Published on January 13th, 2013 | by Henri


Installing new roofing over old

Q.   While my existing roof shingles are sound, they will need replacement in three to four years. I am a retired builder and would like to do the job myself while I am still agile and steady enough to handle it.

The existing shingles are an architectural grade. They are 33 years old but lying flat. There is a light coating of moss over most of the front side of the house and garage. Can I nail the new shingles over the existing ones? And if so would you advise installing a layer of roofing paper over the existing shingles first?

Should the roofing paper be regular or heavyweight?

Finally, do you recommend a membrane on the lower edges of the roof? My roof pitch is 5 in 12.

A.   Although putting a second layer of shingles over sound existing shingles is an approved technique, there are several considerations that affect the decision to do so. The type of shingles on your roof now are heavy and you would be adding another layer of heavy shingles if you choose architectural shingles again.

Unless your roof framing has been designed to carry such heavy loads you would be putting it under great stress, and you might even risk structural failure in case of a heavy wet-snow year. I realize that removing a layer of shingles and disposing of them is a very difficult and messy job but it’s better than having the roof cave in on you. So you would be wise to have a structural engineer calculate the dead load of your roof framing unless you are confident that you can do the calculation yourself. I am sure that you know that any given size rafters on a shallow or medium-pitch roof cannot carry as much weight as rafters on a steep roof.

However, if you decide that a lay-over is fine, you do not need to install a layer of roofing felt. But, unless you are familiar with the special procedure for a lay-over, be sure to read the instructions for installing new shingles over existing ones. The first two courses are the most critical to obtain a smooth roof without ripples.

The advantage of removing the existing shingles is that you can, and should, install an ice and water protective membrane such as Grace Ice & Water Shield or equivalent on clean sheathing at the eaves, in valleys, around skylights, where a lower roof meets a wall, around chimneys and any other roof perforations.

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