Slate Roofing near you

Color permanence tells you nothing useful about durability, and the Park Service says so directly. What the record shows instead is that Vermont and New York slates will last about one hundred twenty-five years, Buckingham Virginia slates one hundred seventy-five years or more, and Pennsylvania soft-vein slates in excess of sixty years, while Pennsylvania hard-vein and Peach Bottom slates, neither of which is still quarried, had life spans of roughly one hundred and at least two hundred years respectively. Those figures are offered as a general guide to whether a roof is nearing the end of its serviceable life, not as a warranty.

The stone weathers from the inside out. Natural weathering appears as a slow process of chipping and scaling along the cleavage planes: paper-thin laminations flake off and the slate becomes soft and spongy as the inner layers delaminate. The chemistry behind it is mineral impurities, chiefly calcite and iron sulfides, reacting through wet and dry and hot and cold cycles to form gypsum, which occupies about twice the volume of calcite and forces the layers apart. Because gypsum leaches and concentrates on the underside, delamination is as bad or worse there, which is why deteriorated slates typically cannot be flipped over and reused.

Geometry matters too. The steeper the pitch, the longer the slate lasts, since water runs off faster and is less likely to be drawn under by capillary action or driven under by wind, which is why spires and the steep slopes of mansard roofs often keep original slate long after flatter planes have been replaced. Areas subject to concentrated water flow and ice damming, particularly eaves and valleys, deteriorate faster than the rest of the roof.

The Park Service gives a repair and replacement guideline that reads like a checklist and is worth using as one. Consider the age and condition of the roof against the expected service life for the type of slate. Count the damaged and missing slates: if the number is under about twenty percent and the roof is generally in good condition, it should be evaluated for repair rather than replacement, with the caveat that the older a roof becomes the more maintenance it will need. If twenty percent or more of the slates on a roof or slope are broken, cracked, missing or sliding out of position, it is usually less expensive to replace than to execute individual repairs, partly because even the most experienced slater will damage additional slates while working on a roof in that condition.

Then look for the real cause. Determine whether there are active leaks and what the source may be, and do not assume the slates are leaking, since gutters, valleys and flashings are more likely candidates. False leaks can be caused by moisture condensation in the attic due to improper ventilation, which is a very different repair. Check rafters and sheathing for moisture stains and poke an awl into the wood to see whether it is rotted.

Finally, ask whether the deterioration is uniform, because often it is not. It may be that only one slope needs replacement while the others can be repaired, which spreads the cost of replacement over many years. And ask whether new slates are readily available: even where replacement is necessary, the existing roof may have to be repaired to buy time for documentation and for ordering, since delivery can take four to twelve weeks and as long as a year for special orders.

A slater removes a damaged slate by cutting or pulling its nails with a ripper, a flat tool slid up under the courses. A new or salvaged slate matching the size, shape, texture and weathered color is slid into place and held by a single nail driven through the vertical joint between the slates in the course above. Because that nail creates a new hole, a piece of copper roughly three inches wide and eight inches long is slid lengthwise under the joint above and over the nail to shed water, a detail the trade calls a bib. Hooks, clips and straps bent over the tail of the slate are the alternatives.

Two habits separate competent slate work from expensive damage. Nails are never driven tight: the slate should hang freely on the nail shank, because a nail driven too far cracks the slate and one left projecting punctures the slate above it. And non-ferrous nails, solid copper or stainless steel, are used as a matter of course, since plain steel and galvanized nails usually rust out long before the slate itself deteriorates. The Park Service names rusting nineteenth century cut nails as a common cause of slate loss on historic roofs.

Mastic is the classic wrong answer. The guidance is that roofing mastic or sealants applied to damaged slates should not be considered a viable repair alternative, because these materials, though effective at first, eventually harden and crack and let water in, make future repairs more difficult, are unsightly and accelerate corrosion when applied to metal flashings. Where several broken slates sit together or a valley flashing is being replaced, the correct technique is to remove a diminishing number of slates from higher courses, forming a pyramid, so only the top slate in the pyramid needs a bib.

Because slate outlives almost everything fastened to it, the metal specification carries unusual weight. The Park Service puts it bluntly: given the permanence of slate, it is poor economy to use anything but the most durable of metals and the best workmanship for installing flashings. Copper is named as one of the best flashing materials, extremely durable, easily worked and soldered, and needing little maintenance, with sixteen-ounce sheet the minimum weight recommended because lighter material will not endure the erosive action of dust and grit carried over the roof by rainwater. Heavier copper, twenty or twenty-four ounce, belongs in gutters, valleys and areas with limited access.

Mixing metals is the avoidable mistake. A roof and its flashings, gutters and downspouts function as a system to shed water, and the guidance is to use a single type of metal for all flashings and the rainwater conductor system to avoid galvanic action, since mixing dissimilar metals leads to rapid corrosion of the more electronegative one. Lead coated copper is described as having similar properties to copper and being even more durable, and is often used in restoration work. Terne is called less desirable because it must be painted periodically, with terne coated stainless steel the modern substitute that will not corrode if left unpainted.

One more detail is specific to slate over metal. Slates that overlap a flashing should be nailed so they do not puncture the metal, which is done by punching a second hole about two inches above the existing one on the side of the slate that does not overlap the flashing. Holes are punched from the back so a shallow countersink is created on the face for the nail head to sit in. These are the small things a crew either knows or does not.

Slate breaks under weight, and the weight most often involved is a person. The Park Service lists the causes of individual slate damage as falling tree limbs, ice dams in gutters, valleys and chimney crickets, the weight of a workman walking on the roof, and naturally occurring faults in the slate. Its maintenance advice follows: as part of regular maintenance, an attempt should be made to keep foot traffic off the roof entirely.

For safety reasons, it recommends that owners and maintenance personnel carry out roof surveys from the ground using binoculars, or from a cherry picker. Where anyone must go up, whether maintenance staff, chimney sweeps or painters, the recommendation is that ladders be hooked over the ridge and that people walk on the ladders to distribute their weight. If slates are to be walked on directly, soft soled shoes and stepping on the lower middle of the exposed portion of the slate are the least damaging approach.

This has a practical consequence when hiring. A contractor who proposes to walk your slate roof freely to give a quote is telling you something about how the repair will go. Installation and repair of slate roofs should be entrusted only to experienced slaters, and an experienced slater arrives with a plan for staying off the stone.

Slate work is priced by craft time and by access, and the numbers behave differently from asphalt. The Park Service notes that an experienced slater and one helper can cover clear roof expanses at about two to three squares per day, that chimneys, dormers and valleys can bring the rate below one square per day, and that one square per day is a good average for estimating, before tear-off. A square is enough to cover one hundred square feet at a standard three-inch headlap. Material lead times of four to twelve weeks, and up to a year for special orders, change the schedule as much as the price.

What separates two slate bids is usually specification rather than labor rate: the source and grade of the slate and how closely it matches, the flashing metal and its weight, whether the sheathing is being repaired with matching boards rather than plywood, how much scaffolding or lift access the roof needs, and whether the quote is for one slope or for the whole roof. Ask for the repair approach in writing, including whether bibs, hooks or straps will secure replacement slates, and whether any mastic will be used, since the honest answer to that last one is none.

LokalMatch is the introduction step and nothing more. Describe the roof once, including its approximate age, the slate color and any history you have, what you are seeing and whether the attic shows stains, and contractors covering your area who work on slate will get in touch. They pay for the requests they receive and you pay nothing to use the service. LokalMatch does not repair roofs, does not set prices and does not vet, rank or recommend anyone, so confirming that a company has genuine slate experience, current insurance and a written scope is your responsibility.