So what’s up with the roof?

By Greg Dodd and Doug Garrett

Your insured claims that the recent wind storms damaged his roof.  That 50 foot high Douglas Fir now resting in the dining room didn’t help it either.  Aside from wind and trees, what else causes roof damage?

If we keep Mother Nature out of the picture, there are numerous roof problems that can be caused by poor workmanship and/or incorrect material selection.  Building codes and manufacturers’ installation guides call for specific roof slopes for various materials; 3-tab asphalt shingles, clay/concrete tiles, wood shakes/shingles, slate, and metal formed roofs all have different requirements.

There are also local building codes in high wind areas.  Roof slope and weather profiles will affect the exposure of the material used. Typically, the lower the slope, the less exposure to the elements, and the longer the roof will last.  For example, using a 3-tab asphalt shingle on a 4:12 pitch (four feet of rise for 12 feet of run) may require five inches of exposed shingle surface.  Some roofers will cut corners to save on materials by increasing the exposure to six or seven inches, which leads to premature aging and makes the entire roof more susceptible to wind.  You don’t see wood shakes installed very much anymore, usually due to their fire hazard.  Newer homes have various sizes and shapes of roofing materials coming in a variety of fire-retardant composites.

Regardless of the material, there is a right way and a wrong way to install them.  Various shapes and sizes of roofing material will also affect the weight.  Here is a table of typically used materials, showing the installed weight in pounds for one square foot:

3 Tab Asphalt                                       2 lbs.

Concrete composition                     3 to 4 lbs.

Clay tile                                      9 to 14 lbs.

Clay tile with mortar             add 10 lbs. more

Slate and Spanish                     10 to 20 lbs.

Wood shingles (1″)                               3 lbs.


It is important to notice the considerable difference in the weight per square foot for the asphalt shingle versus the clay, slate or Spanish.  Not all roofs are built the same nor designed for the same loads.

Traditionally there are two types of residential roof structures: stick built and manufactured trusses.  Both have their benefits and drawbacks.

Whether stick-built or prefabricated trusses are used, both must be designed to support the weight of the roofing material.  Failure to consider the weight differences among different roofing materials will cause eventual roof collapse.  If the structure is in a mountainous area, then snow loads also have to be taken into account.


In general, asphalt roofs will last 15 to 20 years, and then will start to leak.  Wood shake roofs last fewer years; concrete and tile are usually good for 50 years, and slate will last longer yet (as evidenced by many old roofs in Europe).  Frequently a new layer of roofing material is added on top of an existing layer that has reached the end of its effective life, instead of tearing off the old layer.  We have seen this for both asphalt and clay tile roofs. In some cases there is enough of a safety factor built into the roof trusses that they will take the additional weight without a premature failure, especially if you are dealing with asphalt shingles.  But beware of the low-priced roofer giving great deals on re-roofing by not tearing off the old layers.  Three layers is the maximum for asphalt, assuming a strong truss system under it.  In many older neighborhoods, you see examples of overloaded trusses with sagging roof lines or a soft spongy feel when walked on.


Another possible cause of roof structure damage is the improper loading (stockpiling) of the roofing material on the roof by the material supplier.  Instead of distributing the roofing materials evenly, a concentrated load, such as pallets of tile or multiple rolls of underlayment, are placed directly on the roof.  This saves the roofer time and labor, but it will damage even a properly designed and built roof structure.  Cracked beams might be immediately discovered, or it might take the stress of high winds or a heavy snow load to reveal the damage caused by poor installer practices.  Consider that a square (enough to cover 100 square feet) of relatively light asphalt shingles weighs a couple hundred pounds.  Stack 10 squares in one corner of the roof and you have a ton of weight concentrated in perhaps four square feet.  No wonder trusses and beams break.



One other overlooked problem with roof integrity is the flashing.  Flashing may be made of thin sheet metal (galvanized, stainless, or copper) or a plastic composite.  They work in conjunction with the roofing material to keep the wind and rain out.  Most flashings are mass produced, but some are built on-site for special situations.  Flashing and counter-flashing seal roof penetrations and divert water from fireplace chimneys and skylights.  Hip and valley flashings divert water and control rainwater sheet flow.


Although, not widely used, crickets (a formed metal tent-like appearing flashing to divert water around a roof penetration) help to wash away pine needles, leaves, and other debris that would accumulate behind chimneys or skylights.


The problem with flashings is they are usually sealed with either polymeric material or a tar based material.  This breaks down over time and then the flashing leaks.  Flashing maintenance requires periodic inspection and resealing.  This is difficult for most homeowners and usually neglected.  Unfortunately, failed flashing seals are a gateway to termites and dry rot.  As with most things in life, an ounce of prevention is worth a pound of cure.