View Full Version : screw length
08-30-2002, 05:50 PM
Is there a rule of thumb for screw length?
There seems to be a lot of info about predrilling, lubrication and different materials, which is all very helpful. However, how far should the fastener penitrate into the board your fastening to. For example, a 3/8th plank to an oak frame?
08-30-2002, 05:58 PM
A general rule is that the screw should be twice as long as the thickness of the board being fastened. To fasten a 3/8" plank to a frame, the screw would be 3/4". This is if the frame is hardwood, e.g., oak. For fastening into softwood, add a 1/4" to 1/2" to this rule.
[ 08-30-2002, 05:59 PM: Message edited by: Bayboat ]
Don't you have to account for countersinking the screw below the surface of the plank? If you countersink 1/4" does the screw have to be 1"?
08-30-2002, 06:07 PM
Theoretically, you can subtract the depth of the countersink or counterbore from the length of the screw. In practise, I don't cut it that fine unless the frame or board being fastened to is on the thin side. In that case you want to take care that the tip of the screw doesn't emerge. A penetration of the screw's head by 1/4" would be a counterbore, with vertical sides that will accommodate a bung. A countersink is when the hole is cone-shaped with the screw head left at or very near the surface of the board.
[ 08-30-2002, 06:13 PM: Message edited by: Bayboat ]
It's all very confusing. I got even more befuddled when I recently sorted 20# of screws from squaredrive. There were woodscrews with different threads, 3" screws with the first inch below the head having no threads. Fat screws and thin screws.
It seems that if it's correct to double the plank thickness with the length of the screw, countersinking or boring(sorry about screwing up the terminology) wouldn't change the formula. The plank is still 3/8. The stresses working against the screw haven't changed.
When I've used woodscrews in non-boatbuilding applications, I've always based the screwlength on the stresses working on the item being fastened down...all the way from a very short screw for items that have very little stress against them, up to the maximum length possible based on the depth of the member being fastened to.
08-30-2002, 09:06 PM
Um, I thought there should be twice the thickness of the plank being fastened into the timber, i.e. if the plank is 3/8" the screw should be 1-1/8" plus or minus as appropriate. The bottom 3/8" of a 3/4" screw doesn't leave much grip. I have no idea where I got that ratio years ago and I've not tested alternatives so I'm in no position to argue. However,....
08-30-2002, 09:28 PM
Thanks for all the reply's. This particular issue seems to be underaddressed in the few books I've read. Your guidence is helpful. I hope to put it to use this fall.
08-30-2002, 09:55 PM
Sonja's frames are 1" x 1" with planks no more than 5/8". I just figured that I wanted as much thread in the frame as I could and still have the head gripping as much plank as it could, so I went 1 1/4" on the screws. I made sure all the counter bores were 1/4" deep. This left about 1/8" of frame left before I messed up and drilled it through. No professional wisdom here, just a "I think this is the way they do it" thought.
08-30-2002, 10:42 PM
From Robert Steward's Boatbuilding Manual, Ch. 6, 3/8 inch plank should be fastened with 3/4" No.7 screws. 5/8 inch plank, with 1 1/4" No. 9. With the latter, the procedure is to counterbore to 1/2 the plank thickness, and use 3/8" plugs over.
08-31-2002, 10:20 AM
Thanks for the info, I swear I read that manual, musta missed it.
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