How To Fix A Sagging Roof

How To Fix A Sagging Roof


February 16, 2022

You look up at your roof and it seems a bit odd. It’s got curves on it that weren’t there before, and you’re not sure how this happened.

You could be dealing with a sagging roof, especially if your roof is old.

What, exactly, is a sagging roof?

A sagging roof is a roof that doesn’t look straight. Instead, it has a wavy appearance because the roof is old or has deteriorated, so it’s structure is not remaining upright anymore.

There are important things to know about sagging roofs, but don’t panic if your roof is experiencing this problem. Read on to learn more about how to fix a sagging roof.   

Why Do Roofs Sag? 

How To Fix A Sagging Roof

A roof can start to sag for a variety of reasons. These include the following:

  1. Too much weight on the roof. This can be as a result of snow, ice, sheathing, or too many layers of shingles, such as if you installed shingles over existing ones to boost the roof’s appearance. 
  2. Faulty roof installation. If the roof wasn’t installed properly to begin with, this can lead to a variety of structural issues. 
  3. Sheathing that’s too thin. This is usually the cause of a roof sagging between the trusses or rafters. 
  4. Foundation that has moved or sunk. A faulty foundation can cause your walls and roof to shift along with it. 
  5. Water damage. Too much moisture, such as if your roof isn’t directing rainwater away, can cause the roof to experience decay or rot. This can then cause the roof to start to sag. 
  6. Aging. A roof will naturally deteriorate over time. When your roof is very old, this can cause its structure to break down, which will cause sagging. 

Can You Repair A Sagging Roof? 

How To Fix A Sagging Roof

You can repair your own roof, but you need to ensure you know what you’re doing and have experience with roofing repairs. If you don’t you should rather call a professional roofer who will be able to apply their skills and experience to repair your roof so that it regains its good condition.

If you want to go ahead with fixing a sagging roof, here are the steps you should follow.  

Start by checking for damage

Before you reach for your tools, you need to check the roof for sagging and damage so you locate exactly where the problem is.

You can do this from inside the home, in the attic. Look at the roof truss construction and any damage in the form of rot, cracks, and other damage.

Check the metal or plywood gusset plates – these are found where the wood makes contact on a truss – and check the roof rafters as well as decking. The damage you should be looking for includes things like rot, breakage, or cracks that could be the reason for the roof sagging. 

How to fix a damaged rafter or truss: 

  1. You will need an L-shaped channel made of steel to fix a rafter or truss. You’ll also have to drill four holes on each side of it. 
  2. Use a chisel to remove any scraps. 
  3. Then mount the “L” channel onto the rafter or truss. Pro tip: start mounting at the end that’s most difficult for you to access.
  4. Fasten the end of the channel to the rafter or truss by using the holes you previously drilled. 
  5. Now that the end is in place you can move the broken piece into position. 
  6. Seal up the gap between the rafter/truss and channel. Choose bolts that are about 1 ½ to 2 inches longer than the gap. 
  7. Insert and tighten the bottom bolts so the washers are flush with the channel, then tighten the bolt that’s furthest from the crack until the other bolt head is a quarter of an inch from the channel. 
  8. Tighten the back bolt so that it’s close against the channel. 
  9. Also tighten the front bolt so that the back bolt is about a quarter of an inch away from the channel. 
  10. You will have to repeat tightening the bolts until the rafter or truss is securely fastened to the L channel. 
  11. Follow up with inserting bolts into the other holes and secure them well. 

How to fix a gusset plate: 

  1. You should remove the gusset plate that has broken away from the pieces of wood.
  2. Replace it with the use of a half-inch plywood gusset. 
  3. With a nail gun and nails, attach the gusset so that it’s secretly on both sides of the wood. This will replace the original gusset plate’s strength, therefore improving the condition of your roof.

How to fix roof decking:

How To Fix A Sagging Roof
  1. Start by measuring the area of damage between the roof rafters or trusses.
  2. Cut a ½-inch piece of plywood that is large enough for the area. 
  3. Glue a 1-inch piece of rigid styrofoam to the plywood. You want to avoid driving nails into the sheathing as that could make the damage worse. 
  4. Place the plywood and styrofoam against the damaged roof decking. 
  5. Now you want to push up onto the plywood so that the foam makes contact with the underside of the roof decking. 
  6. Then, use a piece of lumbar strapping to secure the plywood until you can have it repaired by a professional roofer.

How to fix a sagging roof beam: 

  1. Attach two chains from one side of the wall to the other with the use of lag screws that have large washers. Using two chains that are spread apart evenly helps to distribute the weight. 
  2. Add a turnbuckle between the chains. This will allow you to adjust the tension.
  3. With the use of a jack-and-post system, jack up the ridge beam. Your two jacks and posts should be set up so that they’re evenly spaced and in the middle, right below the ridge roof beam.
  4. Carefully jack up the roof ridge. Tighten the chains whenever you jack it up.
  5. Repeat this process until you’re done. Make sure that you only jack up the ridge one inch per day otherwise you could damage the structure of the roof. It will take about a week or two to fix the roof.
  6. To prevent the roof from sagging in future, add rafter ties and ceiling joists. These will give the rafters more protection. You can also support the ridge by adding 2×4 boards on top of the joists of the ceiling, but make sure you nail them directly to the ridge. 

How Much Does It Cost To Repair A Sagging Roof? 

If you need to have your sagging roof repaired by a professional, you can expect to pay quite a bit of money for it.

On average, this will be between $750 and $3,000. If it happens that you need to repair your roof, this will increase the costs to $10,000 or more.

The cost to repair a sagging roof will depend on the size of the roof, the materials and tools required, and how much damage has been done. 

How To Prevent A Roof From Sagging

How To Fix A Sagging Roof

Prevention is better than cure! You can take proactive steps to prevent your roof from sagging in future. Here are some measures to consider.

  • Clean your roof regularly. You should check your roof for debris and clear it away ASAP. This will prevent moisture and snow from building up on the roof, and it also prevents moss and algae from forming, both of which can damage the roof shingles. 
  • Clear away ice and snow. When your roof has 12 inches or more of snow buildup, it’s time to clear it away with a roof rake. If you leave the snow on the roof, it will be heavy and put strain on its structure. 
  • If your roof is very heavy, you should consider replacing its materials with lighter alternatives, such as metal or asphalt. 


Dealing with a sagging roof can feel overwhelming. Luckily, understanding the reasons why it’s happened can help you to take charge of the situation.

While you can DIY repair your sagging roof, if you lack experience or you don’t have the tools you need to do the repairs it’s best to call a professional roofer so you can have a roof that’s as good as new in no time.

Reference Articles:


James Weldon is a professional roofing contractor with many years of experience on the job. He does not only handle large projects and provide excellent services for his company’s many clients; James Weldon also dedicates his spare time to teaching others useful tricks of the trade. Using BuildCampus as an avenue to reach many roofers and aspiring roofing contractors, James Weldon continues to provide high-quality educational posts and buying recommendations for anyone who visits the website.