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Extension roof timber sizes

Discussion in 'Roofing and Guttering' started by Splin, 13 Dec 2017.

  1. Splin

    Splin

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    I've read lots here and found it a very helpful site and learnt more than I ever thought I'd want to know about roof design and construction but I've got to the point where I need some help. I'm building an L shaped extension across the rear of my East Midlands house; part is a lean to roof, the other, deeper end is gable ended with a valley where the two join. It will be all be warm roof, insulation above and between, with the ceiling plasterboard nailed under the rafters, so no ties.

    The building control officer won't specify the timber sizes but seems to be easy going and would be happy to accept generous sizes with C24 timber. My builder would also be delighted to over specify the timber. This would be a simple solution but I really want to minimise the rafter depth as I want 200mm over the rafters (75mm of insulation plus counterbattens, tiling battens and interlocking double roman concrete tiles). If the rafters get too deep then I have to compromise on the thickness of insulation, which I really don't want to do, or risk the pitch dropping below 22.5, requiring even deeper rafters.

    Also, I don't want the facias to be any deeper than necessary for appearances sake.

    The lean to roof is 1705/1805mm horizontally from the abutment wall to the inside/outside of the wall plate, with a total rise (from the top of the wall plate) up the abutment wall of 1160mm to the top of the roof covering. I have a spreadsheet to calculate the pitch, which with 100mm deep rafters birdmouthed by 1/3 to the wall plate and 1/2 to a 50mm deep pitch plate is 25.6 degrees.

    I would like the rafters spacing to be 600mm for better thermal performance.

    Unfortunately I don't have a copy of Eurocode 5 span tables (4th edition currently in force?) but I did find this:

    http://www.buildingcontrol-ni.com/assets/pdf/TechnicalBookletD2012.pdf

    which has comprehensive tables which I'm guessing come from the 2nd or 3rd edition of the TRADA document?

    Marley tiles have a load of .47kN/m^2 so I assume the dead load with insulation and battens will be less than .5kN/m^2 and I assume an imposed load of .75kN/m^2 applies (only 25m above sea level and very rarely see snow).

    I calculated the clear span to be 1954mm, so from Table 4B.19 (page 41 of the 2012 NI document I linked above) shows a C16 38x100 to have a span limit of 2.01m but a 47x100 has a more comfortable margin at 2.23m as does a C24 38 x 100 @ 2.17m

    So:

    1) Where exactly is the clearspan measured? Is it along the bottom of the rafter between the points where it just meets the wall and pitch plates? Or is it along the rafter between the deepest part of the birdsmouths?

    2) Could I get away with 38 x 100 C24 @ 600mm spacing (better than 47 x 100 thermally)? I understand that the later editions of the span tables reduced the maximum spans, especially for smaller timbers?

    3) The gable roof has a ridge beam span of 3382mm (abutment wall to inside of inner leaf of gable), with rafter spans approx 1600mm either side, pitched a bit less then 30 degrees. The BCO says he would be happy to see 9" x 3" C24 for the beam but looking at the tables this might be marginal. Do I look at Table 4B.22 (page 42, purlins span, c24 for pitch 22.5 to 30) looking for purlin spacing >= 1600mm?

    4) How would I select the valley beam dimensions, which would have a span of around 2550mm? Is this one of those cases with no standard solutions (theoretically) requiring formal calculations? Would I just use the same size as the ridge beam and assume that would be more than adequate without calculating loads?

    5) Should I be able to find a copy of the current span tables at a local library easily enough or should I go to the local council and the BI dept and ask if I could have a look at their copy?

    I've found plenty of local council build reg advice documents which publish limted tables, but they all seem to have different values - I'm not sure if that's because they are based on different editions of the Eurocode span tables or because they have decided to locally tighten the standards for roofs built without formal structural calculations?

    Sorry for so many questions but I'd appreciate any help/insight you can provide,

    Tony
     
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  3. noseall

    noseall

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    Er.......why hasn't your designer dealt with this?
     
  4. Splin

    Splin

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    Good question - I didn't mention it because my post was already so long that it is is serious danger of being filed under TL;DR

    The designer (a building surveyor/ex LA building inspector) copped out by specifying that roof trusses should be designed and supplied by a specific company. At the time I didn't pay any attention as I assumed it was a formality and something for the builder to sort out. I was only supposed to get involved in the electrics and plumbing aspects.

    In defence of the designers, they assumed, quite reasonably I suppose, that the gabled roof part would be a be a conventional cold roof with flat ceiling and ceiling joist ties for which trusses would be OK, if not typical, for this type of job - a small extension. I don't recall saying I wanted a vaulted ceiling in the gable roofed room so fair enough a design change on my part. However the other lean to roof part was specified to have Velux windows and it never occured to me that this would be anything other than an open, ie. vaulted roof.

    Not that any of the above really matters; I have a high regard for the builder and put the roof design issue into the "batteries not included" category meaning that, as wth almost anything you purchase, there will be surprises and things you expected to be included which turn out to be extras. So I don't have any real beef with the designers, just a bit disappointed. Especially that neither my designer, nor the BCO, (both of whom I am paying), when asked as to the best way to proceed, didn't think it worth mentioning the existence of the Eurocode 5 tables. It seems to me that the whole purpose of these is that they are designed to cover typical situations such as my extension roof.

    Is my roof really that unusual?
     
  5. noseall

    noseall

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    Why don't you go for a vaulted 'cold' roof? The insulation sits between and beneath the rafters, is easily ventilated above (breather membrane, O.F. vents etc) and won't cause any unnecessary height to the roof.
     
    Last edited: 13 Dec 2017
  6. Splin

    Splin

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    Insulating below is an option but then I lose headroom and I still need to know the minimum allowed timber sizes.
     
  7. tony1851

    tony1851

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    You are over-thinking this and getting too bogged down. You should also not attempt to go right down to the published limit of the timber, to allow for bad workmanship or unforseen loading (eg your dead load is a little low; and wind can cause snow to drift into valleys increasing the load locally).
    Just go with the 47x100 C16 @ max 600 centres (better if you could reduce the spacing a little, say 550). C24 timber gives slightly reduced deflection, but has a tendency to warp more than the C16.
    For the valley, use a 225x50.
     
  8. Splin

    Splin

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    That's very helpful thank you - I'm happy to go with 47x100. But the problem now is how to convince the building inspector - I can't just tell him that tony1851 said it woud be alright. :D

    Can someone tell me the maximum spans for 47x100 C16 and C24 rafters @ 600 centres, between 22.5 and 30 degress with imposed loads of .75kN and dead loads of .5kN and .75kNs from the current Eurocodes 5 span document? If I can point to the spans in the relevant table of the code I can't see why that wouldn't satisfy BC.
     
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  10. Mr Chibs

    Mr Chibs

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    c16
    .5kN-.75kN = 2.10
    .75kN-1.00kN = 1.96

    c24
    .5kN-.75kN = 2.25
    .75kN-1.00kN = 2.14
     
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  11. Ian H

    Ian H

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    I did, and they were fine with it (y)
     
  12. tony1851

    tony1851

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  13. tony1851

    tony1851

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    This is from the old (pre-EC5) copy of AD A. It's based on the BS 5268 timber code, but is still acceptable. The upper band is for C16 and the lower band for C24.
    Scan0185.jpg
     
  14. Splin

    Splin

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    Hmm, interesting replies. I had a chat with a very helpfull BI at the local council today. He kindly let me take this photo of the relevant tables from his copy of the 4th edition Eurocode 5 book.


    To paraprase an old computer game, "you are in a maze of twisty little span tables, all(most) alike" :cautious:

    600mm spacing 47x100:

    Dead weight ............................... 0.5 to 0.75kN .. 0.75 to 1kN
    Grade ........................................... C16 . C24 ...... C16 . C24

    Mr Chibs......................................... 2.10 2.25 ....... 1.96 2.14
    tony1851 (pre-EC5) copy of AD A...... 2.10 2.18 ....... 1.95 2.07
    NI BC 2012 (doc linked in first post)... 2.10 2.28 ....... 1.95 2.07
    Eurocode 5 (4th edition)................... 1.73 1.98 ....... 1.63 1.86

    (Sorry about the formatting - couldn't get tables to work.)

    So it looks like the NI building control document is based on the same pre Eurocode table shown by tony1851.
    The EC 4th edition spans are reduced quite a bit. Note also that the 4th ed. table is for .6kN imposed load wheras the others are for .75kN; this apparently is the one that applies to me - there is a snow load table earlier in the document along with a UK map.

    So to be certain my BI (not the one I spoke to today unfortunately) will accept it, I guess it would be prudent to use the EC 4th ed. spans and thus use 47x100mm C24s which just meet my 1.95m span. I'm sure that C16s would also be acceptable but the price difference isn't huge so probably not worth arguing about.

    I also got a couple of other questions answered, vis:

    1) What exactly is clearspan? It is the minimum distance along the rafter between where the rafter and supporting elements meet.

    2) Are there any standard tables for determining valley beam size? No. In theory, each one should be specified by a structural engineer, but in practice common sense and experience applies - using timber which is obviously oversized is a simple solution.


    Unfortunately I forgot to ask which document / edition if any is in force or are just recommendations, but from tony1851's reply it looks like any of the above are acceptable. Is this correct or is it down to the judgment of individual BI's?

    He also stated that many builders get it wrong by butting the rafters to the pitch plate rather than birdmouthing them - a couple of skew nails is not adequate to resist the tension / spreading forces. I've certainly seen a few pictures here like that.
     
  15. tony1851

    tony1851

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    Strangely, all the tables you quote are acceptable, and there is a reason for this.
    When engineers calculate a timber joist or rafter, the main considerations are 1) limiting the bending stress, and 2) limiting deflection. In domestic floors and roofs, deflection is usually the critical aspect and must be kept within set limits. Excessive deflection of floor joists is obviously more of an issue than deflection of roof rafters.The Eurocode and BS code have slightly different allowances for deflection.
    Generally, floor joists in EC5 may be slightly bigger than those in the BS (for equal spans/spacings) while EC5 often gives lighter timbers for roof structures.
     
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