Replacement Basement Stairs

[Rwh56]

After wobbling on the lower flight of basement stairs, I replaced the stringers and treads in our small 1920s dutch colonial. For the stringer, I used a 7 foot length of treated 2 x 10. (We have had some minor flooding.) I copied the original design by using blocks attached to the inside of straight sided stringers to support the treads. I attached 10 inch lengths of 2 x 4s with 2.5 inch decking screws (suitable for treated wood). Six screws for each 2 x 4. Deck screws are very convenient as they screw in easily with a drill. After I completed the job, I was removing a real wood screw from some scrap and was reminded how stout a wood screw is compared to a slender deck screw. I thought that deck screws are meant to keep deck boards down as opposed to supporting something vertically.

Did I "screw" up by using deck screws instead of wood screws to attach the tread supporting blocks?

[Nestor]

Yeah, it woulda been better if you'd used wood screws or long sheet metal screws and predrilled for each screw.

Screws that are meant to be driven into wood without predrilling (like drywall screws) are called "low root" screws because the minor diameter of the thread is much smaller than a regular wood screw. And, that means that the screw has very much less "shear" strength, which is the way you're using the steel in those screws for support. (Remember that cross sectional area increases as the SQUARE of the screw diameter, and so too would the shear strength of the screw. So, if you halve the minor diameter of the screw, you quarter it's shear strength. approximately)

Presuming that deck screws are made from the same strength steel as wood screws, what matters is the total cross sectional area of steel that is supporting each step. So, you could put in more screws to compensate for the fact that each screw has a smaller cross sectional area.

If it wuz me, I'd just take each screw out, pre-drill and drive in a wood screw or long sheet metal screw. With a reversible variable speed drill, it wouldn't take long.

And, not to put a geo-political spin on this post, if it wuz me, I wouldn't use anything but ROBERTSON drive screws cuz they pi$$ me off less.

[HayZee518]

screws

it really doesn't matter because the screws are holding down the treads. there is NO shear force involved. you use deck screws to hold down the flooring of a deck - again no shear forces involved. the weight is supported by the joists. now the ends of the joists do exhibit shear forces as they are trying to hold a horizontal member to the fascia board all around the deck.

[Nestor]

Hayzee:

You musta missed the part which read:

"Did I "screw" up by using deck screws instead of wood screws to attach the tread supporting blocks?"

Deck screws are being used to attach 10 inch long pieces of 2X4 to the stringers. One end of each tread rests on each 10 inch long 2X4 "supporting block". So, yes indeed, there is a shear force on those deck screws.

If we were talking about the fastener(s) holding the treads down, that wouldn't matter. You could almost use double sided tape for that.

Still, 6 deck screws on each supporting block for a total of 12 deck screws holding up each tread should be enough to support the weight of a full grown adult. But, if it was me, I'd still use my reversible variable speed drill to replace the deck screws with wood screws. That way each tread could support much more weight, like two full grown men hoisting something really heavy up the stairs like an old washing machine on a hand truck, for example.

For the half day it's gonna take you, I'd replace the deck screws with wood screws. That way you don't have to worry about the treads being sufficiently well supported next time you have to move something really heavy up or down those stairs.

"The best way to plan for an uncertain tomorrow is to do the best job you can today."
- I dunno who said that, but it's more true than false.

[HayZee518]

stairs

no, I didn't miss it. Matter of fact I have the exact thing on my partial basement. normally a stringer is cut out to support the tread. my span is only 24 inches stringer to stringer. I use a 2x6 for each step tread. I weight about 240 lbs and it supports me!

[Nestor]

And both Hayzee and I agree that if you used 2 1/2 inch deck screws, you should probably use 2 1/2 inch flat head sheet metal screws to replace them. Check to see if 2 3/4 inch are available. I expect not.

The only difference between a wood screw and a sheet metal screw is that a sheet metal screw will be threaded over it's entire length, whereas a wood screw will only be threaded for the first 2/3rds of it's length.

And, of course, the only thing better than a Robertson drive screw is a Torx drive screw, so don't kick yourself in the butt by buying Phillips or slot drive.

(We Canadians get pi$$ed off because all of the Chinese-made stuff we get up here is assembled with Phillips drive screws because most of what China produces is destined for the American market where Robertson drive simply isn't popular. But, I've had the opportunity to drive every kind of screw you can think of, and Robertson is simply a better drive than either Phillips or slot.)

[pushkins]

It is very common place that exterior screws like your mentioning are used for purposes just like you did. An exterior screw by nature of what it's designed and used for does have torsional strength and are code compliant for use in almost all facets of building. In other words you could use exterior rated screws in replacement of nails in building a house (this does not include drywall screws).

Torx heads are about the only screw drive we use now.

If I had a $ for every time I've seen drywall screws used wrongly

[Nestor]

And now for something completely different:

This web page says that the root diameters of a standard #8 and #10 wood screw is 0.112 and 0.130 inches, respectively.

Screw Dimensions

This company that makes deck screws says that the root diameter of their #8 and #10 deck screws are from 0.116 to 0.123 and from 0.152 to 0.158 inches, respectively.

American Fastener - Deck, drywall, and particle board screws and fasteners

So, what we have is the contradictory situation where deck screws that are meant to be driven without predrilling have a larger root diameter than standard wood screws that normally require predrilling. Go figure. That's bass-ackwards.

Rwh56:
What you should do is measure the root diameter of one of your 2 1/2 inch deck screws to see what it actually is. You may in fact have as large or even a larger root diameter on your deck screws than you'd get with conventional wood screws. And in that case, replacing those deck screws with conventional wood screws would be a step in the wrong direction.

[pushkins]

Simple reason is torsional strength !

Pre drilling a hole basically removes a section of the wood material so if it was drilled to the correct size of the screw you only have to drive the thread into the surrounding wood, with self drilling screws your basically forcing the entire body (thread and body) to wedge itself into the wood doing this with a drill puts an enormous stress on the screw shaft.
Drywall screws are designed to not be driven flush with lumber surfaces and if the right size is used in relation to drywall thickness your only driving them in about an 1" (even then they can be a pain in the butt driving into old hardwood studs/joists).

[Nestor]

Well, I can't disagree with anything you said.

But I disagree with your conclusion, which means I disagree with your logic.

You seem to be saying that it stands to reason that a deck screw would need a larger root diameter because it needs to be stronger in order to be driven directly into wood without predrilling. Otherwise it might break cuz of the greater friction and torsion forces involved.

The problem is that it's not necessary to make a screw bigger to make it stronger.

Where I live I can buy grade 2 bolts, grade 5 bolts and grade 8 bolts, each of which is progressively stronger. That's because the steel is stronger, not cuz of any difference in the size of the bolts.

So, a much better design for a deck screw would be to use a stronger steel than a wood screw, not more steel. That way your deck screws wouldn't be more likely to split the wood.

I think the explanation for the apparantly contradictory dimensions in my last post is that wood screws have standard dimensions whereas deck and drywall screws don't. I know I've bought drywall screws that call themselves a #6 or a #8, but their root diameters and threads per inch were completely different. So, I expect every deck and drywall screw manufacturer makes deck and drywall screws their own way, and they subsequently classifying them according to the closest wood screw diameter rather than making them to size to begin with. So, what we might have here is a company making #9-ish and #11-ish deck screws and calling them #8 and #10, respectively. That would explain why a #8 or #10 deck screw has a larger root than a #8 or #10 wood screw.

Besides, your explanation completely forgets the fact that the reason for using a smaller root diameter on drywall screws is to spread the wood less (thereby reducing the likelihood of splitting the wood) and to reduce the force needed to drive the screw through the wood. Why would deck screws need larger root diameters when drywall screws work better with smaller root diameters. I really don't see any reason why the depth the screw is driven into the wood would make any difference at all in the physics or mechanics of the situation. What works best for one should also work best for the other.

[pushkins]

Ya know, every time someone with years of in the field experience posts something you don't like, you end up in a dissertation on why you are right. I think that fastening companies have something that you don't have, an R&D department, in many cases particular fasteners have specific code compliant requirements and this includes nails as well as screws.
Drywall screws have almost zero lateral strength evident by hitting one driven into wood on the side, they break off just the same if you try to drive in a long drywall screw into old hardwood, it will twist and break.
You are correct in the fact that they could make the screws out of different metals to increase strength and they do, but they also balance out the cost of production by tweaking diameter with metal used to get the specs. they require.
To your comment that one is as good as the other>>> code technically allows every nail in framing a house to be substituted with an exterior grade size appropriate screw, in fact the newest method of rafter tie down is with just such a screw driven without drilling up through the top plates and into the rafter/truss. Code will not allow drywall screws.

In the reality of the OP a size appropriate wood screw used with pre drilling on tread blocks is sufficient, however the use of treated lumber (ACQ or ACC) mandates exterior grade screws approved for use with treated lumber. Once in contact with ACQ or ACC wood screws degrade rapidly.

[HayZee518]

engineered stairs

the OP wanted an opinion of what is best. He did not request a complete history of screw application, stresses on pull out strength or shear forces involved. the manufacturer of screws has years of experience in his particular field. you will never find something that will match your application to a "T" there will always be some areas of doubt. You, Nestor, may have years of schooling, more than us "simple folk" - we do a job with what means we have. I'm sure no inspector will even question what we use to do a task.
Keep your answers simple and to the point. I have electrical knowledge and experience in doing my job. I won't go into theory or basics of electron movement through a wire or "skin effect" of high frequency energy along a conductor. Besides, no one would have any idea of what I was speaking about. Keep it simple.

[Nestor]

You two both make good points. But you're both missing a more important point; and that is that I did what had to be done.

You've got an expert in here saying that deck screws needed to have a larger root than wood screws because they need to be stronger. And that makes perfect sense at first glance, so the newbies in here are gonna believe it.

Rather than sit quiet and let them believe it, I felt I should disagree and explain my reasons so they could understand that reasoning and decide for themselves if it the "torsional strength" explanation made sense.

If I didn't disagree, but sat quiet in stead, then lots of people would now have a misconception in their heads. And my preventing that from happening was the right thing to do, and someone needed to do it.

[pushkins]

Well I'm not an expert in chemical engineering that's for sure and I never said exterior screws had to have larger shanks than wood screws.

I did however say that exterior screws are used on a very common basis in the building trade for this kind of work and I did clarify that statement by also posting that with the use of treated lumber it is not only expected but code compliant to use exterior rated screws with either ACQ or ACC lumber for interior or exterior use. That is a simple fact and not erroneous information.
The replacement and use of standard wood screws with treated lumber that you suggested the OP to do is a dangerous situation full stop.

Now I'm sure somewhere I could find ACC/ACQ rated "wood" screws however for the average DIYer it is much quicker to just buy a screw rated to do the job and required to do the job from their local big box store.

[Rwh56]

Thank You

I appreciate everyone's thoughtful advice and discussion about the use of deck screws.

I didnt disclose another big motivation to do this job right, is workers are coming in two weeks to remove my old coal steam boiler, so I didnt want to have a nasty accident as they lugged the heavy parts up the stairs.

I replaced four of the six deck screws that hold each 2 by 4 blocks with 3 inch length, quarter inch lag screws with washer. The treads are a full inch thick and 28 inches long and 9 and a half inch wide. So I think I am good. I dont have risers.

Thanks again,

Rob