I thought this addittional information might help you understand the facts.
The ROOF-OVER Professional consulting available.
"ROOF-OVER", type roofing: the poor man’s roof. “Roofing-Over” is the practice of installing, nailing or placing new roofing directly over old roofing material, to avoid the rip-off and the disposal of the old roofing.
For the customer willing to trade approximately a 10% cost savings for a roof with rough appearance, a potentially short roof life, the cost of additional venting and a doubly difficult tear off and disposal in the future; “roofing-over” is a manufacturer approved practice. This applies to “soft” roofing only, (asphalt shingles, roll roofing, bitumen, and modified bitumen “flat” type roofing). This process is most often done on sloped (shingle) roofs, but it is also common on “flat” type roofing. An increasingly common practice, this procedure became popular in the early 1970’s in the USA. With many such “roof-over” applications in place, there is now a growing body of experience and information centered on this abbreviated approach to “re-roofing”.
An increasingly common practice, this procedure became popular in the early 1970’s in the USA. [size=3]
It is generally assumed that only an aging roof in sound condition should be “roofed-over”. As long as the old roof is not literally disintegrating, some consider it a candidate. Extreme curling and cupping of the shingles in the case of “flat” roofing, deep material or very uneven materials makes “roofing-over” difficult but not impossible. The first step in “roofing-over” is to remove excess damaged parts, so as to “level” the roof. On sloped roofs, the top peaks or ridges have their “cap” shingles removed, as do any hip ridges (the top of the downward sloping intersection of adjacent “sloped” roof planes). Areas that are not severely damaged may require some simple patching with roofing material and roof cement. At this point, the installation may begin.
On sloped roofs, the actual installation of shingles begins with trimming. A starter shingle cut lengthwise down the middle leaving a 5 inch wide strip which is nailed through and over the bottommost row of shingles; followed by a full shingle with its top edge butted against the lower edge of the old row of shingles above it. After this, the shingles are nailed over the rest of the roof in the same manner until the old roof is covered. At the top, or the ridge, again new shingles are cut, or trimmed, lengthwise to run even with the ridge and a new set of “cap” shingles are cut and installed to cover the peak.
The finished “roof-over” more often than not, will have a rough appearance. This is a reflection of the physical condition of the underlying deteriorated roofing.
Of the many factors that affect the service life and durability of a “roof-over”, is the length of the nail, or other type fastener, which is used. Depending on the quality, weight and type of shingles, or roofing material, used on the original roof, the mechanical fasteners for the “roof- over” (nails or screws for some “flat” roofs) should be one half of an inch to three-fourths of an inch longer than the fasteners in the original roof. This means a one and one half of an inch to two inch roofing fastener is generally used on a “roof-over”. Since, in the case of sloped roofs, the original shingle provides little support for the portion of the nail passing through it, these longer new nails can suffer lateral, downward “pull”. “Nail-pops” caused by expansion and contraction of the underlying material can pull a nail head through the new material thereby loosening it, or push a nail head through the material above causing a hole in the shingle (a guaranteed leak).
Another factor is the continuing deterioration of the original roof. Three basic and interacting multi-factor processes are introduced to the new combination old roof/new roof lamination termed collectively as the “roof-over”.
Asphalt shingles, asphalt “flat” roofing, and modified bitumen products are manufactured from an amalgamation (a mix) of petroleum products ranging from tar-like solids and semisolids to liquids (oil-like) and volatiles (solvent-like). These substances diminish over time by evaporation, solidification (embritlement) and by a photochemical process whereby direct exposure to sun breaks down the asphaltic compounds into carbon gas (carbon dioxide, carbon monoxide, methane, etc.) and carbon solids (black, dirty stuff). Once covered, the old shingles, or “flat” roofing material continues to break down by solidification (congealing) and evaporation of the volatiles.
When the roof (of the “roof-over”) needs to be replaced, all layers of roofing must be removed and hauled to a dump. [size=3]
Trapped between the new shingles, or “flat” roofing material, and the underlying roof felt and/or roof deck (usually wood or plywood), the old shingles gather and hold moisture. During the hottest days with direct sunlight, this moisture steams off cooking the new shingles, or asphaltic “flat” roofing. In freezing weather the moisture turns to ice, swelling and tearing or breaking up the old underlying material. These combinations of actions caused by gathering and releasing moisture also involve expansion and contraction of the old roofing.
So, with the continued deterioration of the underlying the old roofing material by 1) material breakdown 2) steaming, and 3) expansion-contraction; the new roofing experiences added stress from heat, mechanical movement (dimensional change from expansion-contraction) and loss of support by deterioration/degradation of the decaying roofing material.
In the unusual case where the new asphalt shingles are installed over a slate, wood shingle, asbestos- cement shingle, or synthetic shingle, life expectancy, performance and secondary affects (side effects) are unpredictable. Such installation and their related problems are beyond the scope of this article, however, needless to say, such practices are not recommended and are rife with negative considerations.
An odd variation on the “roof-over” is the installation of plywood directly over the old shingles followed by roofing over the plywood. There is too little experience with this approach to provide knowledgeable information. It is known that moisture is trapped in the old shingles. This suggests the possibility that some of the types of moisture related problems, mentioned earlier, are likely to occur.
If you are buying a building that has a “roof-over”, the primary consideration is that when the roof (of the “roof-over”) is worn out and needs to be replaced, all layers of roofing must be removed and hauled to a dump site or disposal point. The expense saved on the “roof- over” is more than exceeded. In other words a “roof-over” transfers the cost of trash disposal to the next “re-roofing” operation. “Three-roofing” or roofing over the “roof-over” has been done and even appears in some literature; however, such practice should be relegated to cartoons and comic books. The results of actual triple roofing practice range from deceptively acceptable to obviously and immediately defective.
There are a number of special considerations when doing or taking possession of a “roof-over” roof. Because of the moisture trapped in the old roofing, and the additional heat trapped in the extra mass, a “roof-over” roof requires additional ventilation --an added expense-- to help reduce accelerated and premature aging of the new roofing. (Which includes bubbles, and lumping in “flat” type roofs.)
Another consideration is the added load (weight) of the combination of the old and new roofing. The added weight may necessitate additional structural roof bracing. (In one instance of the author’s experience involving a one-story cottage, the added weight actually snapped the horizontal cross-ties, and ceiling joists, “flattened” out the roof, bowed the front and rear exterior walls literally ripping the exterior walls away from the interior partitions – virtually destroying the cottage. In another case an 1880 vintage sloped roof (114 years old at the time) suffered lengthwise splits in its (unsupported and unbraced with no cross ties, which were not needed in the original construction) rafters. One more heavy snow load on this roof, and collapse was likely.
With “flat” type roofing, an older roof generally provides a poor surface for the new installation, especially if the newer roof requires total adhesion. Usually when roofing over a “flat” roof, a non-adhesive mechanically fastened (nails of screws) approach is far more likely to provide adequate performance.
With both “flat” roofing and sloped, shingled roofs, “roofing-over” works best with considerable additional venting of the roof cavity and/or membrane type vents for “flat” roofs. This additional venting, when done, often offsets the shortsighted financial savings in forgoing the rip-off and dump costs.
There is only one incentive for doing a “roof-over”, to save money, in the short run.
[size=3]Is that what your Codes are for?