In Lansing, our building code requires that you install no more than two layers of roofing material on your home. If you already have two layers, it's an easy choice. But what if you only have one layer of roofing installed? We are presented with this question a lot, and you always have to ask yourself a few questions before making a decision.

Are the Current Shingles in Good Enough Condition to Install New Shingles on Top of Them?

If your shingles are curling or covered in moss, you will probably not want to install new shingles over top of them. When your new roof is completed, you want it to seal down nice and flat. Any imperfections in the layers below could show in your new roof overtime.

Are There Any Leaks in the Existing Roof That May Have Caused Damage to the Structure?

If you install new shingles on top of the existing shingles, you will not be able to see any wood repair that may need to take place. Rotted wood not only weakens the overall structure, but can be a breeding ground for insects. You always want to make sure you take care of any damaged wood when investing in a new roof.

Is the Current Decking up to Code and in Good Condition?

Even if there are not any leaks in the roof, your decking may still be lacking. Over time, the glue in the plywood can break down if the roof is not ventilated properly. This process is called "delamination," which can be very detrimental to a roof and can sometimes only be seen when you tear off the existing roof.

Is Ice and Water Barrier Protection in Place to Prevent Future Damage?

Installing ice and water barrier on eave edges and in valleys has not always been in the Michigan Building Code, so some of those old roofs may not have what you need to protect from the elements. The only way to appropriately protect yourself for the long term is to remove everything down to the decking and make sure your ice and water protection meets up-to-date code requirements.

Does the Attic Have Adequate Ventilation?

When the original roof was put on, proper attic ventilation may not have been taken into account. Or it’s possible that ventilation technology has changed and needs to be updated. If you have old roof vents, your home may benefit from upgrading to a ridge vent system to create more airflow. This is much easier to do when tearing off the old roof because you are able to appropriately fill the old vent holes in with wood.

Will Adding a Second Layer Affect the Manufacturer’s Warranty?

Of course every manufacturer is different, however, putting a second layer of shingles overtop the existing may very well affect the manufacturer’s warranty. If you can’t meet the underlayment or ventilation requirements, it’s possible that it could drop your warranty down to 10 years or lower on the life of your shingles. Removing the existing roof and installing a new one will ensure that you can meet the proper requirements to maintain the roof’s integrity and warranty restrictions.

Summary

Even though you are able to put a second layer of shingles over the existing layer, it doesn't always mean that you should. The best roofing job you are going to get is one where you are able to take a look at all aspects of the roof, make sure they are in good condition, meet updated code requirements, and enjoy an unwavering warranty.