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    • CommentAuthorEd Davies
    • CommentTimeMay 28th 2014 edited
     
    In another thread:

    http://www.greenbuildingforum.co.uk/forum114/comments.php?DiscussionID=12133&page=1#Comment_202682

    Posted By: Paul in MontrealScrews are not approved for applications where there are shear forces, such as joist hangars. Only an approved fastener should be employed - usually a nail.
    I can see why you wouldn't want a threaded part of a fastener to be used in shear but typical woodscrews have an unthreaded shank. If you make sure that goes right through the joint is there a problem? It seems to me that screws can easily have a larger cross section than nails so be a lot stronger. Also they can be extractable while being resistant to being pulled out.

    What are coach screws made for?
    • CommentAuthorBeau
    • CommentTimeMay 28th 2014
     
    I was of the belief that these where the correct sort of nail for joist hanging aplications http://www.screwfix.com/p/square-twist-sheradised-3-75-x-30mm-1kg-pack/12788
  1.  
    Posted By: Ed DaviesIt seems to me that screws can easily have a larger cross section than nails so be a lot stronger.


    Screws are made of a harder metal which is more brittle and thus has a much lower resistance to shear loads compared to the more ductile nails. Screws are not to be used for applications which have shear loads, unless the screw itself is rated for that application. This is particularly important for DIYers - there have been cases of injuries where people have built decks themselves and used screws for the joist hangers, only to have the whole thing fail under load with people on the deck. The manufacturer of the joist hangers usually specify the approved type of fasteners, but sometimes they have special screws rated for this application. Using regular wood screws (even though the shanks may appear thick) is a very very bad idea. If you don't believe me, fasten two pieces of wood with screws and another two pieces with nails. Whack them with a hammer perpendicular to the fasteners and see how long the screws survive. They usually break quite easily whereas the nails will bend at worst.

    Screws are fine for non-structural applications (e.g. the blocking in stud walls or holding the top and bottom plates in place for compression loads) but professional framing contractors only use nails for two main reasons: (1) Nails are always approved for this application and (2) nails can be installed much much faster than screws. No professional would use a hammer - they all use pneumatic nail guns. I cringed watching some episodes of Grand Designs where the contractors were using handsaws and hammers for framing!

    Paul in Montreal.
  2.  
    Ive seen plenty of screws break/sheer, never seen a nail "break". Only nails should be used where its structural.

    The reason some screws have an unthreaded part is so that when screwing two bits of wood together you want to pull one bit tight against the bit of wood below. The part of the screw that is threaded is drilling and pulling into the bit of wood behind/below, whilst the head of the screw is pulling against the bit of wood infront/above. You dont want any threaded screw to be passing through the 2nd bit of wood or it will resist the pulling force of the head of the screw.

    not sure if that is clear!
    • CommentAuthorowlman
    • CommentTimeMay 28th 2014
     
    Posted By: Paul in Montreal............ they all use pneumatic nail guns. I cringed watching some episodes of Grand Designs where the contractors were using handsaws and hammers for framing!

    Me too Paul, I cringe when they are hammering anywhere where wood and masonry are in close proximity. You know damn well they are loosening all the mortar joints. Many UK roofers for instance now use gas powered nail guns but IMO the UK joinery trade is woefully ill equipped compared to their Stateside cousins regarding pneumatic nailers.

    Ed. part of the problem with screws v. nails is that to many, esp. DIYers some pros too don't know the difference between chipboard pattern screws, ( threaded all the way to the head ), and genuine woodscrews, ( with a plain shank ). The difference in relative strengths is really apparent with SS, where the chipboard pattern SS screws very easily shear if overtightened, - easy to do with modern power drivers. But you still get hinge suppliers for instance supplying them with their hinges. When you think that most SS hinges are used on Oak doors for instance the shearing problem is made worse with the very hardness of the wood. I always fit SS, pre drilled, onto oak with stronger mild steel screws to start with, changing them one for one when I'm finished.
    Most modern screws even woodscrew variants are meant for power driving and are very sharp and, I'm guessing, case hardened. This brittleness makes then unsuitable for shear loading. Even where a shank exists they are made nowadays quite thin again to facilitate power driving.

    Coach screws are generally much stronger because of the shank thickness, to which you refer but SS esp. heads will still shear if over tightened with a socket wrench for instance.
    •  
      CommentAuthorfostertom
    • CommentTimeMay 28th 2014
     
    All enlightening.
  3.  
    I disagree with some of the comments about nail guns being the best way. I always screw studwork together for example - A nail gun gives you one chance at getting the stud in perfect alignment - A nail is in whether its aligned or not - I used to use nails many years ago but I guess I've turned into an amateur:cool:

    As mentioned on the other thread I agree with the comments about only using nails for joist hangers etc.
  4.  
    Posted By: Mike GeorgeI disagree with some of the comments about nail guns being the best way.


    If you're doing new construction, where there are a lot of pieces to be joined, a pneumatic nailer is about 10x faster than the fastest impact screwdriver. This makes a huge difference to productivity. If you're just doing some renovations, then speed might not be so important and screws (assuming they are the right kind) with a decent impact driver is fine. It's very tiring on the arms using a non-impact driver.

    DIYers tend to use screws for everything (since they're easy to take out when mistakes are made). A big no-no for load-bearing joist hangers. For small loads like dropped ceilings, they're probably OK, so long as they are not thin screws such as ones for drywall (some people use these for everything).

    Paul in Montreal.
  5.  
    Indeed - impact driver is essential - My new one 18V with 5ah batteries - fantastic.

    Most of my work is renovation so I'm not looking to put thousands of fixings in a day. Quality more important.
    • CommentAuthordaserra
    • CommentTimeMay 28th 2014
     
    I've been using screws in timber roof and decking and haven't had them break except when overtightened. That's what the screw gun clutch is for. Then again I use German torx head screws, I'm not sure if that makes the difference. I would never use a fixing to take a shear load anyway unless I really had to.
  6.  
    Thinking more about Joist Hangers. I'd be surprised if they would fail whether screwed or nailed if the arms are wrapped over the top and behind the bearer. Each hanger typically having what 20 -30 fixings? Can't see all those sheering off under normal loading conditions - which is what Span Tables are designed to accommodate.....

    (not that I'm advocating using screws of course) :wink:
    • CommentAuthormarkocosic
    • CommentTimeMay 28th 2014
     
    You've actually seen a builder use more than three of the holes on a joist hanger Mike? Blimey! ;-)

    This is all new/useful info for me.

    Generally I like threaded fasteners because you can pre-tension them and avoid differential movement (squeaks) when things are loaded up. You release the pretension on the joint as you load it rather than opening it up. Friction (or your adhesive) carries the loads, not loose fasteners in shear.

    Buildings would appear to be a different world entirely!

    Screws in a joist hangar are far less likely to fail than those directly between bits of wood as the joist hangar acts to average out the loads on the screws. It'll deform (and unload the screws/load them up evenly) before the screw heads break. (beam would go from moment at centre first rather than shear at the ends if designed in accordance with span tables) You won't get this if you screwed two pieces of wood together directly though, and slight misalignment will overload the first screw to buggery where a nail made of cheese and with zero tension in it just flexes to accommodate the new position.

    Other than using traditional gravity construction, where the screws or nails just stop things from sliding about or falling over, what's the secret to avoiding sounds in timber framed structures?
    • CommentAuthorMike George
    • CommentTimeMay 28th 2014 edited
     
    Posted By: markocosicYou've actually seen a builder use more than three of the holes on a joist hanger Mike? Blimey! ;-)

    yep...me:) If they weren't intended to be used they wouldn't all be there :)

    Re: squeeky floors there is some discussion here about not screwing weyrock flooring at all - though I looked the other day for a manufacturers spec and couldn't find one.

    What I do is:

    1: Silcone the timber into the joist hanger
    2: Paint loads of pva onto the joist as youmlay the boards
    3: Put 3 screws across aboard into each joist
  7.  
    Posted By: Mike GeorgeRe: squeeky floors there is some discussion here about not screwing weyrock flooring at all - though I looked the other day for a manufacturers spec and couldn't find one.


    No-one uses chipboard for flooring over here.

    We use exterior grade tongue and grooved plywood that is screwed and glued with polyurethane construction adhesive to the joists. It will not squeak when done this way. Of course, the proper type of floor screws are used and a sensible schedule is used (something like every 16" or so).

    Posted By: markocosicwhat's the secret to avoiding sounds in timber framed structures?


    Using decent quality kiln-dried wood that doesn't shrink and twist after it's installed.

    Paul in Montreal.
    • CommentAuthorMike George
    • CommentTimeMay 28th 2014 edited
     
    Posted By: Paul in Montreal

    Using decent quality kiln-dried wood that doesn't shrink and twist after it's installed.

    Paul in Montreal.


    Doesn't exist over here. You must be keeping all the best stuff over there for yourselves :bigsmile:
  8.  
    Posted By: Mike GeorgeDoesn't exist over here. You must be keeping all the best stuff over there for yourselves


    Must be a supply and demand thing. Since most construction over here is stick framing, there's a big demand for materials. Conversely, bricks are expensive compared to the UK because they're relatively rarer.

    Paul in Montreal.
    • CommentAuthordaserra
    • CommentTimeMay 28th 2014
     
    We get loads of German and Polish kiln dried (and treated if you want) here in Portugal. Sometimes I visit our equivalents of B&Q and laugh at the twisted timber they sell.
    • CommentAuthordickster
    • CommentTimeMay 29th 2014
     
    Blimey, been using screws instead of nails for all sorts of jobs where it is apparent I should have used nails. I thought nails were used instead of screws to cut costs and for speed. Enlightened2.
    •  
      CommentAuthorSteamyTea
    • CommentTimeMay 29th 2014
     
    Ronan Point used screws, look what happened there.
    • CommentAuthortony
    • CommentTimeMay 29th 2014
     
    I thought that was ss bolts and paid per panel, they found three bolts were enough, sold all the ss ones and used ms instead making profit three ways .. until.....
    •  
      CommentAuthorSteamyTea
    • CommentTimeMay 29th 2014
     
    Yes, was shoddy workmanship and bad design.
  9.  
    For Joist hangers I would only ever use Simpson nails, they look like masonry nails. Practically need to destroy the timber around them to get them out.

    I think the danger with using screws is that you they are so convenient that you end up using them for everything.
  10.  
    Posted By: bot de pailleFor Joist hangers I would only ever use Simpson nails, they look like masonry nails.


    Exactly, and also remember to use the double-galvanized (Z grade) when using AQC treated timber.

    Paul in Montreal.
    • CommentAuthorEd Davies
    • CommentTimeMay 29th 2014
     
    Just been talking to a just-about-retired builder who's been using stainless screws for everything for years. He avoids galvanized nails in treated timber because the galvanizing is so thin and the chances are you'll scape it off on the joist hangers anyway. Never had a problem.
    • CommentAuthortony
    • CommentTimeMay 29th 2014
     
    Crikey, he musn't mean stainless steel screws, more likely shiny ones that look like ss, ask him for a couple and see if they go rusty!
    • CommentAuthorEd Davies
    • CommentTimeMay 29th 2014
     
    Why do you say he mustn't mean stainless?
    • CommentAuthortony
    • CommentTimeMay 29th 2014
     
    No one can afford stainless steel screws especially not builders!

    He would also be bald and covered in bruises as they are very frustrating to use, makes you rear your hair out and hit yourself, they sheer off dead easy, heads cam our etc
  11.  
    ''Why do you say he mustn't mean stainless?''

    'Cause if they are, he must have very deep pockets!

    As an example, box of 200 4 x 50mm Turbo Gold: £2.33

    Box of 200 Turbo Ultra A2 stainless: £10.99
  12.  
    I use Spax screws and nails, and forby I have "dismantled" quite a bit of scrap joinery from temp jobs (work related) where the joiner used screws.
    THEY DO NOT FAIL IN SHEAR!
    Not even when repeatedly struck by my 2' long shafted 2ld wee sledge/hammer, which is one mean joinery demolotion tool.
    They WILL bend and distort, but!
    The wood invariably fails first.
    PS
    Try "ringshanked" pallet nails, presumably air driven, they are the absolute God's curse to extricate!
    marcus
  13.  
    <blockquote><cite>Posted By: orangemannot</cite>Try "ringshanked" pallet nails, presumably air driven, they are the absolute God's curse to extricate!
    marcus</blockquote>

    Thast why God invented the reciprocating saw!
   
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