Testing the fit of a filler piece. Garage Remodel:

Repairing Shallow Rot Damage In Solid Wood Siding

In This Article:

A rectangular cut is made in solid wood siding, and a new piece of wood is glued and screwed in place.

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Skill Level: 3-4 Time Taken: About 1 Hour

By , Editor

While remodeling and repainting an old garage, I found one small spot where the redwood siding had some rot damage. Luckily the decay was shallow, just over half of the way through the 3/4" thick board.

One repair option was to cut away a section of the siding long enough to span between adjacent studs, and replaced it with a short piece of new siding. While I did have some new siding that was a close match, I decided to try to graft a piece of treated pine onto the damaged area.

The section of rot damage.

I had already power-scraped the paint from the siding and given it a coating of primer.

Redwood siding with rot damage.

I could have filled the entire void with some sort of filler compound, but my experience is that neither epoxy nor polyester fillers work well for large and deep voids. The stuff will droop out of place before it hardens, or I won't be able to get a uniform thickness and I'll have to apply a second coating, plus there can be lots of time spent sanding down the lumps to make the patch look flat like wood.

Even if I managed to get a good-looking patch from filler compound, there's a good chance that it will fall off someday.

I poked at the wood to make sure the rot didn't go all the way through. 

Even if the rot did go all the way through, I could still make a patch, but my approach would be different.


I used a cat's claw to remove the nails from the area.


To prevent the heel of the cat's claw from making a dent in the wood, I placed a putty knife beneath it.

I laid out the 3 sides of the patch with a pencil and a speed-square.

I made a series of shallow cuts with a circular saw. I used a cordless saw with a 5½" blade, but a regular circular would have worked just fine.

I like this tool because it doesn't kick out as much dust as full-sized saws. I can get my face closer to see what's happening.

Cutting out rotted wood with circular saw.


Series of saw cuts made to weaken the wood for removal. My initial thought was to make just one cut to define the upper edge of the graft, but then I realized that I could make a series of parallel cuts that would weaken the wood I wanted to carve out.


I used a sharp 1 inch wide chisel to slice away the thin strips of wood that remained. Removing rotted wood with chisel.


Usually I would be able to hammer the chisel this way to cut across the grain, but this old redwood was really hard.

I noticed the hardness of the siding before I started working on the garage, and it led me to believe the siding was Douglas Fir. Only after power-scraping an entire painted wall did I realize that this was redwood. The near-complete absence of knots was one major symptom, besides the red color.


I used one of my secret weapons: The Poor Man's Roto-Zip. This is a Dremel Moto-Tool with an accessory base plate that enables the tool to make cuts of a precise depth.

The adjustable base plate (Sears Craftsman, about $20 including two cutting bits) just screws onto the end of the rotary tool, after the nose piece is unscrewed.

These tools accept 1/8" diameter cutting bits, which is the standard size Roto-Zip bit.


Using Dremel tool to cut square hole in wood siding. I set the cutting depth and made vertical cuts along both layout lines.


I used a chisel to clean out the bottom of the "excavation".

My cuts were about ½" deep, so they went two-thirds of the way through this siding, which is 3/4" thick.


The completed cut. I don't know if this qualifies as a "mortise".

Since this is tongue and groove siding, the tongue of the board below was exposed.

My patch will need special preparation to handle this.

Note: Since writing this article, I have purchased a Fein Multi-Master, which is similar to those inexpensive detail sanders, but it also has scraper blades and (the most useful) a fine-toothed saw blade. Since the blade rotates back-and-forth only a fraction of a degree, it can make a square-sided plunge cut and quickly accomplish what I did above with a moto-tool and a chisel. At $200 plus, it's not cheap (hey, it's made in Germany) but it's worth the money just for the saw feature.

Cutting The Patch:

Using a circular saw I ripped a piece of treated pine 5/4 x 6 deck board to the desired width. I had brought a miter saw along, but not my table saw. I cut the board a couple of inches longer than needed, just in case something went wrong during the tricky part.

Splitting The Board Down The Middle:

I placed the board on edge on the miter saw. This is a risky cut to make, and I'm sure that the saw manufacturers would "have a cow" if they knew I did this.  

Since the fence has a gap adjacent to the blade, I used a small block against the fence (red arrow) to keep the strip from moving around.

I cut the piece very s-l-o-w-l-y and held on tight. I cut one end and then flipped the board around to cut the other end. 

But the cuts did not meet in the middle, so I finished the cut with the circular saw. A hand saw would work just as well for this final cut.

In fact a hand saw would work just as well for the entire cut, but it would take a while. If I had planned my project better I would have made this cut at home on the table saw, or I could have purchased a ½" thick piece of hardwood at Home Depot.

The filler piece after being cut to the desired thickness.


Remember that I mentioned the tongue from the siding below?

Using a circular saw with a ripping guide (red arrow) I made a couple of cuts to form a rabbet (rectangular notch) in the back edge of the filler.


The profile of the rabbeted filler piece.

Again, this would have been easier to cut on a table saw. I think I need to buy a small table saw to keep in my truck. Hmmm...


Installing wood block in notch in wood siding. After I had cut the board to width, I used the miter saw to cut the board to length. I often start with an extra-long board, just in case I muck up one end or something.

I tested the board to see how well it fit the hole.


I pre-drilled a hole at each end, and drove in a 2" deck screw, just to make sure the filler board would lay right without rocking too much.

It might have been better to drill two holes at each end, which would have given me better control in making the piece lay straight. Luckily I didn't have any problems. 


I applied a hearty dab of urethane glue. It's important to use outdoor-rated glue for something like this. Ordinary carpenter's glue will hold fine... until the first time the wood gets wet or really damp.


I installed the filler board, drove in the screws, and let the glue dry.

This urethane glue takes 4 hours to cure. It oozes foam while it hardens, which helps to fill the gaps.

Once the glue was dry I sanded the area with a belt sander.

Sanding wood patch with belt sander.


Filling small holes in siding with epoxy putty. I applied some fast-setting polyester filler to the small voids around the filler. 

This is essentially "Bondo" auto body filler, though it's marketed as a wood filler. It hardens in about 10 minutes and doesn't shrink.

I've also used WoodEpox from Abatron, but it takes a lot longer to harden.


I covered the bare spot with oil-based primer.  Covering patched wood siding with oil-based primer.


The next day I "painted" the wall with oil-based solid-tone stain. It's actually a tan color, though it doesn't show up very dark here.

It's impossible to see the patch unless you get really close.

This is the garage after all  the trim was replaced and the painting was completed



Tools Used:

  • Cordless Drill/Driver
  • Basic Carpentry Tools
  • 1" Chisel
  • Rotary Tool With Base Plate
  • Circular Saw
  • Miter Saw
  • Belt Sander


Materials Used:

  • Treated Deck Board, Scrap
  • Urethane Glue
  • Deck Screws
  • Primer


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Written February 5, 2003