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Thread: proper woodscrew pilot hole size

  1. #1
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    proper woodscrew pilot hole size

    I am mounting a 2-axis rail system in the ceiling of my shop to hold a half ton hoist and want to properly size the mounting screws for maximum holding power. I will use 5/16" x 4" hex head lag screws into 2x8 rafters. I went on the internet to find the proper pilot hole size for soft wood and the first hit was 9/64", nearly 3/16" smaller than the screw size. It split the (test) wood. I went back to the internet and found pilot hole sizes for soft wood ranging from 9/64" to 15/64". Really? What do the sages recommend to optimize holding power without splitting in a 2x8?

  2. #2
    With lags I usually drill the lag shank size, You could experiment with slightly smaller .

  3. #3
    I would drill the first inch at 5/16" since that size usually has an unthreaded shank, then 7/32", and wax the screws.

  4. #4
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    Forget lag screws and pilot holes, USE SPAX® 5/16" x 4-1/2" T-Star Drive HCR Exterior Coated Washer Head Lag Screw
    no pilot holes, don't twist in half if hole is too small, hold better, built in washer head. Just flat out better
    Ron

  5. #5
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    Reading through the thread, I was getting ready to post the exact same thing that Ron did. Old style Lag screws have been absolutely awful quality for the last 20 years or so. I quit using them for anything.

    edited to add: I keep boxes of these in various lengths to have for all sorts of different uses on the farm here. I've never wrung the head off of one. I'd probably use some of both of these.

    https://www.amazon.com/gp/product/B0...e?ie=UTF8&th=1

    https://www.amazon.com/4571820801005...74&sr=8-5&th=1
    Last edited by Tom M King; 01-08-2023 at 7:48 PM.

  6. #6
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    Quote Originally Posted by jim gossage View Post
    I am mounting a 2-axis rail system in the ceiling of my shop to hold a half ton hoist and want to properly size the mounting screws for maximum holding power. I will use 5/16" x 4" hex head lag screws into 2x8 rafters. I went on the internet to find the proper pilot hole size for soft wood and the first hit was 9/64", nearly 3/16" smaller than the screw size. It split the (test) wood. I went back to the internet and found pilot hole sizes for soft wood ranging from 9/64" to 15/64". Really? What do the sages recommend to optimize holding power without splitting in a 2x8?
    Forget lag screws, I've seen far too many that fractured at the transition from from threads to shank to ever trust them for holding a load. Use Tmberlok screws or other high strength screws. They don't require pilot holes and are much easier to use than lag screws. You can view the Lateral Design values for Single Shear Connections as compared to nails and lag screws here.
    Last edited by Lee Schierer; 01-08-2023 at 8:15 PM.
    Lee Schierer
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  7. #7
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    I agree with other recommendations to avoid lag screws. In addition, if it's at all possible, I would find a way to attach the rails so the fasteners are loaded in shear rather than tension. In other words, fasten into the sides of the supporting structure rather than in the bottom.
    --I had my patience tested. I'm negative--

  8. #8
    I agree, (although I still drill pilot holes) but the lags may already have been purchased.

  9. #9
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    Quote Originally Posted by jim gossage View Post
    I am mounting a 2-axis rail system in the ceiling of my shop to hold a half ton hoist and want to properly size the mounting screws for maximum holding power. I will use 5/16" x 4" hex head lag screws into 2x8 rafters. I went on the internet to find the proper pilot hole size for soft wood and the first hit was 9/64", nearly 3/16" smaller than the screw size. It split the (test) wood. I went back to the internet and found pilot hole sizes for soft wood ranging from 9/64" to 15/64". Really? What do the sages recommend to optimize holding power without splitting in a 2x8?

    IF the 2x8 rafters are dried out and brittle THEN definitely consider a bracket mounted over halfway up the 2x8, bolted to the sides of the 2x8, with a washer on the other side, with an all thread or long bolt down to the hoist frame
    you don't want to split the rafter if it is dried out from the heat and brittle. Timberloks work good, I have used them in the past and like them. MAKE SURE the Timberlok head doesn't interfere with the trolley system you are using.
    Last edited by Ron Selzer; 01-08-2023 at 9:30 PM.

  10. #10
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    I use a lot of lag screws around the farm and haven’t split wood yet. I consider several factors when drilling pilot holes including of wood, dryness, density, and orientation of the rings on the board. I drill a larger pilot hole for dense, dry wood or if the board is plain sawn with the rings patallel to the screw - if perpendicular (quarter sawn) the wood is more resistant to splitting. Experience is helpful here but if unsure I do some tests like you mentioned.

    If the timbers are hidden above a ceiling it might be hard to evaluate the wood so I would drill to clear the minor diameter - I usually just hold a drill bit up and sight for sizing but the threads are coarse enough on lag screws to measure the minor diameter with calipers.

    Are you actually fastening into the rafters (the angled boards holding up the roof sheathing) or into ceiling joists (the horizontal boards above a horizontal ceiling)? Are the boards exposed or covered with sheet rock or something? If attaching to angled rafters that are not covered I might consider adding some wedges to allow the screws to be vertical. In that case or if fastening into ceiling joists through sheet rock or something I’d probably consider using longer lag screws in 2x8s, maybe 6” instead of 4”.

    One thing you can do to help the threads on the lag screws to cut more cleanly into the wood and decrease the chance of splitting is to make a “tap” to cut threads at least part way into the pilot holes. I do this often with all kinds of screws into wood and some other materials. I make the “tap” from one lag screw - cut some grooves across the threads with a metal-cutting disk on a Dremel or for small screws, with a triangular file. I might cut one to four grooves depending on the size of the fastener and the material. With either the file or the cutting disk I angle the groove so the cutting (trailing) edge is more or less vertical. Just drill the hole and drive in and remove the modified screw.

    Just as with driving all screws, I prefer to use hand power so I can feel how much force it’s taking. This lets me judge better if I need to do something different.

    To me a 1/2 ton load doesn’t sound like too much for a sturdy rail fastened to multiple timbers but to be sure you might want to consult with an engineer to make sure you don’t need to add reinforcement. I had a positive experience with one recommended by an architect friend when sizing a roof support beam.

    JKJ


    Quote Originally Posted by jim gossage View Post
    I am mounting a 2-axis rail system in the ceiling of my shop to hold a half ton hoist and want to properly size the mounting screws for maximum holding power. I will use 5/16" x 4" hex head lag screws into 2x8 rafters. I went on the internet to find the proper pilot hole size for soft wood and the first hit was 9/64", nearly 3/16" smaller than the screw size. It split the (test) wood. I went back to the internet and found pilot hole sizes for soft wood ranging from 9/64" to 15/64". Really? What do the sages recommend to optimize holding power without splitting in a 2x8?

  11. #11
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    Another consideration is whether the span of your 2 x 8 ceiling joist can handle the added weight from your hoist and the potential 1,000 pound load. Even though you will have a 2 axis rail system, working the hoist from one end will still apply more load to the closest supporting fastener into the joist.
    Lee Schierer
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    My advice, comments and suggestions are free, but it costs money to run the site. If you found something of value here please give a little something back by becoming a contributor! Please Contribute

  12. #12
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    Thinking about this more, I'd have some plates welded to the top rails to through bolt them through the sides of the joists, even if you still use lag screws straight up. You can buy precut pieces of steel plate all sizes and thicknesses off of ebay for not much money. Predrill the bolt holes, and have them welded where they should be. I would offer to do it if I was closer. It's an easy job for someone set up to do it.

  13. #13
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    That's a lot of ideas and a better response than I was expecting. I apologize for not giving more details on the project. I have included a drawing (not to scale and not a nice CAD) for better explanation. Three parallel superstrut channels will be held into the ceiling by 6 heavy duty channel hanger brackets. There is sheet rock between the hangers and the 2x8 fifteen foot ceiling joists, so no option for shear mounting or plates. It is the hangers that need to be screwed into the joists. A 4th superstrut will ride on 3 trolleys perpendicular to the 1st 3 channels, and the hoist will ride within the 4th channel, providing 2 axes of travel. I have done due diligence on load bearing for all items and believe the system is sound for at least 1000 pounds, especially as load will be somewhat distributed over multiple channels, hangers, and joists (but I have no engineering experience and so could be way off). Once up, I plan to do some load tests to make sure that the joists do not sag too much under load. I need to use hexhead screws to mount the channel hanger brackets so as to allow minor adjustments after the channels are mounted. I have personally never had a hexhead lag screw of 1/4" diameter or greater fail in soft wood, though have had small screws fail in hardwood (including spax). I am currently leaning towards drilling a pilot hole 1/64" less than the minor diameter of the lag screw, though I will check out the hexhead spax. Is it the general opinion that the spax are stronger than similar size lag screws?

    trolley.pdf

  14. #14
    I agree lag screws or lag bolts are not the way to go. I would use a rated structural screw like the Simpson SDS type screw, which is rated for construction. Simpson has a whole line of rated structural fasteners, as do other manufacturers. I don't know if the SPAX screw mentioned above is a rated screw.

  15. #15
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    What is "superstrut"? If it's what I think it is, is it rated for a static load or a moving load?

    Does the manufacturer of the strut and trolleys have mounting requirements for their products?

    I'm not an engineer, but I have plenty of experience working with and mounting overhead lifting equipment. If the manufacturer of the equipment you plan on using doesn't have ratings for overhead load limits and mounting requirements I wouldn't use them.

    You could use shear mounting or plates. It would require a hole in the sheetrock to allow a graded threaded rod to pass through. Simple enough to do with some measuring and a drill.

    The problem with regular lag screws/bolts are how soft they are and have no tensile rating. I've had them twist off while driving them in.
    Last edited by Jerry Bruette; 01-09-2023 at 12:06 PM. Reason: added info
    Confidence: The feeling you experience before you fully understand the situation

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