Edge joining plywood to make a long cornice board strong

The finished cornice board is very stiff even though very longedge-joining-plywood-2127.jpgStaple the boards together with alternating joints and then hammer in ‘dog ears’

   When interior designers ask us to build upholstered cornice boards that are over 110″ long we need a way to make them strong so they won’t break while being transported to the customer’s home.  Plywood comes in 96″ long sheets so if we have an order for a cornice board 185″ long and 12″ high we could just edge join two boards-one 96″x12″ board to another board 89″x12″  with a joint roughly in the center.  But that would be the weakest point of the board if two people each lifted an end.  A much stronger solution is to cut five boards 2@96″x6″, 2@ 44.5″x6″ and one at 89″x6″.  The two 44.5″ pieces would be attached at each end of a 96″ piece producing a board 185″x6″ and this would be used for the bottom half of the face.  The other 96″ piece would be joined to the 89″ piece producing another 185″x6″ piece which would be used for the top half of the face.  Then the two 6″ sections would be joined together to make a 185″x12″ board.   Most cornice boards have a dust cap or top board that lays on L-brackets for when the cornice is installed.  The joint(s) in the dust cap boards would be located at least a foot away from the joints in the top face board.   By alternating our joints this way we can build cornice boards up to 20 feet long with very little loss of strength. 

   The five boards are laid out on a table to make sure we have made the bottom three  6″ boards the same length as the top two 6″ boards.  Next we measure to make sure they add up to 185″.  There is a good old saying “measure twice and cut once”.  When we are sure the five boards will make one board 185″x12″ we lay a long straightedge on top of the boards at the long edge to make sure the joined boards will be nice and straight (we can’t just assume we made  perfect 90 degree cuts so the straightedge shows us where we have to adjust the lineup).  We will first staple all the boards together on one side, then flip the whole thing over and staple them together on the other side.  Then we hammer in aluminum edge joiners (dog ears) every two to three feet along the horizontal middle joint and spaced every two to three inches along the vertical joints since these joints will have the most stress.

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2 Responses

  1. QUESTION? My cornice is 84″” long. Where should I seam it. The material is only 60″ wide.

  2. Hi Grace, There are two ways to approach it. If your piece of material can be turned on its side (railroaded) you can do the cornice without seams. If there is a definite pattern that runs up and down and the fabric requires seams you would have a center section as wide as the fabric (in your case 60″) and the seams would be about 12″ in from each side. Burt

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