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KeLP
Stringers to convex beam
KeLP

I'm putting stairs to the second story of a barn/garage. The beam face the stringers will attach to is convex (the center is further out than the top and bottom). Is it acceptable to use shims to fill in the gaps behind the metal brackets and stringers?

Mastercarpentry
Re: Stringers to convex beam
Mastercarpentry

The stringers need to attach directly to the beam*, or have another means of vertical load support given them so that all you need to be concerned with is pull-away forces. This gets into some rather advanced carpentry skills as the stringers will not be all alike as they usually are. If the end stringers are balanced (identical but on opposite sides) you can set them temporarily, then use straight-edges or strings across them to calculate the distances for the center stringer's top while keeping the bottom edges in plane together. One you locate and create the top cut and it's tread cut, measure along the bottom of the center stringer and make the bottom cut. Now temporarily set the center stringer and check for alignment at the top tread and underneath too. If all is well then mark the bottom riser height, drop the center and one outside stringer, then use the outside stringer as a pattern aligning it at the top tread and the bottom riser, which may vary from the bottom edge where you usually pattern off of. The bottom difference should still be parallel, just not congruent. Trace and cut as usual.

* It is permissible but not optimum to slightly shim the top of a stringer. This should be done with solid materials, not wedging, cut to the needed thickness and cut along the grain, not cross-grain, for structural integrity. I'd limit this to about 3/8" or less. If its minimal and the situation allows you might consider trimming the other stringers to match the gap instead. The entire top vertical bearing area of all stringers needs to be supported even if you have to add backing to achieve this. That is now code in most places and is good practice regardless. If for some reason this cannot be done then you must add something like a wall or supported beam underneath to give the stringers adequate vertical support at the top.

I have gotten into the habit of using joist hangers to attach stringers at the top for the most secure connection possible. Lay the hanger on the stringer until it just meets at the bottom of the vertical top cut, then draw a line under the bottom of the hanger- there's your notch pattern. My local inspectors love this.

Phil

KeLP
Re: Stringers to convex beam
KeLP

Thanks for answering. I knew the entire vertical area needed support, but was not sure if shims were kosher. Now I know they aren't. Thanks.

The plan is to use engineered lumber for the stringers and the documentation for the lumber recommends Strong-Tie A35 framing anchors, or equiv., to make the connection, which I plan on doing. My convex beam is quite different in profile where each stringer will need to be, and matching the profiles is definitely beyond my skill level.

Sometimes in deck building they attach ledger boards with bolts or lag screws to overcome irregular surfaces at the home. Could a similar method work here? Put a 2x board onto the beam, taking care to keep it plumb and unwarped when tightened, and connecting the stringers to it?

Or maybe better, bolt a 4x board of the right dimensions to the beam, with 4x4 supports under the board, to carry the vertical load?

Mastercarpentry
Re: Stringers to convex beam
Mastercarpentry

Then the easy way would be to pad out the beam where the stringer heads will fall, thus giving you identical stringers. Scribe-fit framing material to the curve, glue and screw to the beam, build out as needed to a flat face.

Cap the face with a 2X board (width as needed, glue and screw again) then do the stringers. It will look a little cobbled-together but will work OK.

If the beam has enough strength (probably not) cut a flat face on it.

Stringer hints: If you're not positive everything is right, cut only the heads and tails and tack up to see how they align, then make the riser/tread cuts when that's right. Having a helper for holding them up is nice for longer stringers (plus you can blame them for your mistakes :eek: )

Phil

KeLP
Re: Stringers to convex beam
KeLP

That looks like what I'll do, build the beam flat and put a good 2x board to it. I'm not concerned with looks, just to put the stairs up securely. Thanks for the answers, it's been a big help.

Sombreuil_mongrel
Re: Stringers to convex beam
Sombreuil_mongrel

I remember an old trick of using 1" wide steel lumber banding to hang stair stringers that had little "meat" to nail them into.Take 2ft pieces of banding and nail them to the plumb cut of the top of the stringer, have them wrap down onto the uncut area a few inches, nail with 12's. Then when you set the stringers, just nail the bands into the face of the stair trimmer or joist. If the subfloor isn't down yet, you can also wrap the band over and down the trimmer.
This doesn't get in the way of adding more blocking or steel hangers later.
Casey

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