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Housing - New Build: SIPS roof v Attic Trusses

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  1.  
    • CommentAuthoraa44
    • CommentTimeFeb 8th 2010
     
    A question about the prices and qualities of roof systems.

    My planning application is in and thoughts are turning to more detailed technical questions. I had intended to have a SIPS roof (the walls will be ICF) but my architect is recommending that I use attic trusses as he thinks that it will be a lot cheaper.

    If you insulated between the roof trusses and underneath them (say 50mm between and 100mm below) I assume that you would get similar U-values to a SIPS roof. As I'm in Scotland, there will be a layer of OSB on top of the trusses before the battens and slates.

    Do you think that attic trusses would perform substantially worse than a SIPS roof?

    Has anybody recently priced up the two systems for a roof? If so, was there a dramatic difference?
  2.  
    How are you proposing to support the SIPs? It's most common to use a ridge beam &/or purlins. I don't have numbers, but the combination of SIPs & ridge beam/purlins is likely to be a lot more expensive than trusses, sarking board & insulation.

    On the plus side, if you have a room in the roof, a ridge beam/purlins roof might make better use of the space than room in the roof trusses. It's also a lot easier to make it airtight & you avoid the thermal bridging of the insulation by the trussed rafters.

    Alternatively you could lay a polyethylene air barrier over the trussed rafters & put the SIPs on top of that. That way you get the low cost truss structure with SIP levels of airtightness & no thermal bridges.

    Or you could achieve the same effect by putting a polyethylene air barrier on top of your OSB sarking board, all your insulation on top & then breathable sarking membrane on top of that.

    What type of SIP are you thinking of?

    David
    • CommentAuthortony
    • CommentTimeFeb 8th 2010
     
    trusses for me -- more economic way to go than SIP.
    • CommentAuthoraa44
    • CommentTimeFeb 8th 2010
     
    Thanks for the replies. The quote that I have for the SIPS panels includes the purlins and a ridge beam. My very, very rough calculation is that an attic truss roof would cost about 30% less than the SIPS one but that is very much back of a fag packet!
  5.  
    Posted By: aa44My very, very rough calculation is that an attic truss roof would cost about 30% less than the SIPS one

    If it's a room in the roof then you need to remember to allow for the cost of the floor structure which is included with a room in the roof truss.

    David
    • CommentAuthoraa44
    • CommentTimeFeb 8th 2010
     
    Thanks for the reminder David. It's already included in the comparison but it's definitely not something to forget. I need quite a few I-joists with a span of 6.6m and they are about 65-70 quid each. I am quite tempted by Posi-Attic trusses for the ease of running the services. They came out at about 105 quid each, compared to about 85 quid for "standard" attic trusses.
    • CommentAuthorTuna
    • CommentTimeFeb 8th 2010
     
    Not much to add here, except that we have experience in using SIPs in this sort of application. We have a SIPs room in roof design, with glulam purlins and ridge beam. SIPs were a logical choice for us, as we've used them for the entire structure. We are delighted with the result.

    The advantages are robust airtightness reliably achieved and a clean inner surface for first and second fix, with the central section open to the ridge which (along with the timber beams) looks fantastic. Construction was efficient too - three days to put the full roof on our house. Details such as rooflights were particularly easy to fit and seal. Waste was a bare minimum. I assume you've factored in the added labour for using trusses and packing them out?
    • CommentAuthoraa44
    • CommentTimeFeb 8th 2010
     
    The labour might actually be one of the advantages as I am planning to do as much of the build myself as I can. Also, being up in Shetland, anything fancy such as SIPS means getting specialists in from down south whereas although the attic trusses have to be shipped in, there's plenty of local building knowledge.
    • CommentAuthorRobinB
    • CommentTimeFeb 9th 2010
     
    We'd budgeted 10k for a SIPs roof (don't ask me how) and quotes came in at 18 - 24k. An old fashioned raftered roof, 200 x 50 rafters at 400 centres with insulation added to bring it up to SIPS standard u values will cost about 10K we think and give us the same headroom. There are steels and gluelams to crane in so I'm not sure how much we will actually save.
    • CommentAuthoraa44
    • CommentTimeFeb 9th 2010
     
    Thanks Robin. That's really helpful. I hear so many conflicting reports about pricing that it's hard to know what to believe. There's no substitute for real life quotes!

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