A sound slate rings when tapped and a weathered one thuds, and the Park Service fixes a broken slate with one copper nail and a 3-by-8-inch bib
The Park Service's slate brief pairs a knuckle tap test with a repair that sets each new slate on one copper nail under a 3-by-8-inch copper bib.
- Ring clear, go back on
- 140of 200 slates, 70% of the batch
- Thud or flaking
- 45need replacements
- Cracked
- 15need replacements
- Breakage allowance
- 610% on the 60 to replace
One mark per slate taken off the lower part of an example slope, by the result of sounding each one. Worked example with round numbers. The test and the reuse rule come from National Park Service Preservation Brief 29 (1993).
Slaters judge an old slate partly by ear. The National Park Service's Preservation Brief 29, written by architectural conservator Jeffrey S. Levine and published in 1993, says a good slate held on the fingertips and struck gives a clear, solid sound, and a badly weathered one gives a dull thud. The same brief lays out how to replace a single broken slate: one copper nail, covered by a strip of copper about 3 inches wide and 8 inches long. A worked example below applies both to a slope of Pennsylvania slate.
Section AA worn slate gives a dull thud and cracks under hand pressure
The brief explains why a slate goes dull. Calcite, the mineral in limestone, and iron sulfides in the stone react through wet and dry, hot and cold cycles to form gypsum, which takes up about twice the space of the calcite. The slate splits into paper-thin layers that flake off the surface, and the inside turns soft and spongy as the layers come apart.
The brief's repair-or-replace guideline adds a second check: press down hard on a slate with your hand. A sound slate is unaffected. A deteriorated one feels brittle and cracks.
| Done | Check | A sound slate | A worn slate |
|---|---|---|---|
| Tap with a knuckle or slate hammer | Gives a clear and solid sound | Gives a dull thud | |
| Press down hard with the hand | Is unaffected | Feels brittle and cracks | |
| Look at the face | Keeps a smooth face | Sheds paper-thin layers and turns soft and spongy | |
| Look at the underside of a removed slate | Is as sound as the face | Flakes as badly as the face or worse |
The checks Preservation Brief 29 describes, in the brief's terms. Source: National Park Service, Technical Preservation Services, Preservation Brief 29. Download the table (CSV).
Weathering shows on the underside too. The brief says gypsum leaches and gathers on the back of the slate, so the delamination there is as bad as on the face or worse, and a worn slate usually cannot be flipped over and reused. It also says old, weathered slates soak up and hold water, which can rot the wood sheathing underneath for a long time without any leak inside.
Section BOwners tap fallen slates from the yard, and a slater sounds the rest on the roof
The brief recommends that owners survey a slate roof from the ground with binoculars, or from a lift, for safety. Its guideline tells owners to tap slates that have fallen off or been removed, and that is the part of the test a homeowner can do from the yard. A slater does the rest on the roof. When slates come off for salvage, the brief says each must be sounded to find cracks, faults and how far it has weathered.
The brief also lists the parts of a roof that wear first. Eaves and valleys, which carry the most water and ice, deteriorate faster than the rest. So does the slope with the harshest sun, wind and rain, often the south side. Steep slopes last longer, the brief says, because water runs off faster and is less likely to be drawn or blown under the slates.
Section COne broken slate takes one copper nail and a bib about 3 by 8 inches
The brief describes the standard repair in four steps.
New or salvaged slate Copper bib Hidden under the course above
- Pulls the nails of the broken slate with a ripper.
- Slides in a new or salvaged slate matched to the old one in size and shape and texture and weathered color.
- Drives one copper nail through the vertical joint in the course above about 1 inch below the tail of the slate two courses up.
- Slides a copper bib about 3 inches wide and 8 inches long under the joint above and over the nail head.
Replacing one broken slate, as Preservation Brief 29 describes it. The brief gives the bib's size as roughly 3 by 8 inches. The drawing is not to scale. Source: National Park Service, Preservation Brief 29. Download the steps (CSV).
The bib, a small piece of copper bent to grip by friction, keeps water out of the new nail hole. The brief names metal hooks, clips and straps bent over the bottom of the slate as other ways to hold a replacement. One photo caption in the brief warns against a copper tab folded over the slate's tail in northern climates, because snow and ice can bend the tab open and let the slate slide out.
Some repairs take more work than one slate. If the roof was laid with steel cut nails, the brief says, pulling them can crack or shift the slates next to the broken one, and those need repair too. Where several slates come off, the slater should check the sheathing. The brief advises new boards of the same width and thickness, shimmed flush if needed, and recommends against plywood, because nails bounce on it and loosen nearby slates, and against pressure-treated lumber, which tends to shrink.
The brief rules out roofing mastic or sealant on a damaged slate. It says those materials work at first, then harden and crack and let water in, make later repairs harder, and speed corrosion when smeared on metal flashing.
Section DIn the example, 140 of 200 slates ring clear and 60 need replacements
The house is made up. A twin in the borough has a rear slope of Pennsylvania slate, and slates near the eaves have started to slide out of place. The brief says sliding slates often mean the old iron nails have rusted through while the slates are still good, and that those slates can be salvaged and laid again.
The slater takes 200 slates off the lower part of the slope and sounds each one. In the example, 140 ring clear and hold up under hand pressure. Another 45 give a dull thud or shed layers, and 15 are cracked. The 140 sound slates, 70% of the batch, go back on with copper nails. The other 60 need replacements. The brief says a quarry typically allows for 10% breakage in shipment, which on 60 slates is 6.
| Reused | Result of sounding | Slates | Count | Goes |
|---|---|---|---|---|
| Rings clear and holds under hand pressure | 140 | Back on, copper nails | ||
| Dull thud or flaking layers | 45 | Replace | ||
| Cracked | 15 | Replace | ||
| Taken off the lower slope | 200 |
Slates taken off the lower part of an example slope, by the result of sounding each one. A tick means the slate goes back on the roof. Worked example with round numbers; the test and the reuse rule come from National Park Service Preservation Brief 29 (1993). Download the data (CSV).
Now suppose three broken slates sit side by side in one course higher up. Replaced one at a time, each needs its own nail and bib. The brief recommends removing a pyramid instead, a diminishing number of slates from each course above, so that only the top slate needs a bib. The slates along the sides of the pyramid get two nails each, one above the other, near the top of their exposed edge.
| Method | Slates taken off | Copper bibs |
|---|---|---|
| One slate at a time | 3 | 3 |
| Pyramid of 3 then 2 then 1 | 6 | 1 |
Worked example. Three adjacent broken slates in one course, replaced singly or with a pyramid of three, two and one slates. The pyramid's shape is an illustration; the rule that only its top slate needs a bib is the brief's. Source: National Park Service, Preservation Brief 29. Download the data (CSV).
The brief recommends the same approach when a slater replaces leaking valley flashing.
Section EWhat to ask a slater
Ask how the slater judged the slates, and whether they sounded any that came off. If they recommend replacement, ask for a count of damaged slates on each slope. The brief says that at 20% or more on a roof or a slope, replacement is usually cheaper than piecemeal repair; the site's article on slate types and life spans works through that guideline.
Ask what nails and what bibs the repair will use. The brief calls for solid copper or stainless steel slater's nails, because plain and galvanized steel nails usually rust out long before the slate wears. It sets 16-ounce copper as the minimum weight for flashing, and 20 or 24 ounces for gutters, valleys and hard-to-reach areas.
If the slater proposes covering the slate with shingles, the borough's Roof Permit Application Addendum, revised January 2, 2025, bars installing a new roof over slate. The old slate has to come off first. The permit guide covers the rest of the form and the nearby townships.
The brief also sets out a maintenance schedule an owner can keep.
| Done | Task | How often | Who |
|---|---|---|---|
| Look over the roof from the ground with binoculars | Every year and after severe storms | Owner | |
| Check rafters and sheathing in the attic for water stains and rot | Every year | Owner | |
| Clean the gutters | Twice in the fall and once in early spring | Owner or a hired cleaner | |
| Inspect the roof up close | Every 5 to 7 years | Professional experienced with slate and steep slopes | |
| Replace damaged slates | Promptly | Experienced slater |
Maintenance for a slate roof as Preservation Brief 29 recommends it. The brief suggests keeping a log book of repairs and filing every bill and sample. Source: National Park Service, Preservation Brief 29. Download the schedule (CSV).
The roof inspection checklist covers other roof types.
RemarksCaveats
The brief dates from 1993 and was written for historic buildings, so its advice leans toward keeping original slate. The sound test is a judgment a slater makes by ear and hand, and the brief describes it in words, with no measured scale. The twin, its slate counts and the pyramid's shape are made up. Whether a given roof should be repaired depends on its slate, its flashing and its sheathing, which a slater has to see in person.