New roof over the top

You can roof over the top of your existing roof. But there are some roofing things you need to know.

If the existing roof has only 1 layer, lays flat, and there are not any problems with the plywood, removal is not necessary.

In contempt of whether you overlay or tear off, you should properly ventilate the attic. Almost always, continuous soffit and ridge vents will provide perfect ventilation.

If any of the following are true, a complete tear-off of the existing roof system should be reviewed.

• Too many layers – If the roof has more than 1 layer of roofing (shingles, shake, slate etc.), the roof should be torn off. In most cases this is a code requirement. Codes do not permit more than 2 layers of roofing. To determine this, you need to contact your local roofing inspections organization.


• Bad decking - if spongy areas are discovered when walking on a roof, or if you see sags between the trusses or rafters, there is a chance that some of the plywood may be worsen or be damaged. Deterioration can be attributed to dry rot or delimitation of the plies in the plywood due to glue failure. Dry rot is wood rot caused by certain types of fungi and if it isn't taken care of, it can spread. If there is suspicion of bad decking, then a full tear off should seriously be considered.


• Ice dams - ice dams can become a big problem. In areas where there the average January temperature can be below 30° F, and no ice and water protection membrane is installed on a roof, then a full tear off should be considered. 20-year old buildings with no ice and water protection and no prior problems, can suddenly experience thousands of dollars in damage when a freak cold front hits.


• Incompatible shingles - if a heavy weight architectural style shingle is used to cover a light weight strip shingle such as 3-tabs, then the roof will look good. However, if a light weight shingle is used to cover a heavy weight shingle, the light weight shingles have a tendency to show all the ridges and bumps and won't look good.


• Existing roof is in poor condition - if the existing roof is in bad shape, such as tabs being severely curled or if the rows are crooked, then complete tear off and replacement should be considered.


• Shorter Life Span - There are no documented researches, but most roofing specialists agree that with an overlay, the average lifespan of the shingles will be shortened by about 10%-20%.

If you don’t like the color of the shingles you can paint them but it's not advisable. Shingles lose their granules over time. What you can easily wind up with is a splotchy looking roof. Paint them every few years and you will be okay. Another thing, shingles are made so that there are variations in coloring. With a painted roof, there will be no variations. It will be all one color.

What type of shingles you choose to use is up to you. The fiber glass shingles are easier to work with. The organic shingles have more asphalt and longer lifespan but it’s more expensive to manufacture.

The minimum slope that is needed to install asphalt shingles is higher  than 2:12 which means two inches of vertical rise for every twelve inches of horizontal run. Anything between 2:12 and 4:12, you should consider installing two layers of #15 felt underlayment, reducing the exposure of the shingle - the exposure being the part of the shingle that remains visible after installation, or both. In lieu of both of these, you may want to install a self-adhering ice and water underlayment over the entire deck surface.

Best regards,
You roofing expert