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Stair Building Details

Froed | Posted in Construction Techniques on October 30, 2003 08:36am

Good afternoon all.  I have a set of straight run stairs that I’m replacing, and I have a pretty good handle on the overall construction, but there are a couple fine points I’m fuzzy on.

The stairs I’m making have housed stringers and closed risers.  Rise is about 7-1/2 and run is around 9-10 inches (I don’t have the drawing in front of me), with a tread width of 37-1/2 between stringer faces. The stringers are 2×12 syp and will be shimmed and screwed to studs, risers are 4/4 poplar, and treads are 6/4 (1-1/4 finished) #1 common white oak.   It’ll be stained treads with painted risers & stringers.  Treads and risers will be rabbetted and grooved, then screwed together as well, wedged and glued into the stringers.

Now for the questions: I believe this width of stairs requires a third stringer according to code, but with the treads and risers glued and screwed, does this eliminate the need for the third stringer?  I saw an earlier comment of Stan Foster’s regarding the extra strength added by gluing and screwing the treads and risers together (http://forums.taunton.com/n/main.asp?webtag=tp-breaktime&msg=24264.1).  There are some occasional knots in some of the treads, but they’re 6/4 stock, so I figure they should be okay with the closed risers.  If it’s questionable, I can add a center stringer.  The current stairs are open riser with 2x syp treads, no center stringer.  They visibly deflect.

I figure washer head screws would be better than flat heads (#10 okay)?  With the joints glued, does it really matter?  With a housed stringer and wedges, should I screw into the end of the treads and risers through the stringer, or is that pointless?

Is epoxy reasonable?  Or will it not have enough initial tack for the wedges, since it’s kind of slippery?  The stairs are for a basement that stays dry year round, but you never know, right?  The wedges will be made from offcuts from the treads, but should they be cut across the grain so they match the tread movement?  I intend to finish the pieces all around before assembly (except of course the glue surfaces) to try to minimize expansion and contraction.

Sorry this is so long, and hopefully I haven’t asked anything too stupid, but stairs being what they are, I want to make sure they’re as strong and stable as possible.  And if you got this far, thank you, I appreciate your attention.

Froed

 

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  1. AndyEngel | Oct 30, 2003 08:52pm | #1

    I've built stairs just like this with 5/4 treads, no center stringer. The risers act like beams front and back of the tread. You should have essentially zero deflection on the treads.

    Andy Engel, The Former Accidental Moderator
    1. Froed | Oct 31, 2003 04:49am | #6

      Thanks, Andy, that's what I thought, I wanted sort of a sanity check to confirm that.  I was thinking of forgoing the screws from the sides, since the treads are essentially "mortised" in, and the screws would be going into end grain anyway, but they would at least help pull the stringer tight to the tread while the glue sets.

      The way I'm planning this, with the back of the tread fit into a groove in the riser, the screws I don't think do much more than clamp the pieces together while the glue dries/cures.  At the top of the riser, the load pushes the tread into it, so the screws are only helping to resist shearing along with the glue (the tread and riser can't slide relative to each other as they bend), stiffening the whole thing by making it sort of a wide flange I-beam.

      The exra stringer I thought was code for stairs this wide, but maybe that's just for the open riser type.  The stringers for the existing stairs are nailed to about every other stud.  They deflect just enough to see it with my 215 lb load, maybe 1/32 or so.

      Thanks again for your responses, and have a good evening.

      Froed

  2. Tryit | Oct 30, 2003 08:56pm | #2

    Froed,  Sorry I have no expertise here, but have a question.  Where do you screw treads to risers of visa versa??????  Can't form a picture in my mind.

    Through the top of the tread into hte riser?  From the back or the riser into the back edge of the tread?

    1. AndyEngel | Oct 30, 2003 10:52pm | #3

      When I made my living building stairs, I used to screw and glue a strip of 4/4 square stock to the back of the tread/top-of-the-riser joint. However, I just did a stairbuilding demo last weekend where I skipped the 4/4 and used a Kreg jig to pocket screw the riser to the bottom of the tread. I'm never doing it the old way again! The other end, where the riser above drops behind the tread below, gets glued and screwed through the riser into the back of the tread.Andy Engel, The Former Accidental Moderator

      1. Tryit | Oct 31, 2003 12:21am | #4

        Thanks Andy,

        The pocket screw up through the top of the riser and into the tread can't give you much strength to counter sagging.  Is it more to just pull it tight?

        The screws through the riser and into the back of the tread would give the tread support.  But boy a middle stringer would be beefy!  I suppose with a middle stringer, great care must be taken to insure that it is a mirror image of the side stringers.  Is that the biggest reason for leaving it out?

        1. PaulParadis | Oct 31, 2003 02:00am | #5

          I can't understand why you would need an extra stringer.  If the current stairs deflect maybe the stringers were not screwed the the walls?We become by effort primarily what we end up becoming

           - Zig Ziglar

        2. AndyEngel | Oct 31, 2003 04:50pm | #7

          The pocket screws pull the joint tight until the glue dries. How beefy a middle stringer do you propose? If you notch 10 in. or 12 in. stock, you're left with 4 in. to 6 in. of material, spanning what, 13 ft. or so? No, I don't think center stringers help much, and they're a pain in the neck to install. Screw, mortise, wedge and glue your parts into one unit, and you'll be fine. If you really want a center stringer, and it will be hidden, save some trees by making a strongback out of 2x4 or 2x6 and screwing it below the stairs, rather than notching a 2x12 or 2x12.Andy Engel, The Former Accidental Moderator

          1. Tryit | Oct 31, 2003 07:07pm | #8

            I get your point on the notched center stringer.  The strong back is a good idea.

            To a pro, such as yourself, this is a silly question, but where to all these "wedges" go?

          2. AndyEngel | Oct 31, 2003 11:05pm | #9

            Not a silly question at all--I'm the one who made the assumptions. The mortises routed in the stringers for the ends of the treads and risers taper on the back side. Wedges are glued and hammered into the taper to lock the treads and risers into place. I wrote an article on this in FHB 114.

            Andy Engel, The Former Accidental Moderator

          3. jimblodgett | Oct 31, 2003 11:39pm | #10

            As far as what's left of the center stringer after you notch for treads and risers, I normally sister a 2x4 each side, flush with the bottom edge of that stringer - now you gain a lot of support.

            Same when you used a notched stringer against a wall - sister a 2x each side of it (has to be assembled to the wall one piece at a time) and you not only strengthen the stringer, you allow for sheet rock and a 1x skirt board to slip in there, no muss, no fuss.

          4. Tryit | Nov 01, 2003 05:10am | #11

            I guess I will have to read that article.  I vaguely recall something along those lines.  Thanks again. 

  3. vineyrdbuilt | Nov 01, 2003 07:50pm | #12

    Froed,

    With 5/4 or 6/4 you DO NOT need a center stringer.  This form of stair building, where the wedge is placed under the tread in the in the acutely shaped dado in the stringer, is very strong.  This form of construction is used usually when stair boxes are built off site.  You can build these in place as long as the top tread is glued, srewed and plugged.  These stairs tend to be hollow sounding so either attach polystyrene rigid insulation to the back or stuff insulation between the treads and strapping, before sheetrocking the underside, I usually leave s portion of wall free of sheetrock so that I can access the underside of the stairs if finshed off so that I get good, and strong attahment for my newel posts also, which allows me to insulate and have my stairs sound thick and not hollow.

    Start at the bottom place your riser in its verticle dado, use glue and tack it in place with either brads or staples from the back, and clamp at the base of the stringer from the back and on the outside of the stringer, and then work your way up placing tread and wedges with glue while tacking and clamping as you go.  the tread is glued to the stringer and screwing, glueing and securing all parts together DOES prevent deflection at the center of the stairwell. 

    The Kreg jig or biscuiting the riser to the underside of the tread are the two most optimal and efficient ways of attaching the riser and tread together.  You can always run a horizontal dovetail at this joint too. jointing allparts together than clamping all treads and risers in the stringer at the same time.

    The wedge is used to force the tread up into the grove tightly.

  4. StanFoster | Nov 02, 2003 12:45am | #13

    Froed:   I agree with the others. You do not need a center stringer. The pocket screws alone will be more than enough to transfer the load across that span.  The pocket screws like Andy said also make this an ideal glue up...a tight glue joint which in itself is very strong. 

    1. Froed | Nov 05, 2003 02:23pm | #14

      Stan, vineyrdbuilt,

      Thanks for adding your 2 cents.  Sorry about the delay.  I started a new assignment, and haven't gotten back to my computer until now.

      I guess being stairs, I wanted to be extra sure what I am planning is a sound approach, and since everyone seems to be unanimous, I'll proceed with a bit more confidence.

      Thanks again to all who chimed in.

      Froed

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