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The Lineup

Interactive board building tutorials

Clearwood - hollow wood surfboards and paddleboards

Internal Blocking

Tutorial: Internal Blocking


The purpose of the internal blocking is to support the fin system, leash and vent plugs, and any tiedowns or accessories you may decide to build into your board. Internal blocking can add significant weight to the board if materials aren’t chosen carefully. Some builders use paulownia or balsa for internal blocking and some use high density EPS foam. Watertight compartments for extended touring (if desired) should be finished prior to the internal blocking. Deck hatches should have appropriate support blocking attached to the underside of the deck.

Five fin surf SUP

The photo below shows the internal blocking for a five-fin surf SUP with the vent and leash set blocking. Single fin touring boards only have one center fin blocking position plus the vent and leash blocking.

Getting the blocking “right”

Careful layout and planning will avoid time consuming and frustrating problems. Selecting the fin system, vent and leash sets you are going to use is critical to “getting it right”. I’ve found Robin Mair at Hanalei Fins to be an excellent source of information about fin placement for surf SUP’s You can also check out and

Carrying grips for SUP’s 

Grips for SUP’s can be as simple and attractive as a block of wood machined for a hand pocket. The block is glued into position along with the other internal blocking. The fore and aft balance point is found after the board is released and flipped and before it is reattached to the assembly table. It’s attached to the underside of the deck with thickened epoxy on top of a locator hole drilled through the deck and on one side or the other of the spar depending on right handedness or left handedness. The placement of the blocking for the vent and leash is determined by the esthetics of the deck layout.


There are many ways to learn to safely use the tools and execute the processes necessary to build your hollow board. But the bottom line is this: Tools and the dust they create are dangerous. If you are a novice woodworker, educate yourself about tool safety and proper shop practices. Power tools are very unforgiving and trouble can happen quickly if you are not prepared and diligent in your approach to how you use these tools. Regarding the health consequences of breathing wood dust, take this issue seriously. Use good respirators with fresh, clean filters when cutting or sanding wood. This is also true for applying and sanding epoxy and other finish products. Uncured epoxy is toxic. Most of the ultra violet inhibiting finishes that go over the epoxy for the final finish are also toxic. Use the tools and materials at your own risk. Educate yourself and work smart and safe!