OT: Construction -- new hard metal screws or old fashioned lag screws?

[henrya]

I'm putting in some posts to support a 26' X 10' shed roof dormer style porch cover. The posts are 6 X 6 cedar and I have some nice welded up bases that will bolt to studs epoxied into the concrete slab. The top will get metal angles screwed on with the same screws. My original thinking was to use 1/2" X 3" stainless lag screws with washers. Then I saw some Simpson products that use the popular hard metal wood screws and a decorative nut/washer, shown here: https://www.strongtie.com/avantcollection_outdooraccents/category

Those look nice! But Holding one of those screws in one hand and a 1/2' lag in the other makes me wonder. They're near 1/4' and seem somehow insubstantial. I've been using similar screws for years now as substitutes for lags and like them. But this is a pretty important connection and I have doubts.

I'd love to hear of your direct experience and informed opinions about one fastener versus the other.

Thanks in advance.

 

[EPAIII]

I am not a structural engineer, so just asking. Is a hardened fastener a good idea in a structural connection where vibration and temperature stress cycles over the years will probably be present. Hardened fasteners are usually used in places where such factors are at least controlled to some extent.

I would think you would want something that has some give.

 

[Mr.Smith]

Lag screws are an engineered fastener. The "holding" force for a specific set of conditions can be calculated. If the same can be applied to the product you are considering you can make a valid comparison. If not, you have your answer.

An example of available data for lag screw connections. https://awc.org/wp-content/uploads/2021/12/AWC-DA1-LagScrew-0702.pdf

I can't quickly find it but I used a similar, but simpler data sheet when designing lift point attachments in my shop.

Thank you,
Mr.Smith

 

[Ries]

I agree, you should check manufacturers specs to see if the smaller construction screw is equal to a 1/2" lag. I have used both, and the new screws are great for lighter use stuff, but I still go back to old fashioned lag screws pretty often. SS is a good idea, I have probably snapped a few dozen plain steel lag screws in my time installing.

 

[gbent]

A high strength fastener in wood is a large mismatch in strength. A small corrosion pit reduces a lot of strength in the small fasteners, and outdoor wood can be corrosive. When it has to hold a grade 2 through bolt is hard to beat.

 

[GregSY]

I dislike old fashioned lags for just about anything. Their root diameter is too large, their major dia is too small.. In other words, the threads are too shallow.


What do I like? Spax Powerlag screws. They have a deep threads that is also well formed...unlike lags which are a crummy thread.

HEX HEAD HCR® POWERLAGS®

For use in heavy-duty exterior fastening applications.

spax.us

 

[henrya]

To clarify a bit. What I’m calling hardened screws are the sorta new things like Timberlocks, Headlocks and all the others (like Simpson’s many fasteners) that have shown up in the last decade or two. Maybe heat treated is a better term. Also they differ in having smaller roots and larger major diameters than standard lags.

The page that Mr. Smith sent is helpful! Now to see of I can find same for the screws I am considering.

Those Spax screws GregSY linked to might be a good call.

And — the only real way these will get tested is in a tornado. So we’re talking uplift forces with the screws in shear. Considering its already survived a 60 mph storm with only 2x4 temps poked under it and not fastened at all, I’m not too worried. But I like to know real answers instead of bs.

Thanks and keep it coming if you have more to say.

 

[redlee]

I hate Lags, as others have stated why, they also shear easily. Ive never sheared one of these.

GRK Fasteners | RSS Rugged Structural Screw

GRK Fasteners

www.grkfasteners.com

Structural rated , bit pricey but well worth it.

 

[Rob F.]

I think you are talking about the SDW screws that simpson markets. The simpson catalog should give you some info on how many to replace a lag or through bolt. If it comes from simpson it is sure to have engineering and testing behind it.
It is after all being sold by them to hold buildings together.
Their SDS screws are are more common but are meant to hold steel components to wood. The SDW screws with large head are for holding wood to wood.

 

[SteelSomethin]

Simpson has been the go-to hardware choice for construction for years. Decades even. But I've never used those skinny lag shaped screws to hold anything substantial.

When attaching raised decks to 6x6 posts we used to notch the posts (to set the framing ON the posts, AND use through 1/2"-13 bolts to secure the porch frame to the posts. 1/2" all thread is also OK. It's the building code.

To find your answer I'd look into the building codes. There's a TON of engineering/math involved with calculating how to hold a roof onto a frame.

I'd use proven technology. Period. But I'm 99% confident those skinny Simpson lags would be fine too.

 

[mjr6550]

I have used many Simpson and similar structural screws. They have good testing programs and make good products. One main advantage is that you don't need a pilot hole.
Lag screw ratings are based on calculated strength, not testing.
In general, it is best to rely on wood to wood support and not just the bolt strength. With concentrated loads at columns screws alone often don't have the capacity to support design loads.