Figure 2-16 re roofing starter course (C) J Wiley, S Bliss Re-Roofing Asphalt Shingle Roofs - Best Practices

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Asphalt roof shingle re-roofing advice:

This article discusses how to prepare an existing asphalt shingle roof for a roof-over or for re-roofing with new asphalt shingles. This article series discusses best practices in the selection and installation of residential roofing.

We also discuss Proper asphalt shingle roof preparation for a roofover job. Roof-over details for multiple layer asphalt shingle roofs.

Characteristics of asphalt shingle roofing materials. Best practices for roofing material installation, flashing, ventilation, nailing, underlayment

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

Reroofing Options on Asphalt Shingle Roofs

Shingles 3 layers (C) Daniel FriedmanAdapted/paraphrased with permission from Best Practices Guide to Residential Construction, chapter on BEST ROOFING PRACTICES:

Reroofing saves the cost, trouble, and risks (water damage while the roof is exposed) associated with a tear-off.

If the roof is structurally sound, most building codes allow for two layers of asphalt shingles and some allow for a third on roofs with a 5:12 or steeper pitch.

Here is a typical excerpt limiting the number of shingle layers on a residential roof to two:

Roofs that already have two layers of 2 shingles should not be roofed again. Remove the existing shingles, then roof with
new shingles.
- FHA Single Family Housing Policy Handbook, cited at REFERENCES

If the original shingles are not badly curled and the sheathing is sound (check for bouncy areas), then a reroof is a good alternative.

Shingle Type Recommendations for Re-Roofing Asphalt Shingles

The heavier the shingle on the new layer, the less likely it is that irregularities in the surface below will telegraph through. Laminated or other heavy-textured shingles work well, as they do not need to be carefully fitted to the existing shingles, and the irregular texture will conceal any small bumps or dips from the original roof.

Prep Work for Re-Roofing Over Asphalt Shingles

When is it Forbiden or even Dangerous to Roof-Over Types of Existing Roof Coverings?

Unsafe roof deck means tear-off (C) Daniel Friedman

[Addition to the original article - Ed.]

The question of when is it necessary to tear-off an existing roof-covering vs. when is roofing-over an option for a leaky roof is one of a sickening amount of argument.

  1. Strip Hard-Roof-Coverings: Model building codes address the easy part of the question: we do not put a new layer of roofing material over rotted, damaged roof decks, nor over wet roofing, nor over hard-material roofing such as slate, fiber cement, clay tiles, or wood shake roofs. [IBC 1510.3].
  2. Limit Number of Roof Covering Layers: Local building departments in many jurisdictions will permit multiple layers of some but not all roofing materials. Most won't permit more than three layers of asphalt shingles on a roof.

    The issues are with nailing security, wind uplift, shorter life of asphalt materials over an uneven surface, effects of moisture trapped between roof layers, and possibly fire and roof covering weight loading safety concerns.
  3. Threshold of Percent of Damage or Worn-Out Roof: Roofing contractors often will opine that if 25% or more of the roof area is at or beyond end of life then it is more-economical and smarter to tear off the existing roof, replace any questionable roof decking (and framing), and install a new roof.

    The contractor won't want to warrant the entire roof against leaks if she is repairing only a limited area of it.

Watch out: Check roof deck condition: if you are stripping roof shingles entirely from a roof deck, either because of their poor and rough-surface condition (not suitable to shingle-over), or because portions of the roof deck itself need to be replaced (perhaps due to rot, insect damage, water damage), be sure that you properly identify the type of roof sheathing used and its condition over the entire roof deck.

Don't even think about roofing-over a structure whose roof deck is in the condition shown in our photograph at left. But a careful inspection may be required to detect more subtle but dangerous roof deck conditions.

Watch out: Life Safety Issues: Walking on an old water-damaged roof with an un-sound roof deck can result in serious injury if a worker steps into a soft area and falls through the roof - a hazard more likely with thin plywood sheathing (on some low-cost homes as thin as 3/8" or even 1/4") and also more likely where the roof is known to have been leaking, or finally, where fiberboard roof sheathing has been used.

Roof deck fall through report: In the 1980's when we [DJF] were working on a roof-over job for a 1960's house we had observed from the building attic that thin 1/4" plywood had been used for roof sheathing and we already had a concern about the nail-holding ability of that material to prevent a roof blow-off.

We had also seen evidence of prior leaks on the north slope of the roof. But we failed to spot badly damaged roof sheathing in one area of the roof. After our in-attic inspection, our worker who weighed the least, D. S., was walking over one slope of this modestly-pitched hipped roof when she stepped right through a section of soft roof decking. Luckily D.S. was not hurt, but a fall off or fall-through injury could have been very serious.

Best Practices Guide to Residential Construction continues:

Starter Course for Re-Roofing Asphalt Shingles

Figure 2-16 re roofing starter course (C) J Wiley, S BlissIf laying three-tab shingles over three tab shingles, it is important to nest the new shingles against the old to create a flat surface as Friedman elaborates just below.

This process starts with a 5-inch starter strip fit along the eaves and set against the second course of existing shingles (see Figure 2-16).

DF Notes:

Notice that in re-roofing the starter strip is an inch less in width than when installing new shingles over a fresh roof-deck, shown in our asphalt shingle roofing guide at ASPHALT SHINGLE STARTER COURSE INSTALLATION

That's to avoid a raised ridge and to let the new full shingle course lay flat atop the original shingles. In essence when I [DJF] installed shingles over an existing roof, for maximum roof life I want the shingles to lay flat - raised ridges in asphalt shingle mean bending that means wear that means shorter shingle life.

So the upper edge of each new shingle course butts against the lower edge of the appropriate existing shingles on the roof above the course being installed. That will let the shingles lay flat on the roof. Only the first course needs support below - that's why we cut a special starter strip of more-narrow width than for a new roof installation.

The special starter strip width is equal to the exposure width (usually 5") of the existing roof.

Even before installing the starter strip or underlayment you should install drip edge flashing along all roof edges.


Next install a course of shingles cut down to 10 inches wide, so they fit against the bottom edge of the existing third course (this creates a new 3-inch first course). After that, shingling should proceed normally, fitting each course up against the bottom of an existing course.

Fastening / Nailing Details When Re-Roofing with Asphalt Shingles

Use galvanized roofing nails long enough to fully penetrate the sheathing, typically 1-1/2 inches for a second roof and 1 3/4 inches for a third. Nesting each new row below an existing one keeps the new nails 2 inches below the existing, which will help minimize any splitting of the sheathing.

Flashings for Re-Roofing Asphalt Shingles

Depending on their condition and accessibility, some flashings can be reused. New shingles may be able to tuck under existing step flashing, chimney flashings, and front-wall flashings. If they are deteriorated, they must be replaced along with vent boots.

See FLASHING ROOF WALL DETAILS and FLASHING ROOF-WALL SNAFU for specifications of roof-wall flashing and for some photo-examples of troubles with existing step and counter flashing when re-roofing an existing structure.

Valley Details for Re-Roofing Asphalt Shingles

Any type of valley flashing will work and simply lays over the existing flashing (except in a tear-off, where all flashings should be replaced).

Unless a metal valley flashing is used, the first step is to line the existing valley with a new underlayment consisting of either 90-pound roll roofing or a more durable modified bitumen membrane. Then install either a closed or woven valley as described above.

See roof valley details at ASPHALT SHINGLE VALLEY FLASHING.

-- Adapted with permission from Best Practices Guide to Residential Construction.

Resources: Roofing Materials & Equipment Suppliers


Asphalt Shingles

Ridge Vents

Venting Underlayments

More Information about Roofing Materials, Methods, Standards

This article includes excerpts or adaptations from Best Practices Guide to Residential Construction, by Steven Bliss, courtesy of Wiley & Sons.

Also see the roofing article links listed at the ARTICLE INDEX the bottom of this article and our roofing home page: ROOFING INSPECTION & REPAIR.

-- Adapted with permission from Best Practices Guide to Residential Construction.


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