October, 2010
Inspection News and Views from the American Society of Home Inspectors

Don't Miss the Shingle Stagger!


Do asphalt roof shingles really stagger? In my part of the country, they do.

Stagger is a term some roofing contractors use for the shingle offset—that is, the spacing between butt joints of adjacent shingles. Some contractors call it "shingle offset" or "edge-to-edge spacing." I like to call it stagger like the local boys do.

No matter what you call it, maintaining shingle stagger is important to prevent roof leaks and to conform to manufacturers' instructions, keeping the warranty intact.

If the shingle stagger is too small — less than 4 inches — water can travel into the shingle butt edge to the butt edge joint of the shingle below (less than 4 inches away) and leak in. You know what leaks cause: rot, mold and lawsuits. Do you check the shingle stagger when you inspect roofs? How do you check it on modern laminated shingles? Checking the old three-tab shingles was easy, but things have changed.



Shingles have changed

I do miss the good, old three-tab shingles! Until a few years ago, almost all the houses I inspected had three-tab asphalt shingle roofs. Now, it seems as if most of the asphalt shingle roofs consist of laminated or architectural shingles or some type of hybrid made to look like shakes or tile or wood shingles. In my market, about 85 percent of the asphalt shingle sales are some type of laminated shingles.

Why do I miss the three-tabs? Because, they were easy to inspect. The tabs are about 12 inches wide and separated by a slot. When installing a roof, the contractor cuts about 6 inches off the edge of the starter of each row of shingles, creating a simple stagger. When there are three-tab shingles on a roof, you can see the alignment of the tabs/slots and the edges of the shingles. This makes it easy to check the stagger. 


Photo above: On this roof, the alignment of the tabs/slots and edges of the three-tab shingles is clearly visible, making it easy to check the stagger.

In fact, unless there was a really unusual pattern of tabs or slots, I never checked the stagger with three-tab shingles because it was always right. Maybe the contractors or DIYs got it right because the three-tab shingles were easy to install, or maybe because mistakes with stagger were easy to see. The slots just didn't look right if the spacing was off … and the DIY's spouse could see it from the ground.

The new laminated shingles have no tabs and no slots. When the shingles are properly installed, it is difficult to locate the shingle butt edge to measure the stagger. In fact, some shingles seem to have laminations that even cover the butt joint. Improper offset or stagger, which will allow leaking, is difficult to detect visually. 



Photo above: Laminated shingles do not have tabs or slots making it difficult to locate the butt edge to measure the stagger. If they are not installed with the correct stagger, the roof may leak.

Stagger is important

If the stagger (offset) is not adequate, rain can move through the edge of the shingle to the edge of the lower shingle, creating a leak. This is especially problematic in wet climates, areas with heavy rain or areas with freeze-thaw cycles, and also with low-sloped roofs.

The old "English" measurement shingles were approximately 36 inches wide and the stagger was about 6 inches. The "metric" shingles are approximately 40 inches wide and the stagger is still about 6 inches. If the stagger is off, the shingle tabs/slots will appear out of alignment and be easily spotted during a visual inspection.

With laminated shingles, the stagger ranges from 6 to 15 inches, depending on the type of shingle and the shingle manufacturer's instructions. In a few cases, manufacturers may allow 4 inches, but that is not common. With laminated shingles, the appearance might change if the stagger does not follow the manufacturer's directions, but often this is hard to see during an inspection.

Check offset and pitch

Shingle offset (stagger) of less than 4 inches makes the roof prone to leaking. On architectural shingles, I look for shingle edges and visually measure the edge-to-edge spacing. In my work as an engineer and roofing consultant, I have seen roofs with stagger of less than 4 inches, and they leak — random leaks right through the roof.

The pitch of the roof is also a factor when the shingle stagger is small. The lower the pitch of the roof, the more it is prone to leaks. Remember, asphalt shingles shed water and do not constitute a membrane. Ice on the roof also limits the roof's ability to shed water and can compound the leak/stagger problem.

What do you do when a roof leaks due to improper stagger? You tear the whole thing off and start over. It's an expensive defect. It's also difficult to detect in a laminated shingle unless you know what you're looking for.

I was called in as a consultant on a huge laminated shingle roof over an indoor pool attached to a 3,500-square-foot home. The stagger was as little as two inches on parts of the 7,000 square feet of roof, and the roof leaked in random locations. The whole roof was replaced, and the attorneys made some money. The shingle manufacturer said the shingles were not installed per instructions, which voided the warranty; I agreed.

Inspecting laminated shingles

To inspect laminated shingles for stagger, locate the edges of shingles in adjacent rows. Often, this is difficult, but with a little practice, you will notice subtle changes in appearance at the edge. At times, I slightly lift an edge to be sure it is an edge and not just a lamination over a base felt. Remember that butt edges are a straight cut, perpendicular to the lower edge of the shingle. Edges of laminations often are cut at an angle.

Measure from edge to edge of adjacent shingles. If it looks too small, use a ruler and start taking pictures. Once you have checked a few shingles, you can almost be assured that the whole horizontal row of shingles will be correct. You must check for stagger on each roof plane.


Photo above: To determine the stagger of laminated asphalt shingles, locate the edges of shingles in adjacent rows, and if the stagger appears to be small, measure from edge to edge of adjacent shingles and take photographs.

If the stagger is off, you may also notice an irregular pattern in appearance. At times, this is visible from the ground when viewing a large area.

Be suspicious of "California" valleys

A "California" or "Long Island" valley is one in which a vertical shingle is used to line the edge of the valley (see photo next page, bottom right). It could be an open metal valley or a closed-cut valley. When you inspect this type of valley, you will notice a shingle laid parallel to the valley and then rows of shingles laid over the vertical shingle to the rake edge. You will be able to see the laminated edges of the vertical shingle in the valley.

A vertical shingle is used to line the edge of this California or Long Island valley, with rows of shingles laid over it to the vertical edge. It looks good, but is prone to leaks.

I anticipate there will be problems with California valleys because the rows of horizontal shingles often are started in the valley and the stagger is too small. The roofing contractor is saving time and material by not trimming the shingle edge for proper stagger. It looks good, but it can leak.


Photo above: A vertical shingle is used to line the edge of this California or Long Island valley, with rows of shingles laid over it to the vertical edge. It looks good, but is prone to leaks when the stagger of the horizontal rows of shingles started in the valley is too small.

The role of manufacturers

Can the manufacturer help? Well, maybe. I have found that manufacturers and their local representatives are good resources for technical information. But when it comes to evaluating a shingle installation issue, often they are reluctant to criticize their customer, the roofing contractor. And that makes good business sense. Manufacturer reps also may waffle a little with their statements. They guarantee their shingles for performance, but if leaks occur because of improper installation, the warranty is voided.

Most manufacturers' instructions for installing asphalt shingles don't mention the California valley, but some references to this type of valley installation can be found in trade journals and instructions.

What's a home inspector to do?

Always check for stagger with laminated shingles. Watch for those California valleys; at least the stagger is easy to see at the edge of the valley. Unfortunately, is it difficult to visually identify a stagger issue unless you are on the roof looking down at the shingles.

If the stagger is 5 inches or less, start to look closely, documenting the condition and warning the buyer. If the stagger is 4 inches or less, document the condition, suggest that there is a potential for leakage, and refer the condition to a professional roofing contractor for further evaluation. Carefully check the attic and ceilings for leaks.

I suggest that the buyer ask the home-owner to document in writing the type of shingles, the name of the roofer and the installation requirements and warranties. Have the buyer make the seller responsible for the roofing issues. You are not paid to be responsible for improper roof installation. Don't accept that responsibility.

Do a little research. Gather and review a set of shingle manufacturers' installation instructions for shingles commonly used in your area. You can find these instructions on the manufacturers' websites. The Asphalt Roofing Manufacturers Association (www.asphaltroofing.org) also provides excellent reference materials.

Carry the instructions with you to use as a reference. I provide the manufacturers' instructions to customers when there is an issue.

Installation instructions are also printed on every bundle of shingles. You can often find a spare bundle of shingles in the garage to identify the manufacturer and type of shingle.

Also, review these instructions in detail to see what they say about stagger. They may call it "offset," but talking about shingle stagger is more fun. Be knowledgeable about shingle stagger because some contractors and some DIYs aren't.

Photographs and illustrations for this article are courtesy of Tom Feiza.

See Tom Feiza, Mr. Fix-It,  at ASHI InspectionWorld 2011 in Atlanta. He will be speaking on Asphalt Shingle Roofs/Attics – Proper Inspection and Reporting and on Water Intrusion into Buildings – Forensic Investigations.