Recently we visited dear friends in England and while we were whiling away a very pleasant afternoon I happen to notice their book shelf was sagging quite considerably.  It’s what happens when heavy books are placed on shelves.  Actually with no books on the shelves it would sag over time but a lot slower.  The human eye can notice even the slightest sag 3/32″ in a yard run. There are two fixes for this once the shelves are made – 1) add a rail to the underside of the front and a support of some kind on the back edge 2) add a vertical divider in the middle that pushed the shelf back into horizontal alignment. Both of these require construction work.  But a quick fix is to simply turn the shelf upside down – the bow will vanish almost immediately with the weight of the books.  Of course in a few years the shelf will again sag – so the ‘flipping’ process will have to be repeated. This leads me on to the topic of this post – I have been building built in bookcase and did not want the shelves to sag. The shelves I am making need to be approx.  39″ x 11″ x 3/4″ (details on this build will appear in a later post) but in the meantime I discovered a wonderful calculator – the Sagulator at WoodBin Woodworking.  That allows you to calculate the sag of any length.