Core Basics

Surfboard design begins before you even pick up a tool.  You need to know what you want your surfboard to do.  Do I want I board that's lightweight or heavy? Do I want flexy or stiffer? 

These are just some of the points when considering which core (blank) will suit your design. 

This page is to help you understand the facts with a bit of technical knowledge for which material will give you the maximum performance in a surfboard blank.

Blanks are at the heart of your board.  Shapers and Designers must choose the right blank for the board to ensure a strong deck with the right flex to create a shape they have in mind.  The right core material is important as there are a lot of different choices so it’s worth doing some ground work before you start.

Below is a guide to some of the most common used cores in surfboard blanks.

Core type

EPS – S Grade

EPS-   M Grade

EPS-  H Grade

EPS – VH Grade

PU normal blank




Density Kg/M3









Water up take









Cell structure










EPS - Expanded Polystyrene , PU – Polyurethane,
XPS - Extruded Polystyrene

PU surfboard blanks are a closed cell structure though if left open to the elements (not repaired) will rot in water

^EPS foam is a closed cell, though if skin is ruptured water will fill the air voids in the core



Types of Surfboard Blanks


Polyurethane (PU) Surfboard Blanks


Polyurethane foam is most popular selection. Polyurethane foam blanks are used in conjunction with polyester resins to produce what is known as a polyester, or PU surfboard.

There are two different polyurethane (PU) foams with slightly different chemical make-ups to be on the lookout for, one is a tolulene-di-isocynate (TDI) base (this is the one that Clark foam was making until they closed down), and the other is made using methylene-di-isocynate (MDI), a newer less toxic chemical that has been used more since Clarks closure.

PU basically replaced the balsa cores in the 60’s because of its ease of shaping compared to the wooden balsa core.  It was lighter with better performance and became the preferred surfboard core almost overnight!

It usually comes from the manufacturer with a stringer to keep the core strong and prevent twist in the blank

This is the most cost effective core as the materials are very accessible and when combined with polyester resin it can minimise the overall cost of a surfboard.






Expanded Polystyrene (EPS) Surfboard Blanks

Expanded Polystyrene (EPS) foam is also known as beaded foam and features a closed cell core.

EPS is made using small expandable beads that inflate with heat, and then are placed into a mould, where they expand to fit the shape. The expanded beads trap air, which gives expanded polystyrene its lightweight properties.  This also leaves it open to water absorption (though new developments are happening with a “fused cell type of polystyrene” to reduce the water absorption), reducing the blank effectiveness over time.

A very important difference between a polyurethane and polystyrene blank is that traditional polyester resins cannot be used with a polystyrene blank during the glassing process.  This would cause the polystyrene foam to dissolve.

The foam is very easy to shape though can get damaged in the shaping process however, generally this isn’t that much of an issue once glassed, as the epoxy resin covering this type of su
rfboard foam offers a great deal of protection due to its hardness.

Extruded Polystyrene (XPS) Surfboard Blanks

There is often a great deal of confusion between expanded (EPS) and extruded polystyrene(XPS), with many people assuming they are essentially the same, but in fact expanded and extruded polystyrene are very different products. 
XPS is made by mixing polystyrene pellets with chemicals that liquefy the pellets. A blowing agent is then injected into the mixture, forming billions of miniature air pockets. The major advantage of this type of an Extruded polystyrene surfboard blank is that it has a closed cell structure.  This makes is totally water repellent. So if you do damage that extra hard epoxy surfboard covering there is absolutely no need to leave the water.

In addition to that impressive fact, extruded polystyrene foam is exceptionally strong, and isn’t damaged as easily as EPS and PU foam. 
There are some reports of the XPS foam shearing or the gases making delaminations, though once you know the different process of shaping this material these issues are reduced.  The foam is also reactive to excessive heat, so don’t leave in the car on a hot summer’s day.

What’s new?

Something to keep an eye on is Airloys. Airloys are a new class of advanced engineering materials that combine the strength of conventional plastics with the low density and super insulating properties of aerogels.  
Airloys are strong, tough and are typically three to fifteen times lighter than conventional plastics, foams and ceramics. Airloys are available in densities ranging from 0.01 to 0.9 g/cc.  Unlike classic aerogels, Airloys are easy to machine or shape, waterproof, non-brittle and they have floatation properties.  Airloys are not restricted in composition to any one particular substance and can be made from ceramics, polymers, carbon, metals, carbides, and combinations thereof.
At the moment Airloys are still in the developmental stage but small size tiles are being sold, with plans to make bigger size sheets with different densities in the future.
The Green Option 
A company using Myco Foam using mushroom materials, have gone commercial with bio-based, biodegradable, and rapidly renewable surfboard blank. These blanks will decompose when broken, discarded, effectively reducing toxic marine debris. The mushroom mycelium materials float similar to other surfboard foams and have other structural characteristics that make it excellent for use in marine applications.
The company which has developed this technology has showcased its first generation surfboard cores, and is collaborating with the industry's top surfboard manufacturers and shapers to advance this sustainable product.

What Skins work with my core? 

Is fibreglass the only skin covering for surfboards?

Are there any other resins except epoxy?

Drawing a blank? Visit the Forum for more info