd still have the thermal bridge to contend with.
    Much better to fill existing full depth and overboard with inso or inso backed PB.

    You will not have to insulate rafters beyond the low height stud walls so you will have 'jiggle room' to notch the timber and manoeuvre it behind the purlin.
     
  6. TatskyPT

    TatskyPT

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    Hi there

    Thanks for your help.

    I would prefer to just bump out the existing rafters with 2x2 as that would be significantly cheaper, and also easier. This is one option I am looking at.

    BTW I need to have 100mm between the rafters and then 25mm kingspan across the rafters to sort the thermal bridge.

    I was speaking to Marsaday on here, and he suggested 75mm between, and 50mm over rafters. Still getting 125mm insulation + 12.5mm plasterboard.

    If BC are happy with 2x2 to the front of the rafters, I will do that. Then I just need to contend with the different between the 90mm+2inch (sorry for mixing metric and imperial) and the 150mm rafters I have to double up for the velux windows. Probably looking at a 10mm difference between the bulked up rafters and the doubled rafters. Should be able to deal with that though.

    Cheers

    Tatsky
     
  7. Deluks

    Deluks

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    If you have breather membrane you can go with mars idea as is. No need to bump anything out, you will have the deeper rafters alongside the windows 'sticking in' to the room but it won't look so bad.
     
  8. ^woody^

    ^woody^

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    You can lap the new rafters on the purlin

    Why not put your insulation between the rafters and then just fit the additional insulation to the underside of the existing rafters and then the plasterboard fixed with some long screws?
     
  9. TatskyPT

    TatskyPT

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    Hi Deluks

    I don't have a membrane or felt etc. However, I am told as I don't have this and my roof is naturally vented then I can get away with a smaller air gap between slates and insulation.

    However, at 90mm my rafters are still too shallow to fit in 75mm insulation. I would like something bigger than a 15mm air gap.

    I think I will double up rafters around the roof window openings, with trimmers head and foot, and then on other rafters just beef up the existing with 2x2.

    Should do the trick

    Cheers

    Tatsky
     
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  11. Deluks

    Deluks

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    If nothing is between the insulation and tiles/slates then you don't need an airgap. (The battens themselves count as an airgap anyway) Only thing to remember is to use a breathable membrane if ever the roof gets re-covered.

    If the rafters are good for the loading, then no need to build them up. and around the rooflight you could probably just double (or triple) the existing using the same depth rafter again. No real need for deeper rafters. (I'm sure Ronny or Static will be along to confirm ;) )
     
  12. TatskyPT

    TatskyPT

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    Cheers Deluks

    based on all the information, help and guidance I have what you are suggesting is most probably the course I will take. i.e. double up around windows with similar dimension rafters to that which is already there. Then draw out the existing with 2x2 as I would like to get 100mm in between rafters if I can for the u rating, with 25mm over the top of the rafters.

    The rafters are solid, so they don't need strengthening. The only reason new rafters were specified in the plans was for insulation depth. And even then, I think the spec on the plans is a generic thing as it specifies to double up existing rafters at 450 centers.... but my existing rafters are 400mm centers. Which leads me to believe that the snippet on the side about rafters is just a generic quote, and not specific to my case.

    I was going to start the windows this weekend, after the plans, BC and party wall has eventually been sorted.

    Looks like nice weather at the weekend..... oh! Snow from Thursday.

    Padstones this weekend it is then!
     
  13. RonnyRaygun

    RonnyRaygun

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    I agree. If the roof structure isn't being altered then no need to increase rafter depth. Woody's suggestion sounds like the best one; by fixing insulation between and then to the underside of the rafters you won't have a cold bridge to contend with.

    And as Deluks says, double or triple rafters around the rooflight, depending on how many have been cut.
     
  14. foamit

    foamit

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    Hi tatsky

    Just read this post,when I did my loft conversion BC asked me to double up rafters where I wanted to take out purlins,I had to bolt through with large toothed washers in between the sandwich,this was to take out any flex on the existing when purlin was taken from under the rafter.
     
  15. RonnyRaygun

    RonnyRaygun

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    Although load to the rafters will be increased by the weight of the insulation and plasterboard. Probably not enough to overstress them but should be considered...

    As rules of thumb go, that's not a great one :rolleyes: . If you double the span of a member by removing the purlin, the stresses go up four fold. So everything being equal, you would need three extra rafters of the same depth. Of course, it's possible the rafters weren't very highly stressed and doubling them was enough in this case...
     
  16. TatskyPT

    TatskyPT

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    Hi All

    Thanks for the advice. Lots of good useful stuff there.

    Just to clarify, I am not removing any purlins, actually there is a steel channel going in to beef up each purlin. So the only additional load on the rafters is the insulation, and the windows of course which will be doubled or tripled up anyway.

    What I am thinking of doing is installing the 150x50mm rafters for the window openings. Then increasing the depth of the existing with 2.5inch timber to give me the depth. So I will get 100mm insulation between the rafters and 25 over the rafters.

    The difference between the 150mm and the beefed up rafters should be around 3mm, so won't be noticeable once insulation and PB goes on.

    RonnyRaygun, you mentioned doubling or tripling around windows depending on how many rafters are cut into. That raises a good point, as the widest window is 780mm. You are supposed to leave 20-30mm per side between the frame and the rafter. My rafters are 400 centers, so sods law that means I have to remove 2 rafters. Is that a case for trippling up the rafters remaining on either side? That would make sense to me.


    Cheers

    Tatsky
     
  17. RedHerring2

    RedHerring2

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    I think the rule is: Remove 1 or 2 rafters then add 2 (1 either side),
    Remove 3 or 4 add 4, (2 either side), etc.
     
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