Repairing a knot using a half-lap


Timber strakes can contain knots. You have tried to eliminate as many knots as possible in the way you have transferred the template onto the strake. However, in the event that there is a knot or a rotten area in the timber you can repair it. That is done using what is known as a ‘half-lap’.


A knot that is secure can be left in the timber. You can fill any cracks using wood glue and fine sawdust and then sand smooth once the glue is dry. However, a knot that is loose (image 7) must be removed from the timber and the hole has to be repaired using a half-lap (image 5). Proceed as follows:

  • Draw a pentagon (or polygon) around the knot, which encompasses the entire knot. Extend the lines of the pentagon by a minimum of 15 cm from the pentagon (image 10). This is done in order to be able to mark off the half-lap, the filler, later. A pentagon (or polygon) (image 2) is used because a square in the timber could be forced out as a result of expansion, which could cause splitting in the strake.

  • Cut away the entire knot to half the depth of the strake using a mallet and a sharp chisel (image 1). Do this very precisely by cutting in five straight lines at right angles to the strake. In this way you will end up with a very precise pentagonal hole into which the half-lap will fit exactly (image 9). The hole is then cut slightly wider as you go deeper into the timber (image  11) so that you can tap in the half-lap so that it is wedged into the hole.

  • Now place a piece of oversized timber over the pentagonal hole. This piece of timber must be at least half of the thickness of the strake. Using a pencil, transfer over the extended lines from the strake that you have already drawn. You will now have a pentagon that has the same dimensions as the hole in the strake.

  • Carefully saw the pentagon half-lap to size.

  • Coat the hole first with a preservative, before fitting the half-lap. After this has dried you coat the inside of the pentagon hole with glue. You now tap in the half-lap carefully using a wood mallet until it is wedged in (image 12).

  • When the glue has dried sand off the pencil lines and at the same time sand the surface of the half-lap until it is level with the surface of the strake (image 6).
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Tools Required

  • Pencil
  • Folding rule
  • Sharp chisels
  • Wooden mallet
  • Saw

Materials Required

  • Piece of timber that is half as thick as the plank containing the knot