Roof-Over Vs. Re Roof (Roof Replacement)
“We have two sets of shingles on our roof in Denver and it’s time to reroof again. Is it possible to put a third set of shingles on top of the present two or would we have to take off the old shingles to put on a new roof? I can save about $2000. if we do not tear off the old shingles. Some say tear it off down to the wood. We have a roofing bidder who tells us we will be wasting money to tear off the old roof. Will the layers be too heavy if we put shingles over what we have?”
You’re going to hear all sorts of advice both pro and con depending upon your sources but as far as I am concerned roofing over an old, worn out layer of roofing is never a good idea and going over two layers is really out. The roofer with the high bid to tear it off is telling you by his bid that he doesn’t want to do the job. Some will tell you that’s it’s OK to roof over .. up to two layers … and that you’ll save some money by avoiding the extra labor of the tear off– which is true, partly. Others will try to strengthen their argument by blathering about obtaining some additional attic insulation quality by leaving an extra layer of shingles on that is pure nonsense. Insulation is above the ceiling not below the shingles.
I’ve heard the argument about the added weight to the roof system that a additional layer of roofing would impose which is also nonsense as a layer of modern roofing material weighs less than 2 pounds per square foot whereas roofs are designed to carry over 10 times that amount. The reason most areas only permit two layers of roofing has more to do with nail length than with weight. If you tried to put a third layer on, the nails you’d need would have to be over two inches long to go through all that old stuff to get down to the wood roof sheathing.
The new roofing material will never lay as flat as it would if laid directly over the wood roof sheathing and it doesn’t look as good as a single layer. Some roofers will strip off the weathered and curled tabs of the old roof shingles they are going over in an attempt to get the new roof to lay flat but it never really does.
Along with the not laying as flat, roof-overs tend to blow off during high winds much easier than a single layer. I know someone who lives on the water and has a roof-over who goes out and picks up pieces of his roof in his yard every time the wind blows out of the North or West greater than 25 miles per hour. One day he’ll bite the bullet and tear both of them off.
Most roof-overs I’ve seen have poorly tied-in flashings at chimneys, dormers and plumbing stacks which end up leaking prematurely. And lastly, when it comes time to replace the roof over itself, you have to tear the whole mess off down to the sheathing at which time you end up paying for that extra labor that you thought you avoided when the additional layer went on. It’ll cost you more to get rid of all those old shingles too. That’s called a tippage fee at the land fill that will accept them.
Common three tab roof shingles are triply redundant which means if you were to core a section out of the roof you’d see three layers of material. The first layer is the tar-paper or felt as it is known by roofers. It has a couple of functions. The first is to add another layer of moisture barrier and secondly it does not permit the roof shingles to stick to the roof sheathing over time making future removal a real chore. There are lines printed on the paper to aid in shingle placement but most good roofers snap their own lines with chalk as a guide to keep the job straight and professional looking. The proper method of installing shingles are printed on the wrapper of every bundle of shingles sold. Amateurs never follow them and it shows.
Next comes successive shingles laid one over another with about 5 inches of exposure. When the roof gets old the exposed layer gets real curly and starts breaking off pieces of shingle tab alerting that the roof is getting towards replacement. Shingle roofs can look pretty ratty before they actually start to leak due to the redundancy and when they do start to leak it’s usually along the edges or where the roof has been hit with a falling limb that damaged just one shingle. Hail can damage shingles and completely ruin even a new roof.
Plan to have your old shingles torn off and hauled away. You’ll then be able to check all of the roof surfaces and make any repairs necessary as well as putting on new flashing at dormers, chimneys and plumbing vent stacks plus drip edge. Add anti ice-damming material, called ice and water shield, at the edges to avoid leakage from roof edge ice build-ups during winter. It’s actually in the building code around here but many roofers don’t do it to keep their prices competitive and they’ll be long gone when it does leak. It’ll cost you a little more but you’ll be glad you did it in the long run.
For some reason, which may have to do with summer heat retention, the top roof-over layer of shingles doesn’t seem to last as long as a single layer so there is a reduction of working life.