1708 Biaxial Fiberglass Explained: What the Numbers Mean

If you've shopped for fiberglass reinforcement, you've probably seen "1708" or "1208" biaxial listed. These numbers aren't random—they tell you exactly what the fabric is made of and how strong it is.

Here's what you need to know about 1708 biaxial fiberglass and when to use it.

What Does "1708" Mean?

The numbers decode like this:

• 17 = 17 oz/sq yd of biaxial fiberglass (the structural layer)
• 08 = 0.75 oz/sq ft chopped strand mat backing

So 1708 biaxial is a 17 oz biaxial fabric with a 3/4 oz CSM backing stitched to one side. The total weight is approximately 24.5 oz/sq yd.

Similarly, 1208 biaxial is 12 oz biaxial + 0.75 oz CSM backing.

We also carry a 17oz biaxial fabric without the stitched in mat as well as the 0.75oz chopped strand mat on its own. 

Why the CSM Backing?

The chopped strand mat backing serves two purposes:

1. Better adhesion with polyester/vinyl ester resins – The CSM binder dissolves in styrene, creating a strong chemical bond to existing fiberglass surfaces
2. Easier wet-out – The mat helps hold resin against the biaxial fibers during layup

If you're using epoxy resin, the CSM backing won't bond as well (styrene-soluble binder doesn't dissolve in epoxy). For epoxy projects, you may want plain biaxial without the CSM backing, or use it CSM-side-up away from the epoxy bond line.

Biaxial Fiber Orientation

The "biaxial" part refers to fibers running in two directions at ±45° angles. Unlike woven cloth (0°/90°), biaxial orientation provides:

• Superior shear strength – Resists twisting forces
• Better impact resistance – Fibers deflect energy at angles
• More uniform strength – No weak bias direction like woven fabrics

This makes biaxial ideal for structural applications where loads come from multiple directions.

1708 vs 1208: Which to Choose?

Use 1708 When:

• Rebuilding transoms
• Reinforcing stringers
• Major hull repairs
• New construction requiring maximum strength

Use 1208 When:

• Secondary bonding and tabbing
• Lighter reinforcement needs
• Tighter curves (conforms easier)
• Weight is a concern

How to Work with 1708 Biaxial

Cutting

Use sharp scissors or a rotary cutter. The stitching can fray, so cut cleanly and handle carefully. Some fabricators tape the edges before cutting to prevent unraveling.

Layup Tips

1. CSM side down for polyester/vinyl ester – The mat bonds to the surface
2. CSM side up for epoxy – Keep the mat away from the primary bond
3. Wet out thoroughly – Work resin through both the biaxial and CSM layers
4. Use a bubble roller – Essential for removing air from the thick laminate
5. Don't over-saturate – Target 1:1 resin to fabric ratio by weight

Resin Compatibility

• Polyester – Excellent (CSM backing bonds well)
• Vinyl Ester – Excellent (CSM backing bonds well)
• Epoxy – Good with caveats (orient CSM away from bond line)

Common Applications

Boat Transom Rebuilds

1708 is the go-to material for transom repairs. A typical layup might include 3-5 layers of 1708 biaxial to rebuild the structural core, providing exceptional strength to handle outboard motor loads.

Stringer Reinforcement

When tabbing in new stringers or reinforcing existing ones, 1708 biaxial creates strong, durable bonds. The ±45° fiber orientation handles the flex and vibration that stringers experience.

Hull Repairs

For structural hull damage, 1708 biaxial provides the strength needed to restore integrity. It's often combined with chopped strand mat for complete repairs.

1708 vs Woven Cloth vs CSM

How does 1708 biaxial compare to other reinforcements?

For more on choosing between materials, see our guide on Biaxial vs Chopped Strand Mat.

How Much Do You Need?

Calculate your material needs:

1. Measure the area (length × width)
2. Add 10-15% for overlap and waste
3. Multiply by number of layers

Resin estimate: Plan for approximately 1:1 ratio by weight. One layer of 1708 over 10 sq ft needs roughly 1.5-2 lbs of mixed resin.

Shop 1708 Biaxial Fiberglass

Ready for your project? We stock 1708 and 1208 biaxial in various roll sizes:

• Shop Biaxial Fiberglass →
• Chopped Strand Mat →
• Epoxy Resin →

Questions about your layup schedule? Contact us – we help customers plan structural repairs every day.

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