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?

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.

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.

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

As mentioned on the other thread I agree with the comments about only using nails for joist hangers etc.

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.

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)

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

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

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.

Ronan Point used screws, look what happened there.

Yes, was shoddy workmanship and bad design.

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.

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!

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

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