Rotten Window Sill Replacement Cost (Ultimate Guide)

Rotten Window Sill Replacement Cost

If you see rot on your window sill, it’s the start of a future disaster. It may not be noticeable right now, but it would spread to the other parts of the window and compromise its security and strength.

You need to know about rotten window sill replacement cost because it can be a bit expensive depending on the amount of damage that it has.

If you are planning to have your rotted window sill replaced, there are a few things that you need to know.

Rotten Window Sill Replacement Cost

Reasons Why your Window Sill Rot

Before we proceed to the frame replacement process and the actual cost of the task, you need to understand the reasons why your window sill rots. The goal is to delay the damage as much as possible so if you know the root cause of rotting, you won’t have to spend too much on the replacement.

1. Excessive Moisture

Moisture is the main culprit in the corrosion of your window frames. Well, moisture is not directly causing the rot, but it promotes the growth of fungi, which is the main reason why your window frames rot.

The rotting of your windows will take time so if you notice a specific spot that is starting to rot, it means that the damage is starting to spread. The first thing to check is the dampness of the frames. You will notice that rotting window sills are damp when you touch them.

There are a few reasons why your window frames become damp:

  • If the window frames are placed in an area where they are exposed to the elements, you can expect them to become damp.
  • Dampness can also be caused by your air conditioning system, especially if it is installed directly above your window. The water from your air conditioning unit will keep the window frames damp and it would promote the growth of fungi.
  • Failure to paint your window sills regularly can also cause water to seep in. Once water seeps into the wood, it can lead to the development of fungi.
  • If the wood is not properly dried before they use it to create your window sill, you can expect the wood to retain some moisture inside. This will lead to rot.
Excessive Moisture
Image Credit: malvern_windows

2. Inadequate Ventilation

Inadequate ventilation can raise the humidity in your house, which would also increase the levels of moisture. Although it cannot directly cause rot or damage to your window frames, this is the root cause of the problem.

With more moisture inside the house, your window frames will start absorbing more water. The water content in your window sill should not go beyond 17%. Some experts say that the moisture content of your wood should only be 9 to 14 percent.

If it goes above 20%, it will promote the growth of fungi. Anything above 17% on the other hand can cause the paint to peel off. Use a wood moisture meter to make sure that your window sill does not absorb too much moisture.

Make sure that the area around your window frames is well-ventilated. This would prevent moisture from seeping into the wood and fungi will not grow in them.

3. Your Wood is Old

Aging can happen no matter how you maintain your wood. Even if the wood is well-treated, you can expect it to rot in a few decades. You can delay it through routine maintenance, but it will eventually come to a point where the wood will start rotting.

If your home is over 10 to 20 years old, you can expect the wood to rot slowly. With proper maintenance, your window frames can last for more than 20 years. Some would even reach 40 to 60 years. It will all depend on the type of wood that you used and how you maintain it.

Your Wood is Old
Image Credit: malvern_windows

4. Improper Treatment of Wood

Wood undergoes a lot of different processes before they are sold. They dry the wood to reduce the amount of moisture inside it. This would make it lighter and less prone to rot.

Once the wood is completely dried, it will undergo different treatments as well. It will include biocide treatment. They might also waterproof the wood during the process.

If there were problems during the drying and treatment of the wood or the manufacturer decided to skip a specific treatment, you can expect it to rot in just a few years. This is the reason why you need to be very meticulous when choosing wood for your window sill.

5. Improper Maintenance

Your window frames are probably rotting because you have no idea how to properly maintain them. You need to check the caulking in your window frames and the paint may start to peel. You will notice that dust will start accumulating as well if you don’t clean it regularly.

Failure to address any of the issues above will cause the wood to rot early. If you failed to handle these issues, it means that you did not properly maintain your windows.

  • Once the caulking in your window frames starts falling out, water will seep through the spaces and the moisture level of the wood will increase. This would lead to fungal growth, which would result in rot.
  • When the paint starts to peel, the wood also loses one of its protection against moisture.
  • Dust can damage the wood as time goes by. Simply wiping it will do, but if you don’t clean it regularly, you can expect the moisture to seep in and cause damage.
Improper Maintenance
Image Credit: malvern_windows

How Much Does It Cost to Replace Rotten Window Sill?

Since rotting can happen eventually, you need to understand how much it would take to replace your window sill. There are two ways to know the cost of window frame repair and replacement:

  • DIY
  • Hiring a Contractor

Let’s find out how much it would cost to replace your window sill by doing it yourself and by hiring a professional.


You need certain supplies if you want to do the replacement on your own. Let’s say you already have the tools to do this job, you only need around $75 to $90 for the supplies. This is the cost of supplies per window.

Take note that the cost mentioned above is only for straightforward replacement. If the window sill is completely damaged, you might require other tools and supplies for the removal of the damaged frame.

There are a few tutorial videos with detailed instructions on how to replace wooden frames. You won’t have to worry about the repair costs because you will handle the labor.

Hiring a Contractor

If you have no idea how to replace a sill on your own, the best choice would be to hire a contractor. The repair cost will be based on how it is attached to your house, the material of your window, the amount of damage that it has, and the contractor that you’re going to choose.

Contractors can ask for hourly pay or they can ask for payment per window.

  • Per Hour

On average, contractors charge around $65 per hour. This is a good choice if the damage in your window sill is not too severe. The repair can be as simple as cutting the area in half, removing it from its position, and installing a new sill. This can be completed in just a few hours.

However, there are problems that will usually require more hours, especially if the window requires more tools and effort. There are instances where the whole process would take 8 hours or more to complete, especially if they are dealing with old windows.

The average price of a single window is $150 up to over $500 depending on how long it would take to repair it.

  • Per Window

You can also find contractors that would ask for a fixed rate regardless of the job size. They would usually ask for $50 to $200 per window. This is a pretty good deal, especially if your windows are very old. They would cost more if you hire a contractor by the hour.

However, the repair cost can go up depending on where the window is located. If it’s located on the first floor, you won’t have to spend too much. If it’s on the second or higher floors, you will have to spend $250 to $350 per window if you hire a handyman.

If the rotted wood goes beyond the sill, you might need a carpenter or a window installation company to fix the problem. The full replacement will cost more than what a handyman would ask for.

It’s better to ask for free estimates before you proceed with the task. These free quotes for replacement windows can help you figure out which contractor offers the best price for the job.


Window repair and installing a new window sill would cost a lot of money if you hire a professional. If you do it yourself, you will need a lot of tools and there’s a chance that you would fail to properly install the new wood window sills.

Remember that it would cost more if you mess up with the repair and replacement. If you have no confidence in doing the job by yourself, it’s better to hire a contractor.

If you have any questions about the repair and replacement of your windowsill, feel free to comment below.

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