Author Topic: Making longer stringers from shorter (stringer scarfing)  (Read 3475 times)

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Brian.Dixon

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Making longer stringers from shorter (stringer scarfing)
« on: March 05, 2010, 10:27:51 PM »
Many have asked about using shorter main stringers (9-1/2" x 1-3/4" x L) to make longer stringers, e.g. if shipping long LVL beams, costs can be prohibitive unless shorter pieces can be shipped.  You CAN make full-length stringers from shorter pieces by using a vertical scarf as shown in the picture below, but make sure that you glue them up with good straight edges along top or bottom, sides laying on a very flat surface.  It's easy to make crooked stringers if you are not careful and if you do ...well, then I guess your savings in shipping costs went down the tubes, right?  And if you do build longer stringers from shorter pieces as shown, make sure you glue them up and glass them as prescribed below.



Gluing and glassing:

  • Cutting the scarf:  CUT the scarf with a circular saw and then DO NOT sand the joint.  You can vacuum sawdust out of the wood if you want, but don't sand it.  Sanding smashes the wood cells closed and epoxy will not soak as deeply into the wood.  If the fit is not perfect without sanding, don't sweat it (or sand it!).  Trust in the gap-filling qualities of the epoxy and move on.  Just make sure the beams are glued straight as described.
  • Inside the joint:  PRE-WET with unthickened epoxy every 10 minutes or so until no more epoxy will soak in, and both glue surfaces stay wet-looking.  Always use wax paper under the joint when gluing, a flat floor, and straight edges (cedar 2x4s?) along top and bottom to guarantee straightness.  For a good gap-filling mix inside the joint, use epoxy thickened with a strong thickener mixture made from 60% wood flour, 20% milled glass fiber, and 20% colloidal silica (Cab-o-Sil or similar).  The glue mixture should be peanut-butter thick.  Small thickness adjustments can be made by adding 80% wood flour, 20% silica.  Slightly soft is better than slightly dry.  You can always fill holes and gaps afterwards.
  • Once glued and cured for 24+ hours, scrape then sand to smooth all squeeze-out epoxy to the wood.  After rounding the top two edges (see Construction Manual, Part 1), glass the joint with 12-ounce biaxial fiberglass cloth.  The cloth should extend beyond the joint by 6" to 8" or so, not less.  The glass should wrap from one side, over the top, and down the other.  Trim any excess that extends beyond the bottom, e.g. with 60-grit sandpaper on a random orbital sander (aka "fast and easy glass trimmer").  Either add 10-oz woven glass over this joint at the same time, wet-on-wet, or fill the weave with epoxy later on ...before glassing the entire stringer with 10-oz woven glass later.  And no, if you add the 10-oz glass now in order to save having to fill the biaxial weave with epoxy later, you do NOT have to add more 10-oz woven glass over this area later on.  Just overlap the already-completed glassing by a couple of inches on either end and call it good.

As far as the economics involved in buying an extra 4 or 5 feet of beam in order to save money on shipping full-length (22 ft +/-) beams goes, I will leave those calculations up to you.  But if you want to make longer LVL beams from shorter, this is the way to do it.

Brian