Tar and Gravel Roofs

Perhaps the  most striking visual aspect , and in fact a defining feature of contemporary homes is the low pitched roof, often less than 2:12.  The most common original roofing material is tar and gravel.  A tar and gravel roof consists of layers of asphalt and tar paper (or newer, more sophisticated materials) adhered with applications of molten asphalt. The laminated layers get a top finish of gravel, some of which becomes embedded in the hot asphalt, and some of which lays loose on the surface of the roof.  The purpose of the gravel is to shield the asphalt from the damaging effect of sun exposure.  It also provides a nice finished appearance if well maintained. The life expectancy of such a roof is 20 years.   

The complexities arise when doing repair or replacement, or addressing poorly done past repairs.  Homes that are 30 to 50 years old will certainly have had some roof work done.  In many cases, multiple generations of tar and gravel will have been applied.  This is problematic for a number of reasons--  primarily that tar and gravel does not do well with multiple layers.  The process for subsequent layers of roofing is to sweep the loose gravel from the roof and apply new layers of tar paper and hot asphalt.  Okay in theory, but in practice, the rough and sharp gravel surface of the original layer/s puts the new layer at risk of damage.   If the new roof is walked on (to maintain the gravel, clean gutters and skylights, remove redwood debris) the new roof membrane is likely to be punctured underfoot.  Once the membrane has a hole, even a tiny one, moisture can get between the layers of roofing, expand in the heat causing a separation between layers, called a blister.  In cold weather particularly, the blisters become extremely brittle and exposed to much greater damage by foot traffic.  If a blister is broken, a large water retaining pocket is opened up, increasing the chance of leaks. A second or third generation tar and gravel roof has a life expectancy of only eight or ten years, but most roofers recommend against doing secondary generations at all.

So what are the alternatives?   Removal of the existing roofing material is virtually mandatory for whatever steps will follow.  Removal is a substantial item, both in terms of labor and disposal fee expenses.  As an example, when the three layers of tar and gravel were removed from this project, it took a crew of six men two days, and  the weight of the refuse was 20 tons!   

The low pitch largely defines the roofing materials that can be used.  Manufacturers of composition shingles, the most common and affordable roofing material for general use, will not warrantee their materials on roofs with a pitch of less than 2:12, which rules out  its use on many modern homes.   Certainly a new tar and gravel roof  is a possibility, though many roofers consider other newer alternatives to be better.  Torch-down is essential the same material as in composition shingles but in roll form, with a layer of asphalt on the back.  That layer is heated with a gas torch to liquefy it and then it is laid onto a substrate ply  which has been tacked to the roof.  Another option is a combination of built up tar and substrate plies, topped with a cap sheet  that resembles composition shingle material in roll form, applied to the roof with a layer of hot mopped asphalt.   Each of these – and other roof types—have their distinct advantages and supporters.   Best to consult with several roofing contractors to get a variety of viewpoints, and then select  based on your own specific criteria.  

Other considerations:

Many roofs, in particular those over open beam ceilings, will have a layer of fiberboard (Firtex) insulation, perhaps several layers, and probably only over  the living areas of the house.  The old roof material will probably separate nicely from the Firtex, so the Firtex layers can be left in tact.  In homes with open beam ceilings, leaving the Firtex in place has the advantage of limiting the amount of filtration of dust and dirt and asphalt particles  through the roof planks and into the home interior  during the reroof project.  (In areas where Firtex is not present, be prepared for a huge and filthy mess caused by filtration). Areas covered by layers of Firtex once roofed over, will be visibly higher than areas with no Firtex—potentially creating an odd stepped appearance to the plane of the roof.   

On homes with open beam ceilings, electrical wires for any ceiling mounted devices will typically be run on the exterior of the roof, under the roofing material.  The wires may be embedded under the Firtex or in grooves cut into the top of the Firtex, or just laying on top of the Firtex, then covered over with the  roofing material.  This suggest a few  things:

  • the existing roofing material must be removed carefully to avoid damaging electrical wires;

  • while the roof is removed, there is a rare opportunity to add or change wiring for ceiling mounted devices; 

  • when the new roof is applied, depending on material, a trace of the wire paths may be visible.  

One last word of advice:  if you’re buying a home with a tar and gravel roof, unless you are given documentation of a recent roof removal and replacement, be skeptical of any report that indicates that the roof is in good condition.  Conscientious roofers and roof inspectors consider tar and gravel roofs to be very difficult to inspect and determine condition with any degree of certainty.  Unless you can personally observe the roof performing in a downpour, assume major roof work to be imminent.  

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 A Removing the old roof material is a laborious task, scraping it up and lifting it off with shovels. 

B All roofing material has been removed, exposing the 2x8 tongue and groove roof planking over the carport and garage.  Firtex insulation over the living areas is visible in the foreground.  Note that the three layers of original firtex insulation is stepped to minimize an abrupt elevation change in the surface of the roof. 

C The asphalt kettle transforms our driveway into the smoldering gates of hell while D the hot-moppers do their thing top side.  

E Rolls of the first ply have been attached to the roof deck with hot asphalt.  Note electrical wires exposed on surface of roof deck being covered by the new roof material. Again, the  layers of original Firtex insulation are stepped toward the edge of roof to minimize an abrupt  elevation change. 

F Subsequent plies have been applied.  A skylight has been temporarily removed to allow roofing material to be carefully wrapped up the surrounding curb. 

G The final cap sheet has been applied.  Asphalt seeping out from the seams looks black and shiny now, but will mellow with age.  New filon skylight material is fitted over the openings in the carport.  Wiggle molding matches the corrugated profile of the filon.