How to Fiberglass Boats: Fiberglass Materials


Regardless of the chemical composition, the glass filaments are made into materials known as "fiberglass". These materials include cloth (or "fabric"), various mats, rovings, woven rovings, and specialty materials that may combine two or more of these materials into one, or orient fibers in one or more directions in a non-woven format. Most of these will be discussed in greater detail later.


Fiberglass cloth is woven from fine yarns of various twist and ply constructions into a wide range of types, weights, and widths. For boat covering purposes, cloth is usually a plain square open-weave type where the lengthwise and crosswise yarns intersect alternately at right angles, much like a basket weave. This type of weave has the maximum number of interlacings, the most crimp, and the highest stability. Other weaves of cloth (such as satin weave) are available and may be suitable, but aren't as common. Cloth is categorized by WEIGHT PER SQUARE YARD, ranging from approximately 2 ounces (very light) to 40 ounces (very heavy) per square yard. Cloth thinner than 4 ounces is not durable enough for most boat work, while cloth much heavier than about 10 ounces is more difficult to wet out and form around corners and contours. Generally, the heavier weight cloths are not necessary; heavier sheathings can be achieved by adding more layers of a lighter weight cloth, or adding a layer of mat, although mat is rarely used on small boats. Both these approaches will be discussed later.

Lightweight 4-ounce cloth is often used on the decks of smaller boats, and hence is often called "deck cloth". It is also ideal where a natural wood finished surface needs protection. This light fabric becomes very transparent when wetted out with resin, with the weave becoming practically invisible. This allows the true beauty of the surface below to stand out through the clear resin. For more general covering purposes, such as hull, deck, and cabin top areas, cloths ranging from 6-ounce to 10-ounce should be used as they offer greater durability and resistance to scrapes and abrasion.

Commonly available widths of cloth are 38", 50", and 60", usually sold by the running yard. Also available are cloth tapes with selvaged edges in widths from about 2" to 12", which are ideal for reinforcement of localized areas and can be applied along seams to help make a boat watertight. While you could cut any fabric up into strips to make your own tape, these strips would not have a selvaged edge and the tendency is for them to unravel.

Cloth is the most expensive form of fiberglass material on a weight basis, and thus is seldom used in the production of all-fiberglass boats. If cloth were the primary material used in building such a boat, it would be very uneconomical to use as it takes too much of the material to build up the thickness necessary for strength and stiffness. When cloth is used in fiberglass boatbuilding (and it seldom is), it is usually used in conjunction with various compositions of mat and/or woven roving. Cloth by itself is used almost exclusively for coverings or sheathing work on hulls, decks, cabins, repair work, and similar applications.


The yarns made from the strands of glass filaments must be protected from abrasion and breaking during the weaving operation by lubricating materials called "sizing". Sizing is not compatible with the resins that will be used later, hence they must be removed. A common way to do this is by baking the cloth or burning off the sizing materials. Various chemical treatments or "finishes" are then applied after this heat treatment.

A wide variety of finishes are used to meet the requirements in current fiberglass laminating procedures including sheathing applications, and to suit the wide variety and types of resins available. Basically, the finishing treatment has to do with "wetability", or the time it takes for the resin to saturate the fabric. The finish also improves the adhesion between the glass and the resin, and one type of finish may yield better results in this regard with one type of resin than with another.

Each fiberglass weaver seems to have his own "special process" that he claims is "superior" to others. Some cloth manufacturers have indeed developed their own "hybrid" finishes which may indeed be better. But for the cloth fabrics most commonly used in sheathing boats, the "chrome" and "silane" (or "amino-silane") finishes are common and proven. Other terms seen on suitable finishes include "Volan", "Garan", and "Sea-Glass".

From the consumer's standpoint, this makes things confusing, especially since many firms selling fiberglass cloth have no idea what a finish is, let alone what type is used. One cannot tell the type of finish used by appearance; they are all the same in this regard. If you ask many merchants about the cloth they sell, the usual reply is, "we've got boat cloth", which will probably suffice. In any case, these finishes are what give the fiberglass fabrics their characteristic "white translucent" appearance and their ability to be wet out by the resin while bonding to a surface material or "substrate".


Fiberglass mat is a reinforcing material made from glass fibers about 1" to 2" in length, or continuous strands arranged in a random swirl pattern formed into a felt-like product. The fibers are held together with a dry resinous binder. Because mat is cheap and builds up thickness (and subsequently, stiffness) quickly, it is commonly used in fiberglass boat production, even though it is heavy. The binders used in the mat must be highly compatible with polyester resin (since this is what's commonly used in the factory) in order to wet out the thick, heavy material quickly.

However, the binder may create a problem if other resins are used; especially epoxy resins, which may be incompatible with the mat binders. Thus if you want to use mat with epoxy resin, you must determine that the epoxy being used will wet out the mat being used. Unfortunately, due to the wide variety of epoxy resin formulations, not to mention mats and their respective binders, this may be difficult; some work, some don't. But from a logical perspective, due to the higher cost of epoxy resins, and the greater amount of resin required to wet out mat, there is little justification or need for this combination. Thus, in general, mat and epoxy resin should be avoided if only to conserve materials.

The type of mat used in boat work is commonly referred to as "chopped strand mat" or "CSM", and is categorized by WEIGHT PER SQUARE FOOT (not per square yard, as are cloth and other woven materials). Weights popularly vary from approximately 3/4-ounce to 3 ounce per square foot, with 1, 1 1/2, and 2-ounce weights most common.

Mat is generally considered the cheapest of fiberglass reinforcement materials on a weight basis, and hence is commonly used. However, mat absorbs a higher percentage of its weight in resin and may tend to be brittle when used alone with resin. Mat is easy to wet out with polyester resin and it gives positive bonding between layers of cloth and/or woven roving while building up thickness. It also shapes readily and can conform to complex shapes easily. Mat, however, is not as strong as cloth or woven roving, and this is why it is commonly used in conjunction with other reinforcing materials; it helps form a laminate with balanced strength and stiffness properties. Again, these combinations are not applied to plywood boats, but would be used for production or "one-off" fiberglass boats.

Mat is commonly available in 38" and 50" widths and is sold by the running yard or by weight. Because the mat fibers are only held with a thin, dry resinous binder, many loose strands may be apparent. This is not important and does not mean that the mat is defective. Even small holes and voids can be tolerated in the mat without detriment, as once the mat is saturated, it can be pushed and shoved around to some extent to cover over these areas. So don't be too hard on your supplier if the mat you receive looks a little like it's falling apart; that's the nature of the stuff.


Woven roving (often abbreviated "W.R.") is a coarse, heavy, open weave fabric that resembles cloth fabrics, but usually does not have a selvaged edge. Because of this, there is a tendency for the rovings to pull apart and away from one another, especially along the edges. A few loose strands won't affect the material, but care should be exercised in handling and use so as not to pull the material apart.

The square weave type is commonly used in boat work. Woven roving is made from rovings that consist of continuous strands of glass fibers grouped together to form untwisted yarns or rope-like structures. These rovings are arranged into a lattice or basket-type weave to form the material.

Woven roving is categorized by WEIGHT PER SQUARE YARD, ranging from approximately 14 ounces to 40 ounces. Commonly used weights of woven roving are 18-ounce and 24-ounce. Widths are usually 38" to 50" and the material is commonly sold by the running yard or by weight. Woven roving gives strength to a laminate at a cost lower than cloth fabrics, which makes it an important element in fiberglass boatbuilding. But because of its coarse appearance, it is not used where appearance is important, since the weave cannot be concealed without an excessive amount of resin build-up. Furthermore, woven roving is more difficult to wet out than either mat or cloth, and all these elements make it not as suitable or popular for most sheathing applications. The material is often used in alternate layers with mat for fiberglass boatbuilding laminates.


Cloth, mat, and woven roving are the three primary materials that this book is most concerned with since they are the most common and readily available materials, and their cost is favorable. For sheathing plywood boats, cloth is used almost exclusively. However, there are other fiberglass materials or combinations that may be available which will be mentioned to inform the reader.

One such product is a combination of mat and woven roving made into one material and commonly used in production fiberglass situations where plenty of labor is available. The primary advantage of this product is that it can save time. "Fabmat" and "Stitchmat" are trade names for specific products of this type. The latter material is held together with stitching instead of the usual binder, and thus is said to be compatible with epoxy resins as well as polyesters. However, because of the weight of this product and resulting difficulty in wetting it out by the individual worker, plus the fact that its properties are seldom needed in most sheathing work, it is seldom used.

Other materials include the more sophisticated, and usually costlier, unidirectional, bi-axial, and tri-axial rovings sometimes referred to as "exotic" materials. While these can and are made from all-fiberglass yarns, "hybrid" materials are also available which include other types of reinforcements in them, such as Kevlar and carbon fiber.

The main difference in these specialty materials as compared to conventional woven rovings is that the fibers are usually non-woven and the strands are oriented into distinct and specific directions. This is done so that strength properties can be oriented in specific directions. Because the materials are non-woven, the laminates made from them will be thinner and lower in weight, but higher in strength on a weight basis. For this reason bi-axial material is often used for reinforcing the planking junctions of high-speed stitch and glue boats.

However, these materials are not really intended for sheathing work. Their specific directional strength properties are usually not necessary in the typical sheathing application. However, they could be used for localized reinforcement, e.g. to reinforce seams, and then covered with fiberglass cloth for better appearance.

More information about Fiberglass Materials