hello, can anyone tell me the differance between screws that are for metal and wood .reason is that on my aluminum storage shed they used self tapping metal screws to thread into the metal and then into the wood frame.they seem to pull out of the wood over a period of time.is it good to use metal screws in wood.is there a differance in the threads?thanks jvl
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Self drilling will leave too big a hole in wood to hold well.
Self Tapping Sheetmetal screws have fairly coarse thread and will hold well.
The self drilling that are the WORST for wood have two "wings" after the drill point. Try to avoid those.
Regular drill point screws are marginally better.
Ideally, just a sharp metal piercing point is all that is required in soft sheet stock like alum or copper or thin galvanized.
Spheramid Enterprises Architectural Woodworks
"Success is not spontaneous combustion, you have to set yourself on Fire"
thanks for the reply,i figured that.i used a flat headed phillips head 8-32 2" nuts and bolts.i needed a fairly large head in order to cover up the prexisting hole.first drilled thru the 2x2 rafters ,inserted the bolt and put thru the roof caulk under the head of the bolt and inserted it in.on the inside part of the bolt used loktight with a washer and a lock washer and tighten down.do they make a flat headed wood screw,couldnt find any,jvl
When you say pulling out, do you mean that they are not holding the metal down tight to the wood anymore - but are still secure into the wood?
If so, the problem isn't with the screws "letting go", the problem is that the rafter wood has shrunk as it has dried. Some boards might lose 1/4". Since the screw doesn't shrink, it starts standing taller than where it was when it was first driven.
Rebuilding my home in Cypress, CA
Also a CRX fanatic!
If your hair looks funny, it's because God likes to scratch his nuts. You nut, you.
they are hex head sheet metal screws 1/2" #8 32s.overtime wind can vibrate the roof pulling out the screws causing mouisture to penetrate into the unit.these screws are backing out.talked to the manufacturer and he said that even his unit was doing the same thing caused by the wind.suggested to rescrew it but anyone would know that the screw would not hold that well.they need to predrill and use glue to prevent backing out.the wood looked to me that it was kiln dried but over time moisture would penetrate and there would be expansion and contractment jvl
There are a book of answers; but in this case, I'm guessing that your problem is that these sheet-metal screws are threaded right to the head , while a "wood" screw would have a smooth shoulder to circumvent bridging.
As another poster mentioned, wood changes dimensions in some proportion to its moister content, wood screws have this smooth shoulder so that the screw is compressed through the top, or attached piece, to the adjacent piece, which receives the threads.
The classic fix is to use wood screws with cup washers, but I'm sure we've all used pan-head or truss-head screws, sometimes with a compressible washer (simple lock washers, with or without a rubber washer to seal it).
The key is the pilot-holes through the roofing surface have to be slightly over-sized to allow for wood movement below AND thermal expansion and contraction of your roof surface.
Phill Giles
The Unionville Woodwright
interesting thanks jvl