The original 120-year-old slate on this circa 1880 building in Cambridge, Mass., was successfully restored.

On older roofs, high-quality clay tile or natural slate frequently outlast the original metal flashings and roofing underlayment, for example, asphalt-saturated felt paper that help to make the roof watertight. The question frequently facing building owners, contractors and architects is whether the old slate or clay tile are worth reusing.

Their anticipated durability and life expectancy is often the crucial question in deciding whether to reuse them, particularly in cold climates with significant freeze/thaw cycling. Visual assessment of gross visual characteristics and defects, such as cracks, surface wear, etc., is useful for screening, but must be supplemented by laboratory testing to make an informed decision on residual durability and service life.

The standard battery of American Society for Testing and Material tests for slate and clay tile, as described in ASTM C406 and ASTM C1167 respectively, can be conducted on existing and salvaged materials, as well as new. On many clay tile and slate projects, we have tested existing materials to help assess the anticipated durability of salvaged slate or clay tile versus new materials, and to help building owners, contractors or architects decide whether to reuse or replace the existing slate or clay tile.

High density, hard firing, deep vitrification and low water absorption are essential to long-term durability of new or used clay roofing tile in a freeze/thaw environment.

Never reuse clay roofing tile salvaged from a relatively warm climate for use in a colder, more severe climate without strict laboratory testing to verify that the absorption and saturation coefficient provide the best prospects for durability in the more severe freeze/thaw environment. Tile imports from Western and Central Europe, which are regions farther north than the northern United States, originate from areas typically not as severe in freeze/thaw cycling as the northern U.S.

Whether new or salvaged, tile for use in the northern U.S should comply with ASTM Grade 1, which is “providing resistance to severe frost action.” In our experience, a high quality, dense well-fired tile will meet the requirements of ASTM C1167 Grade 1 easily. ASTM Grade 2 clay roof tile that we would not expect to be durable in New England or the Midwest, may last for many years in Florida, California or the Southwest.

On some projects, our roofing design has called for the removal and reinstallation of 100-year-old clay roofing tile, with all new underlayment and flashings. In doing so, some replacement tile is invariably needed to account for loss and breakage. In such instances, it is often preferable to segregate the new tile from the original on discrete areas of the roof, e.g., dormers hipped ends versus main roof, for color consistency. It also allows these tiles or slate populations to be managed separately in the event that one population proves more durable than the other. In addition, segregating the tile is an advantage for installation if the new and old tile vary in thickness or dimension.

As with clay roof tile, removing and reinstalling slate is an option when flashings and underlayment are deteriorated, but the slate is still in good condition and absorption tests are comparable or better than new slate. However, if the absorption and saturation coefficient of the existing slate indicate relatively limited durability and remaining life, reuse of the slate may not be worthwhile.

In some cases, salvaged clay roofing tile may offer a technical advantage over new tile. Dense, well-fired, quality tile salvaged from a roof in a cold climate has already stood the test of time, weathering and natural selection, i.e., any inferior or underfired tile from the original roofing have already been eliminated by exposure to the elements. In the roughly 2000-year-old text “The Ten Books on Architecture,” the Roman architect Vitruvius noted this phenomena, and observed that consequently the strongest structural “burnt brick” walls were those built of old roofing tile.

As with clay roof tile, removing and reinstalling slate is an option when flashing and underlayment are deteriorated, but the slate is still in good condition, and lab tests results — particularly absorption — are comparable to or better than those for new slate. However, if laboratory test data for the existing slate indicates relatively limited durability and remaining life, reuse of the slate is probably not worthwhile.

Salvaged slate can be a worthwhile option for the spot replacement of durable slate that are no longer quarried and difficult to match visually, such as the remarkably durable Monson black slate from Monson, Maine.

In some cases, salvaged slate can offer a technical advantage over some new slate in flexural strength and resistance to breakage. In much new slate, the grain, a secondary direction of compression not visible to the naked eye – not to be confused with the readily visible bedding planes, is not oriented parallel to the long direction of the slate. This results in generally lower flexural strength of the individual slates than in older slate in which the grain typically runs parallel to the long direction of the slate.

For all slate and clay tile roofing projects, whether new or old, we recommend the following:

• Thoughtful design of reliable underlayment and durable, solderable flashing is essential to having a reliable leak-free roof over the long term. Design of soldered metal flashings, such as copper, must provide for thermal expansion and contraction to avoid cracks and fatigue failure at soldered seams which can become sources of leakage.

• Any necessary rooftop repair and maintenance items, for example, chimney repointing and wood repairs to dormers, should be undertaken prior to or in conjunction with the roofing work, in order to reduce future foot traffic on the new or relaid roofing. When walked upon, even the best slate or clay tile are prone to breakage.

• Roofers should “ring” every slate/tile prior to installation and discard any slate/tile that emit a dull thud rather than a ringing noise when struck; this is done by holding the slate/tile loosely at the top corner and tapping down low with a knuckle or wood handle. This is a test to find the egregiously poor slate/tile, not a test to prove that slate/tile that ring are necessarily durable. Slate/tile that do not ring tend to have internal defects or delaminations that will severely limit their durability.

• Slate and clay tile should hang from the nail, with a slight gap between the head of the nail and the slate/tile, to prevent stressing and possibly cracking.

Matthew B. Bronski is a staff engineer/architect with Simpson Gumpertz & Heger, an engineering firm based in Waltham, Mass.