I used my own house as an experiment to determine what is the best method to re-roof a house is it better to remove the shingles down to the sheathing of the roof or go over the existing shingles if there is only one layer of shingles. This is what I will discuss in this article.
On the roof of my garage I removed the shingles down to the roof sheathing and installed new no tab shingles on the roof at the time if installation they were called JET shingles over 30 pound felt paper. That will be 35 years ago this July that I did the garage roof and it should be re-shingled this year.
On the house roof the west side I removed the shingles up 6 feet to the sheathing, then installed Ice and snow shield on that portion of the roof I left the remainder of the roof with that one layer of shingles on it and re-shingled the roof with 3 tab shingles. That work was done 15 years ago this summer.
The rest of the house I just re-shingled over the shingles that were there using 3 tab shingles, with no tear off. Now 15 years later the house roof has to be done again
After my 35 year experiment on my property I can tell you that ripping off the roof although it is the hardest way to install the a new roof it is also the best way my garage roof lasted 35 years compared to only 15 years when I did the roof leaving the old shingles on and going over them.
More over when I only removed the bottom 6 feet of roofing and installed ice and snow shield this protected the lower part of the roof and the shingles in this area have another 10 years of life left to them, but the roof will not look good leaving the lower 6 feet and re-shingling from there up.
Since I started this experiment on my property the code in our area has changed. The first 6 feet of the roof has to have ice and snow shield on it along with any rake 3 feet of ice and snow shield has to be installed. The remainder has to be at least 15 pound felt, but preferably 30 pound felt. The valleys also need ice and snow shield installed another area that we install ice and snow shield is around vent opening and skylights.
Flashing should all be changed when you do a new roof along with the counter flashing. All boots should be caulked under the flanges in such a way that water will run away from the penetration.
As far as roof ventilation goes that is for another article to discuss this item.
So in closing I am a strong believer of always stripping a roof and installing all new shingles preferably architectural shingles with ice and snow shield up the roof at least 6 feet and if the roof has a pitch of less than 6/12 the Ice and snow shield should go up to at least 9 feet. I would like to hear any comments that anyone would have on this article.