proper woodscrew pilot hole size

[jim gossage]

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?

[Kevin Jenness]

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

[Cameron Wood]

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

[Ron Selzer]

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

[Tom M King]

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/B018JQFFMY/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&th=1

https://www.amazon.com/4571820801005-Washer-Exterior-Powerlag-Pieces/dp/B00DSSLIBG/ref=sr_1_5?crid=32R66KEHYVDTS&keywords=spax%2Bhex%2Bhead%2Blag%2Bscrews%2B5%2F15 %2B3-1%2F2&qid=1673224840&sprefix=spax%2B5-1%2F2%2Bhex%2Bhead%2Caps%2C74&sr=8-5&th=1

[Lee Schierer]

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. (https://www.jwlumber.com/wp-content/uploads/2020/10/timberlok-spec-sheet.pdf)

[Paul F Franklin]

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.

[Cameron Wood]

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

[Ron Selzer]

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.

[John K Jordan]

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



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?

[Lee Schierer]

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.

[Tom M King]

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.

[jim gossage]

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?

493099

[Rob Sack]

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.

[Jerry Bruette]

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.

[Tom M King]

https://www.amazon.com/AIRAJ-Drywall-Wallboard-Bimetal-Sheetrock/dp/B0BJDWWDYL/ref=sr_1_2_sspa?crid=1D6ILQ9FOPEZ4&keywords=sheetrock+saw&qid=1673287450&sprefix=sheetrock+saw%2Caps%2C99&sr=8-2-spons&psc=1&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUEzVUVPWFVaRTNHUERRJ mVuY3J5cHRlZElkPUEwNzY3OTMzMTBEQ09GSUhZQVdKVCZlbmN yeXB0ZWRBZElkPUEwMTE0MTI5Mk0yVDU2WDNWRFoxNyZ3aWRnZ XROYW1lPXNwX2F0ZiZhY3Rpb249Y2xpY2tSZWRpcmVjdCZkb05 vdExvZ0NsaWNrPXRydWU=

[Jim Becker]

What is "superstrut"?

Unistrut; may be a brand name. I know my local HD carries it.

[jim gossage]

Like Jim said, it's another brand name of strut channel and appears nearly identical to unistrut in all dimensions. Re another query above, this stuff looks pretty sturdy and according to their engineering tables can support 1000 pounds over a 36" span with minimal deflection. They don't really tout it as trolley channel for a hoist but they sell trolleys and brackets for it that are rated at 1000 pounds and there are several youtube videos of people installing substantial trolley hoists in their ceilings, but none in two axes. Might have to make a video...or maybe it will be an epic fail!

[Jerry Bruette]

Go for it if you feel safe. I'd try to contact the company and float your plan past them, see if they endorse it.

You might need two trolleys on each rail to keep the traveling rail from racking and binding.

Here's a link to a store bought system similar to your idea. Check the weight rating and how it's built. https://www.globalindustrial.com/p/hubbell-heavy-duty-overhead-tool-crane-kit-6ft-250-lb-capacity
It's similar but different.

[jim gossage]

Go for it if you feel safe. I'd try to contact the company and float your plan past them, see if they endorse it.

You might need two trolleys on each rail to keep the traveling rail from racking and binding.

Here's a link to a store bought system similar to your idea. Check the weight rating and how it's built. https://www.globalindustrial.com/p/hubbell-heavy-duty-overhead-tool-crane-kit-6ft-250-lb-capacity
It's similar but different.

Jerry, Thanks for the two trolley idea and the link. I can easily retrofit my system to two trolleys per rail if I get racking with the one trolley version.

[Bill Dufour]

I have to wonder how much drilling screw of any kind into a 2x beam derates the load capacity. Screwing into the thin web seems like it is a more serious problem then into the side.
Bill D

[Ronald Blue]

You do you but.... Here is a link for trolleys from McMaster Carr. The maximum weight rating is 300 lbs for a trolley. In my experience they are intended to support lines, cords, or tool balancers but not be used for overhead lifting. Looking at the Unistrut site they mention that strut hangers must be used to keep the track clear. Looking at their imagery it appears there isn't sufficient clearance for a bolt head to be in place. I'd proceed carefully. The problem in this sort of thing is when you exceed the physical strength of the structure the failure is catastrophic. Good luck with this.


https://www.mcmaster.com/trolleys/strut-channel-trolleys-5/

https://unistrutstore.com/blog/post/unistrut-trolley

[jim gossage]

Yes, I fully plan to use hangers, 5 per channel, each of which has a 1200 pound rating. It will be uncommon that I might lift anything more than 200 pounds - The heaviest tool in my shop is my grizzly planer at 600 pounds. I just want to be prepared in case. Except for the ceiling joists, I feel like I have overengineered everything. If load testing of 4-500 pounds shows too much deflection of the joists, I can always add some braces above.

[Ron Selzer]

Jim, In 2014 I fastened 5- 10' 1 5/8" Struts parrell to garage door in one bay of my two-car garage, this made them perpendicular to the roof trusses. Secured them with the Spax screws I recommended previously to you. Then fastened a 10' stick of 3 1/4" strut perpendicular to the 1 5/8" strut, could not get the outside hangers locally at that time and trolleys could not pass the 3/8" bolt heads when 1 5/8" strut was used. two trolleys support a 30" long piece of 1 5/8" strut to which a Harbor Freight electric hoist is bolted to. Initially installed for unloading my SawStop ICS, gets used every year to hoist up front of zero turn to remove blades and clean under side of the deck. Has been used to unload other tools as they have shown up. Keep thinking I need to add 3 more struts hanging from trolleys in the 1 5/8" struts allowing side to side movement. Definitely would be nicer for unloading the truck just never spent the money to do it yet. Both trolleys used are the 600 lb rated
Ron

[jim gossage]

Hi Ron,
Your set up sounds interesting. Could you include a photo? What is the heaviest tool you have lifted?

[Ron Selzer]

pm sent
500+- lbs off the top of my head
Ron

[jim gossage]

FYI, I made a little you tube video testing Spax vs Lag screw shear strength (I'm not an engineer) to satisfy myself. Spoiler alert - which will likely surprise almost no one on this site - Spax won handily. Link below. Thanks again for all your help. I'll post some pics when the trolley is finished.

https://www.youtube.com/watch?v=Ek4O9If9t10

[Tom Bender]

The traveling beam is called the bridge in a bridge crane. As Jerry suggests you will have a racking problem.

I have never seen a bridge crane with a middle rail, it may just increase the racking problem and will require all three rails to be perfectly level. Best to delete it from your plan. The only result is the need to make the bridge stronger.

Since the load can travel to all corners the fasteners must be able to carry the max load at all points.

Safety factor for this should be 5. That means design or testing should be 5,000 pounds plus the weight of the rails, hoist etc. Why so much you ask? Part of it is shock loading. Imagine how much force it would take to drive a nail with just a static load, without the shock of impact. Same for driven screws and sheared bolts.

End stops need special attention. They sometimes fail, In industry they are attached by chains to catch them if they are sheared off, so they don't fall on your head. And there are double stops so the first failure isn't fatal.

[jim gossage]

The traveling beam is called the bridge in a bridge crane. As Jerry suggests you will have a racking problem.

I have never seen a bridge crane with a middle rail, it may just increase the racking problem and will require all three rails to be perfectly level. Best to delete it from your plan. The only result is the need to make the bridge stronger.

Since the load can travel to all corners the fasteners must be able to carry the max load at all points.

Safety factor for this should be 5. That means design or testing should be 5,000 pounds plus the weight of the rails, hoist etc. Why so much you ask? Part of it is shock loading. Imagine how much force it would take to drive a nail with just a static load, without the shock of impact. Same for driven screws and sheared bolts.

End stops need special attention. They sometimes fail, In industry they are attached by chains to catch them if they are sheared off, so they don't fall on your head. And there are double stops so the first failure isn't fatal.

Hi Tom,
Thanks for your thoughtful comments. I just completed the system and as predicted racking is present, but tolerable with careful movement. If I remove the center rail I'm not sure my "bridge" would adequately support a 8' span but something to look into. I'm not sure exactly how much I'll use the system, but if racking becomes untenable, I have already imagined some 2 trolley attachments to each rail that should improve racking. Not much to do about the safety factor of 5, just have to hope OSHA doesn't make a surprise visit! Thanks for the pointer about the end stops - I'll have to consider some sturdier options.

[Tom Bender]

Hi Jim
You can make good end stops by drilling cross holes and putting in bold. These would not need safety chains.

The center rail should not cause racking so if it moves well enough as is then it's good.

Racking will probably be worst with the load at one end of the bridge. A pole with a hook and a helper might ease the travel.

As to failure, you could load test it at 1000 pounds with the load at each corner of the range (while staying out from under). Then divide 1000 by whatever safety factor you choose and use that as your system capacity.

[jim gossage]

Hi Jim
You can make good end stops by drilling cross holes and putting in bold. These would not need safety chains.

The center rail should not cause racking so if it moves well enough as is then it's good.

Racking will probably be worst with the load at one end of the bridge. A pole with a hook and a helper might ease the travel.

As to failure, you could load test it at 1000 pounds with the load at each corner of the range (while staying out from under). Then divide 1000 by whatever safety factor you choose and use that as your system capacity.

Thanks Tom. I unhooked the trolley on the center channel and racking was not better. I am leaning towards improving the racking by replacing the 3 bridge trolleys with 3 angle irons each with 2 trolleys in line with the ceiling channels. Any recommendation how far to space the trolleys along the angle iron to minimize racking? Seems like the farther apart the better for racking, but the downside will be loss of travel distance for the hoist. would 12" be enough?

[Tom Bender]

It's going to depend on the trolleys. How well do they travel when there is a side load on them? Industrial trolleys sometimes have side guide rollers and run on patented rails. The trolleys you are using are best used for a monorail crane. You have made an underhung bridge crane.

Also, they will handle differently when loaded.