Overview  The core of a ski is made by laminating pieces of wood together and profiling the core to get the desired shape.

There are several reasons why a core should be laminated, as opposed to using a single piece of wood.  First, a laminated core minimizes irregularities such as knots and grain patterns that run in various directions.  These irregularities are often found in a single piece of wood, and they can affect the strength and durability of a ski.  Therefore, a laminated wood core is more uniform in density and grain pattern, and more lively in terms of performance.  Second, the lamination process allows the core's characteristics to be custom designed by mixing different types of woods.

There are several types of laminated cores: vertical, horizontal, and symmetrical; they are discussed below
 

Vertical Lamination  A vertically laminated core is one in which the adhesive interface (shown below in red) is vertical:

In this arrangement, the wood grain boundaries run lengthwise along the core.  This configuration provides strength and rigidity (stiffness) in the longitudinal direction, but very little in the transverse direction.  As a result, a vertically laminated core relies heavily on composites such as specially braided fiberglass for torsional support.

Step 1:  Rip Wood Into Strips  Begin by selecting your wood of choice and rip it lengthwise along the grain boundaries into thin strips (1/4" to 1" wide).   Use a table saw to rip the wood and be sure to use appropriate safety precautions when operating the table saw.  Note that thinner strips reduce the size of the irregularities (e.g., knots) but it also increases the amount of adhesive during the lamination process, potentially making the core weigh more.  Epoxy is almost twice as dense as wood, and the increase in density adds more weight for the same volume.  Thicker wood strips weigh less but are not as torsionally stiff.  The strips we use for our skis vary in width from 1/2 to 3/4 of an inch.

Rip enough strips so that when placed side by side they collectively measure wider than the ski's width by a few inches. 

Step 2:  Laminate Strips  Apply a generous amount of high quality epoxy or wood adhesive to the strips and clamp them together as shown below.  Some claim that epoxy is the better choice for core lamination because it is more durable and forms a stronger bond.   On the other hand, some believe wood adhesive is better because it is specifically designed for wood applications.

Make sure that a generous amount of adhesive is used, such that it overflows from the seams of the core.  Let the adhesive dry for a period of 24 hours or until completely cured.  To prevent your work space from becoming a mess, use painter's plastic to cover tables and work benches during the lamination process.
 

Horizontal Lamination  A horizontally laminated core, or plywood, can be identified by the horizontal direction of the adhesive interface (shown below in red) between the wood layers:

Although not as common in ski construction, horizontally laminated cores can be found in some snowboards and is very popular in skateboard construction.   The direction of the wood grains are varied with every layer.  This configuration provides rigidity in several directions (not just the longitudinal direction as in vertical lamination).  It's debatable whether this provides for a better core. 
 

Symmetrical Lamination  This core is a version of the vertically laminated type but symmetric around the longitudinal axis.   The goal of symmetrical lamination is to create cores with consistent flex and density.   

Step 1:  Laminate Planks  Symmetrical cores are made by first laminating several planks of wood together.  The height of lamination should equal at least half the width of a ski.

Step 2:  Cut Planks  After the adhesive has cured, the laminated piece is run through a table saw to cut out planks, e.g.,

Step 3:  Create Symmetrical Core  Two of the planks are then laminated side by side to produce a symmetric core.  

As you can see the core is mirrored about the longitudinal axis (highlighted in red).  This process ensures that wood from the same batch is used at the same distance from the ski's center, resulting in a symmetric core.
 

NOTE: A laminated core doesn't have to use the same wood throughout.  It is possible to combine the properties of a hardwood (stiffness, strength) with those of a softer wood (less weight, more shock absorption).  In fact, this configuration is very common.   Many manufacturers create cores that have alternating strips of two different types of wood to meet certain specifications, such as weight, stiffness and cost.

Often, strips of harder woods are placed underneath the binding areas to maximize energy transfer.  

Another configuration involves placing stiffer woods near the edges to increase edge stability, e.g.,