Green Building Forum |
Home Books GBEzine Subscription News HelpDesk Your Cart Register |
Welcome to the Green Building Forum. Reading any of the public conversations is free but if you want to join in the discussions then you need to register first to obtain a code for which there is a small charge. Please follow the link on the left. OR:
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.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.
Logout | ||
© Green Building Press |
||
Comments
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.
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!
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.
As mentioned on the other thread I agree with the comments about only using nails for joist hangers etc.
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.
Most of my work is renovation so I'm not looking to put thousands of fixings in a day. Quality more important.
(not that I'm advocating using screws of course) :wink:
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?
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
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). Using decent quality kiln-dried wood that doesn't shrink and twist after it's installed.
Paul in Montreal.
Paul in Montreal.
I think the danger with using screws is that you they are so convenient that you end up using them for everything.
Paul in Montreal.
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
'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
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
marcus
Thast why God invented the reciprocating saw!