Size of wood beams? Corrugated roof?

Discussion in 'Misc DIY' started by Harry Bloomfield, Jun 19, 2006.

  1. Hi,

    Thanks for the previous help with cement bag sizes, that bit is in
    place now.

    Q1. The beams are to support a light weight corrugated flat roof of
    internal size 8' x 15'. Beam spacing will be a little under every 2
    foot with the 5 beams running the length of the 15 foot with two sets
    of bracing noggins at 5' in from each ends. Unsupported span is 15
    feet.

    Ends will be firmly fixed down to structure and protruding through
    walls.

    Will 5" x 2.5" (actual sawn size) be adequate?

    Q2. What is a good cost effective alternative to asbestos cement
    corrugated roof panels?





    --

    Regards,
    Harry (M1BYT) (L)
    http://www.ukradioamateur.co.uk
     
    Harry Bloomfield, Jun 19, 2006
    #1
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  2. Harry Bloomfield

    Phil L Guest

    Harry Bloomfield wrote:
    > Hi,
    >
    > Thanks for the previous help with cement bag sizes, that bit is in
    > place now.
    >
    > Q1. The beams are to support a light weight corrugated flat roof of
    > internal size 8' x 15'. Beam spacing will be a little under every 2
    > foot with the 5 beams running the length of the 15 foot with two sets
    > of bracing noggins at 5' in from each ends. Unsupported span is 15
    > feet.
    >
    > Ends will be firmly fixed down to structure and protruding through
    > walls.
    >
    > Will 5" x 2.5" (actual sawn size) be adequate?
    >


    15ft unsupported span is far too long, the beams will sag, is there no
    possibility of going the opposite way and using 8ft beams?....failing that,
    I fear it will require a purlin in the centre, possibly steel.

    > Q2. What is a good cost effective alternative to asbestos cement
    > corrugated roof panels?


    OSB boards, overlaid with torch-on roof felt
     
    Phil L, Jun 19, 2006
    #2
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  3. Harry Bloomfield

    Roger Guest

    The message <>
    from Harry Bloomfield <> contains these words:

    > Q1. The beams are to support a light weight corrugated flat roof of
    > internal size 8' x 15'. Beam spacing will be a little under every 2
    > foot with the 5 beams running the length of the 15 foot with two sets
    > of bracing noggins at 5' in from each ends. Unsupported span is 15
    > feet.


    > Ends will be firmly fixed down to structure and protruding through
    > walls.


    > Will 5" x 2.5" (actual sawn size) be adequate?


    Probably. If it was a floor you would want 9" x 2" joists to span 15'
    but you don't want to load it with anything more than wind and your own
    weight as you fix it down.

    > Q2. What is a good cost effective alternative to asbestos cement
    > corrugated roof panels?


    I think you will find that modern corrugated sheet does not contain asbestos.

    Plastic covered steel is an alternative. I can't remember what it is
    called but it has a section something like (IIRC)
    _____ ______
    _/ \_____/ \_

    and is a doddle to fit.

    You don't say which way the roof slopes but the beam layout suggests it
    is down the short dimension. However should you want it to slope down
    the long dimension the steel sheets can be ordered in in quite long
    lengths, I think in excess of 15'.

    --
    Roger Chapman
     
    Roger, Jun 19, 2006
    #3
  4. Harry Bloomfield

    Owain Guest

    Chris Bacon wrote:
    > Harry Bloomfield wrote:
    >> Q2. What is a good cost effective alternative to asbestos cement
    >> corrugated roof panels?

    > Corrugated iron over Sterling board. You could use Onduline
    > or similar, but... well, I really wouldn't.


    Two layers of corrugated iron with fertiliser bags laid in between. The
    bags (overlapping like tiles) make the holes in the corrugated iron
    waterproof.

    (If your corrugated iron doesn't have any holes in it, you've paid too
    much for it)

    Owain
     
    Owain, Jun 19, 2006
    #4
  5. Harry Bloomfield

    Guest

    Harry Bloomfield wrote:

    > Hi,
    >
    > Thanks for the previous help with cement bag sizes, that bit is in
    > place now.
    >
    > Q1. The beams are to support a light weight corrugated flat roof of
    > internal size 8' x 15'. Beam spacing will be a little under every 2
    > foot with the 5 beams running the length of the 15 foot with two sets
    > of bracing noggins at 5' in from each ends. Unsupported span is 15
    > feet.
    >
    > Ends will be firmly fixed down to structure and protruding through
    > walls.
    >
    > Will 5" x 2.5" (actual sawn size) be adequate?
    >
    > Q2. What is a good cost effective alternative to asbestos cement
    > corrugated roof panels?


    sagulator will give you a figure for how much sag you can expect under
    any given load.
    http://www.woodbin.com/calcs/sagulator.htm
    Then you can decide of the amount of sag will look ok. Over 15' it
    would have to sag a foot or so before breaking.

    You dont say whether this needs to be BR compliant, if so there are
    different rules to follow.

    Dont forget to calculate for snow loading, roofs must survive that ok.


    NT
     
    , Jun 20, 2006
    #5
  6. Harry Bloomfield

    Autolycus Guest

    "Harry Bloomfield" <> wrote in message
    news:...
    >
    > Q1. The beams are to support a light weight corrugated flat roof of
    > internal size 8' x 15'. Beam spacing will be a little under every 2
    > foot with the 5 beams running the length of the 15 foot with two sets
    > of bracing noggins at 5' in from each ends. Unsupported span is 15
    > feet.
    >
    > Ends will be firmly fixed down to structure and protruding through
    > walls.
    >
    > Will 5" x 2.5" (actual sawn size) be adequate?
    >

    I've just built a garage with 16ft span, using 8x2 at 4ft centres. My
    gut feeling is that 5" is a bit light, and the joists will sag visibly
    under their own weight. Bearing in mind that deflection is inversely
    proportional to the cube of the depth, even a modest increase in depth
    makes a big difference (e.g. 5" deflects 4 times as much as 8" for the
    same total load). If you decide to do proper calcs, the deflection and
    maximum stress are the easy bits - deciding on the wind load is the
    tricky bit.


    > Q2. What is a good cost effective alternative to asbestos cement
    > corrugated roof panels?
    >

    Box section steel sheet is about 6 quid a square metre, and is available
    in long lengths, so you could get away with no end laps at all. Minimum
    suggested pitch is 5 degrees iirc, or about 1 in 12. Fibre cement
    sheets are only available in shorter lengths (< 3m ?) and really need a
    steeper pitch, particularly if they're laid such that there are end
    laps, despite the practice of makers of cheap concrete garages.

    The problem with steel sheets is that the insulation value is minimal,
    and you'll get condensation dripping off the underside in some
    conditions. The design of insulated roofs with vapour barriers etc is
    quite a complicated business: my compromise was to deck the roof with
    secondhand 18mm ply then cover it in polythene, then clad in steel.

    The SLE Cladding site is helpful.


    --
    Kevin Poole
    **Use current month and year to reply (e.g. )***
     
    Autolycus, Jun 20, 2006
    #6
  7. Harry Bloomfield

    Guest

    Autolycus wrote:
    > "Harry Bloomfield" <> wrote in message
    > news:...


    > > Q2. What is a good cost effective alternative to asbestos cement
    > > corrugated roof panels?
    > >

    > Box section steel sheet is about 6 quid a square metre, and is available
    > in long lengths, so you could get away with no end laps at all. Minimum
    > suggested pitch is 5 degrees iirc, or about 1 in 12. Fibre cement


    I'm not sure what you refer to here, can you give us a link or
    soemthing?

    thanks, NT
     
    , Jun 20, 2006
    #7
  8. Harry Bloomfield

    Autolycus Guest

    <> wrote in message
    news:...
    > Autolycus wrote:
    >> "Harry Bloomfield" <> wrote in message
    >> news:...

    >
    >> > Q2. What is a good cost effective alternative to asbestos cement
    >> > corrugated roof panels?
    >> >

    >> Box section steel sheet is about 6 quid a square metre, and is
    >> available
    >> in long lengths, so you could get away with no end laps at all.
    >> Minimum
    >> suggested pitch is 5 degrees iirc, or about 1 in 12. Fibre cement

    >
    > I'm not sure what you refer to here, can you give us a link or
    > soemthing?
    >
    >


    http://coruspanelsandprofiles.co.uk/

    then

    Products and services > Roofs > Site assembled > Standard roof profiles
    > View Profiles > R32-1000


    takes you (I hope) to some basic data on one of the most common
    profiles. I was wrong: it's 4 degrees (1 in 15). Note that it's also
    often found in 0.5mm thickness, which imho is a bit thin unless you
    support in continuously, as I did.


    --
    Kevin Poole
    **Use current month and year to reply (e.g. )***
     
    Autolycus, Jun 20, 2006
    #8
  9. Harry Bloomfield

    Guest

    Autolycus wrote:
    > <> wrote in message
    > news:...
    > > Autolycus wrote:
    > >> "Harry Bloomfield" <> wrote in message
    > >> news:...


    > >> > Q2. What is a good cost effective alternative to asbestos cement
    > >> > corrugated roof panels?
    > >> >
    > >> Box section steel sheet is about 6 quid a square metre, and is
    > >> available
    > >> in long lengths, so you could get away with no end laps at all.
    > >> Minimum
    > >> suggested pitch is 5 degrees iirc, or about 1 in 12. Fibre cement

    > >
    > > I'm not sure what you refer to here, can you give us a link or
    > > soemthing?
    > >
    > >

    >
    > http://coruspanelsandprofiles.co.uk/
    >
    > then
    >
    > Products and services > Roofs > Site assembled > Standard roof profiles
    > > View Profiles > R32-1000

    >
    > takes you (I hope) to some basic data on one of the most common
    > profiles. I was wrong: it's 4 degrees (1 in 15). Note that it's also
    > often found in 0.5mm thickness, which imho is a bit thin unless you
    > support in continuously, as I did.


    thanks Kevin. What a horrible website.


    NT
     
    , Jun 20, 2006
    #9
  10. Harry Bloomfield

    Guest

    Harry Bloomfield wrote:
    > Hi,


    >
    > Q1. The beams are to support a light weight corrugated flat roof of
    > internal size 8' x 15'. Beam spacing will be a little under every 2
    > foot with the 5 beams running the length of the 15 foot with two sets
    > of bracing noggins at 5' in from each ends. Unsupported span is 15
    > feet.
    >

    Depending on the time and trouble you want to take, you might want to
    use a structure like designed by the mathematician John Wallis

    "Wallis tackled a practical problem - how to provide a flat structure
    such as a roof to span a large square open space, using only wooden
    beams much shorter than the required span and only supported around the
    edges. He devised a repeating pattern of short but interlocking beams,
    which could be extended to span any sized space."

    See <URL:http://www.soue.org.uk/souenews/issue4/wallis.html>

    As it turned out, this design wasn't used for the internal roof of the
    Sheldonian Theatre, Oxford - Wren used a different technique - a
    composite roof truss.

    Cheers,

    Sid.
     
    , Jun 20, 2006
    #10
  11. Harry Bloomfield

    Guest

    wrote:
    > Harry Bloomfield wrote:
    > > Hi,

    >
    > >
    > > Q1. The beams are to support a light weight corrugated flat roof of
    > > internal size 8' x 15'. Beam spacing will be a little under every 2
    > > foot with the 5 beams running the length of the 15 foot with two sets
    > > of bracing noggins at 5' in from each ends. Unsupported span is 15
    > > feet.
    > >

    > Depending on the time and trouble you want to take, you might want to
    > use a structure like designed by the mathematician John Wallis
    >
    > "Wallis tackled a practical problem - how to provide a flat structure
    > such as a roof to span a large square open space, using only wooden
    > beams much shorter than the required span and only supported around the
    > edges. He devised a repeating pattern of short but interlocking beams,
    > which could be extended to span any sized space."
    >
    > See <URL:http://www.soue.org.uk/souenews/issue4/wallis.html>
    >
    > As it turned out, this design wasn't used for the internal roof of the
    > Sheldonian Theatre, Oxford - Wren used a different technique - a
    > composite roof truss.
    >

    Actually - you might not - further investigation here shows:

    "However, if the roof structure had been used, it may not have stood
    the test of time. Every wooden beam in the structure was vital because
    it supported the weight of other wooden beams. If one beam failed, the
    whole structure would collapse. So if woodworm nibbled away, or a
    lightning strike or earthquake damaged one of the beams, the roof would
    fail. Most other roof structures can withstand a bit of a battering,
    but in John Wallis's design every beam was doing a vital job and none
    of them could afford to be damaged."

    from <URL:http://www.noisenet.ws/pages/m_carticle2.php?cid=9&aid=216>
     
    , Jun 20, 2006
    #11
  12. Harry Bloomfield

    Guest

    wrote:
    > wrote:
    > > Harry Bloomfield wrote:
    > > > Hi,

    > >
    > > >
    > > > Q1. The beams are to support a light weight corrugated flat roof of
    > > > internal size 8' x 15'. Beam spacing will be a little under every 2
    > > > foot with the 5 beams running the length of the 15 foot with two sets
    > > > of bracing noggins at 5' in from each ends. Unsupported span is 15
    > > > feet.
    > > >

    > > Depending on the time and trouble you want to take, you might want to
    > > use a structure like designed by the mathematician John Wallis
    > >
    > > "Wallis tackled a practical problem - how to provide a flat structure
    > > such as a roof to span a large square open space, using only wooden
    > > beams much shorter than the required span and only supported around the
    > > edges. He devised a repeating pattern of short but interlocking beams,
    > > which could be extended to span any sized space."
    > >
    > > See <URL:http://www.soue.org.uk/souenews/issue4/wallis.html>
    > >
    > > As it turned out, this design wasn't used for the internal roof of the
    > > Sheldonian Theatre, Oxford - Wren used a different technique - a
    > > composite roof truss.
    > >

    > Actually - you might not - further investigation here shows:
    >
    > "However, if the roof structure had been used, it may not have stood
    > the test of time. Every wooden beam in the structure was vital because
    > it supported the weight of other wooden beams. If one beam failed, the
    > whole structure would collapse. So if woodworm nibbled away, or a
    > lightning strike or earthquake damaged one of the beams, the roof would
    > fail. Most other roof structures can withstand a bit of a battering,
    > but in John Wallis's design every beam was doing a vital job and none
    > of them could afford to be damaged."
    >
    > from <URL:http://www.noisenet.ws/pages/m_carticle2.php?cid=9&aid=216>


    Perhaps one could use 2 layers of this structure, with woodwork bolted
    together at each crossing of top and bottom layer.

    NT
     
    , Jun 20, 2006
    #12
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