Can anyone there tell me where someone could get some guidance on maximum stair riser spans for given dimensional lumber? I want to build a 15 step (10″/7″ rise) staircase in one span. The staircase will be hanging in space, attached only at the top and bottom. Roughly 4 feet wide. I can’t figure out if I can do this with 2X10s over this span, and if so whether I can still cut the treads into the risers. Alternately, if I cannot cover this span with plain lumber, is there a clever way to re-enforce lumber? Like attaching some steel channel down the inside for stiffness?
Thanks
Replies
If you are worried about the stringers not being strong enough, you should consider using Micro-lams.
T
Do not try this at home!
I am a trained professional!
Good idea, I thought of that, but it is an outside application. I assume glue-lam becomes de-lam in rain. Right?
O.
XXXXIII,
Cut your stringers out of a 2x12 The sister a whole 2x12 on the inside. Use plenty of exterior screws and exterior glue. This should make heap strong stair!
T
Do not try this at home!
I am a trained professional!
Hey, I like that even better!
I hope by exterior screws you mean structural screws right?
Brass could be considered exterior but is too brittle for such a structural application. It's a pet peeve of mine how many guys use SR screws for structural applications when half to two thirds of them will be snapped off almost immediatley. I've even seen the results of some idiots who assembled some 2x staging with Sheet Rock screws. They changed their minds about using what is cheap, fast, and immediately available while in the hospital.Excellence is its own reward!
I assume that most everyone knows this, some learned the hard way but screws (the kind that almost everyone uses - non structural) have very little shear strength and therefore are virtualy useless in most applications. Screws were designed not to pull out. The snap very easily under loads.
Todd
The screws are used to tie the two boards togeather while the gue is drying. But I agree. Use structural screws and glue. If its outside use brass or stainless steel.
Mr T has the best suggestion; I would maybe add a center stringer.
That would give 43 a clear span and if it bounces check the ground.
I clear spanned some stairs about 30 years ago they were inside and I'm sure,(I think), that I used 4 2x12s
However 43 good luck and let us know the results.
Charlie
I would use full 3x12 stringers with 3x12 treads cleated to both sides with galvanized stair cleats and bolts. I'd post a picture but I don't know how.you can go to my website (http://www.pro-dek.com) and look under portfolio, then stairs.
Bob
"Rather be a hammer than a nail"
Edited 6/18/2002 9:07:39 PM ET by Pro-Dek
I'm reminded of a trampoline. stair stringers shouldn't deflect. Any! Aside from the danger involved is a comfort factor.
I'm not certain what you are trying to achieve here, do you want cheap? Some kind of design concept? wide stair in a narrow area?
>> stair stringers shouldn't deflect. Any!
I assume you mean no deflection perceived by the person using the stairs, rather than no measurable deflection.
Well let me eleborate,
In a similar width run/rise I broke it in two with a landing at midpoint. I then used 5 per flight 2"x14" (actual 2inches by 14 inches ) stringers. The treads are 2"x12" (actual) Pretty close to my definition of no deflection..
I don't know anything about how the physical characteristics of wood get translated into building codes, but I did have some fun thinking about this, and it may be helpful.
The bending load on a tilted beam, or stair stringer in this case, is some fraction of the actual vertical load, varying from 100% if the tilt is zero to zero if the tilt is 90 degrees. (I'm not saying the load disappears at 90 degrees, but it becomes simply a compression load, no longer a bending load.) That much is obvious. The rest of what follows assumes that the beam reacts as if the load were being applied perpendicular to its axis. I'm nearly 100% sure this is true, but I can't justify my assurance or site an authority to back me up.
Imagine a vertical line 40 inches long rising from the top edge of the stringer. The actual number isn't important, I just picked 40 to represent 40 psf of live load. Now imagine a line perpendicular to the stringer rising from the top edge of the stringer to meet the end of the first line. The length of the perpendicular line represents the bending load that a 40 pound vertical load imposes on the stringer. More generally, the ratio of the length of the perpendicular line to the vertical line is the ratio of the bending load to the vertical load. Mathematically, this ratio is the cosine of the angle between the beam and a horizontal line. 7/10 defines a 34.99 degree angle, and its cosine is 0.82. 40 times 0.82 = 32.76, so if you can find a span table for a 33 psf live load, you can find your maximum span directly.
Another way to think of it is that since the actual load is vertical and we think about a beam bending under a perpendicular load, the effective length of the stringer under a vertical load is the horizontal length, or the run of the stairs. The result is the same, since the ratio of the run to the actual stringer length is also the cosine of the angle. But this way is even easier, since we don't have to calculate anything, we can just check to see if the horizontal run is less than or equal to the maximum span in the table.
Next question is, which span table? For calculating load on the stringers, 4" wide stair is equivalent to floor joists 24" OC, because each stringer is supporting 24" of floor. The only difference is that in the stair, each stringer is supporting its 24" from the end instead of in the middle.
If I haven't misunderstood your description, your stair has 15 risers and 14 treads, so the run is 140", or 11'8". The first span table I found with a Google search, http://www.mcvicker.com/resguide/page013b.htm says the maximum span for #2 douglas fir 2x10 on 24" centers is 11'-5", and for #1 douglas fir, 12'-3".
If the maximum span for your choice of wood doesn't quite make it, I would consider sistering a 2x4 on the inside of each stringer, flush with the bottom edge, under the treads. Or if that looks too clunky, steel strap (not banding strap, something like 1/8" x 1) let into the bottom edge.
Edited 6/10/2002 7:21:06 PM ET by Uncle Dunc
43
Use 4 2x12's for stringers equally spaced, with 2x12 threads and 3/4 inch plywood risers. If you're using oak.for threads that will work as well. The 2x12 will have to be about 16-18 feet to span the distance. I do not have a square to check it.
charlie
Bull, they'll bounce like hell.
Qtrmeg:
How do you know they will bounce?
No bull. If you have good lumber, properly crowned and installed correctly they will not bounce.
Charlie
charlie, I agree with your design of 4 2x12 crowned and glued risers and treads. But it will have some bounce. To take out the wiggle we can place some barcing aka kneewall about a third of the way up the span, add angle iron like the original post, or add more meat to it. Any of those can be done as an addon after getting it built.
To my minds eye that idear of adding a 4x4 dadoed up into the bottom of the stringer is a good one but labor intensive. Also it would have an open joint facing up to let rain get trapped in it so it should be assembled with PL Premium glue and then caulk those joints good to avoid rot. Or acheive the same effect by sistering 2x4s to the side of the notched stringers, flush with their bottoms.
I think it was dunc who said something about the strenght based on size of 2by and span but he left out the fact that you lose wood to the notches. These two by twelves will effectively be only 2x6 in application so that's how you should look at span tables, but he's right about using the horizontal span of the openning to figure from.
Excellence is its own reward!
Depending on your application or asthetic value...maybe two glulams (say 3 1/8" X 12" or 13 1/2") for stringers that you could sand and finish, with same material for treads. This has an open riser effect and looks nice...but is a few more $$$$. This has no bounce, which I also can't stand.
Call the lumber yard. A real one. Tell them Jeff said to get their stair guy out for a measure. He'll build them......probably at the cost of materials and half your time......and they'll fit good and look good. If ya wanna save money...get them dropped off in the driveway. If ya spend a litle more...they'll send his helper out and they'll install them right quick.
For freestanding.......always quicker and sometimes cheaper to have the pro stair guy do the measure and build. Or let the fingers do the walking. Jeff
"That's like hypnotizing chickens........."
Build it out of 2x12's, if your span is only 11 feet then it should be fairly stout, provided you weigh under 500 pounds, cover the treads and risers with 3/4 plywd be sure to use constuction adhesive to glue all the risers and treads together) this will eliminate squeeks and unitize the whole structure. If your still afraid use 2x14's for your stair horses and hang the treads with "Simpson" hardware (I think A10's). Good Luck, Hank
Could you live with 2 - 4 x 4 's half lapped on the stringer half way down the stairs. This would essentially cut the "moment" (structural engineers word for deflection) in half. Of course you will need to put a footing under the 4 x 4's. Good luck!