How to Install Fiberglass Gutters

Q: Our historic district says our replacement gutters must be wood, but we’d like to avoid the maintenance. Any ideas?

— Ashley Guir, Worcester, Mass.

Tom Silva replies: You may want to consider fiberglass gutters like those chosen by the owners of the latest TOH TV project house, in Arlington, Massachusetts. Made and hung by The Fiberglass Gutter Company, they look just like traditional wood, but they’ll never rot or need paint. And because the seams are fused in the field, they won’t leak, either.

Many historic districts in New England have already approved these gutters as replacements for wood. When your district sees them, maybe you’ll also get a green light.

The company ships gutters in 26-foot sections all over the country, so take a look at the steps at right to see if the project is something you’d like to do yourself or hire out.

Pictured: Installer Peter Robinson makes sure the gutter slopes ¼ inch every 10 feet.

Step 1

Attach Stand-Off Blocks

With a 12-inch miter saw, cut 5/16-inch slices from the end of a 5/4×6 piece of cellular PVC trim to use as stand-off blocks. Nail them vertically to the fascia at each rafter location with 18-gauge stainless-steel brads, as shown. Next, snap a chalk line over the blocks where you want the top of the gutter’s back edge to go. It should slope toward the downspout location ¼ inch per 10 feet.

Step 2

Cut Sections to Length

Wearing a dust mask, gloves, and eye protection, cut the gutter with the miter saw. For runs that end at a corner, cut the gutter ends square to match the length of the fascia. Sections that meet at a corner need to be mitered. For each outside miter cut, add the spacer-block thickness to the fascia length. For each inside miter cut, subtract the block’s thickness.

Step 3

Mount the Gutter

Sand the trough at each cut with 80-grit paper. Then, with a helper, lift the gutter into position against the spacers and line up its back edge with the chalk line. Drive a 3⅛-inch stainless-steel structural screw through the back edge, through the stand-off block, and into the rafter. Repeat at each stand-off block.

Step 4

Seal the Joints

To join miters and attach end caps, superglue the pieces together and tape over the joint on the outside of the gutter. Cut two 6-inch-wide strips of fiberglass mat long enough to cover the joint in the trough. Put on an organic-vapor respirator and gloves, and wipe the sanded surfaces with acetone. Brush a mix of polyester resin and gelcoat into the trough and onto the strips until the fibers are saturated. After 30 minutes, snip off any projecting fibers and remove the tape.

Step 5

Set the Outlets

At each downspout location, use a 2½-inch hole saw to cut through the bottom of the trough. Wearing latex gloves, squeeze a bead of fast-cure marine-grade polyurethane sealant onto the flange of the downspout outlet. Then, holding the outlet as shown, set it into the hole, sealant side down. Wait about an hour for the sealant to set before attaching the downspout.

Step 6

Fasten the Downspout

Fit the downspout over the outlet and join them with three stainless-steel sheet-metal screws, as shown. Repeat at each downspout joint. Finally, wearing gloves and a respirator, fill the joints on the outside of the gutter with a gelcoat-resin mix thickened with a microballoon filler. Clean the joints with acetone, then apply the putty with a gloved finger. Wipe up any excess with acetone.