You need to know the name and location of the actual quarry. That’s important because some quarries have better reputations — and warranties — than others. (iStock)

When Tsion Messick started talking to roofers about replacing the slate on her 90-year-old Colonial in Greenville, Del., something didn’t seem right.

“We ran across roofing contractors who did not have much experience in slate roofs,” Messick said. Worried that she would end up with a shoddy job, she delved in and did her own research. “There was a lot to learn and finding the right resources was key.”

One of those resources was Joseph Jenkins, a slate roofing consultant and author of “The Slate Roof Bible.” Jenkins warns the worst thing uninformed roofers do is tell homeowners their slate roof needs replacing when it really doesn’t.

“The main thing people need to understand is that a slate roof can literally last for centuries,” Jenkins said. “I’m annoyed that a roofing contractor would destroy somebody’s roof and it’s regretful that the property owners don’t know what they lost.”

How long your slate roof will last depends largely on what type of rock was used. According to an identification guide on the Slate Roof Central website, some of the hardest, heartiest types are purple slate, non-fading green slate and grayish black Buckingham slate, which can all last 150 to 200 years. Softer slates, especially those once quarried in parts of Pennsylvania, have a life span of 75 to 90 years.

That’s the type on top of my 1946 house, so I am in the market for a new slate roof. The median age of houses in Washington is 75 years, so I’m not alone.

A slate roof can cost from $12 to $40 per square foot, according to websites and roofers I consulted. Your price will be on the lower end if you use recycled slates or reinstall your own. The price will rise if your house is complicated, your slate is extra fancy or you live in an expensive area. That’s two to four times more than an asphalt-shingle roof. Then again, a slate roof can also last four or more times as long, making it a good value over the long haul. But for a slate roof to last, it must be installed correctly.

Here’s what you need to know:

Where it comes from

It’s not enough to know the color and cost of slate you’re putting on your roof. It’s not even enough to know the name of the slate manufacturer. “The most important thing you need to know is where did it come out of the ground,” Jenkins said.

In other words, you need to know the name and location of the actual quarry. That’s important because some quarries have better reputations — and warranties — than others. Look for slate that is warranted to last at least 75 years. One hundred is even better.

That warranty should contain a guarantee that if the slate develops pyrite stains — ugly, rust-colored streaks — the slate manufacturer will pay to replace your roof. The Slate Roofing Contractors Association maintains a list of reputable manufacturers and quarries on its website. You should also ask your installation company if it can reinstall or recycle your old slate and to adjust the price accordingly.

Sidelap and headlap

A critical factor is the amount that slates overlap each other to keep water out of your house. Knowledgeable roofers make chalk lines on the felt underlayment to assure the overlap is adequate.

Slates should overlap by at least three inches on the sides, called sidelap. Even more important is the overlap at the top of each slate, called headlap. If your roof is steep — with a pitch of at least eight inches in height for every 12 inches in width — you need three inches of headlap.

You can confirm your roof’s pitch using an online calculator. If your roof is shallower, between 4:12 and 8:12, the slates should have a headlap of at least four inches. Roofs shallower than 4:12 should not be made of slate, according to Traditional Roofing Magazine.

One final note: There should be an additional one to two inches of headlap near the eaves to help guard against ice damming.

Metal on metal

Copper is the most common metal used in slate roofs, but there are other options, such as coated stainless steel, that cost less. Regardless of what metal your contractor uses for flashing and other purposes, the real key is consistency because of something called galvanic corrosion.

Put simply, some metals, when they come in contact with one another, will “eat” each other over time. This is one reason the metal parts of slate roofs often fail long before the rock itself.

This caution applies to metals used in flashing, ridges, valleys, nails, rivets, drip edges, gutters and downspouts. You do not need to worry so much about the metal composition of your snow guards, because they don’t come in contact with other metals.

Choosing nails

Slate nails should be copper, stainless steel or at least hot-dipped galvanized steel to give them a fighting chance of lasting as long as the slates themselves. Electro-galvanized nails are a no-no because they can rust in just a couple of years.

Nail length also is important. The correct length is double the thickness of your slates plus one inch. If the nails are the correct length, they will penetrate the roof sheathing boards but not punch through to the other side.

Once hammered in, the nailheads should sit flush with the slate. If they’re pounded in too far, they can crack the slates they are supposed to be holding; if they are not pounded in far enough, they can crack the slates layered over them.

Choosing underlayment

In this era of artificial materials, underlayment for slate roofs has become a confusing and disputed subject. All but one roofer I interviewed wanted to use what is called ice and water guard in the valleys of my roof underneath the slate. Jenkins said ice and water guard is not only unnecessary but can actually be harmful to slate roofs.

He said this product prevents your slate roof from breathing because it’s synthetic and makes it hard to perform repairs later because it’s sticky. So what underlayment is appropriate? Traditional, 30-pound organic roofing felt.

The felt is mostly there to protect your home from the elements if there’s a storm before the contractor installs the slate. It then disintegrates in a few years and Jenkins said that’s perfectly fine. “I’ve worked on well over 1,000 slate roofs in my lifetime, and none of those had any functional underlayment,” Jenkins said. The rock itself is what sheds water and protects your home, not the material beneath it.

The correct sheathing

The sheathing, also called decking, is the wood to which the slate is attached. Ideally, the wood will last 150 years — or long enough to support an average initial slate roof and replacement slate roof.

The right sheathing choice is individual plank boards. The boards can be anywhere from three-fourths to 1½ inches thick and many different species of wood will work. Plywood, particle board and laminated woods are not ideal because they are made with glue and don’t last long enough, according to Slate Roof Central.

If you are replacing a lightweight roof, you will also want to make sure your rafters and sheathing are strong enough to hold up a heavy slate roof without sagging.

Proper equipment and technique

Your roofer should choreograph the project so that the workers walk on the slate as little as possible, which can crack the stone. To achieve this, contractors should use roof brackets — called “roof jacks” in the trade — which are like mini scaffolds roofers can work from.

Other traditional tools any good slate roofer should use: slate cutters, hammers, hooks and rippers. To test prospective roofers’ slate knowledge, you can ask them whether they use these tools. Good roofers also know to mix up their supply of slate as they work, rather than pulling from a single pallet, to avoid sudden changes in color.

What your contract should include

As you have read, there’s a lot to know about — and insist upon — especially because replacing a slate roof can be a five- or six-figure investment. And yet, when I requested bids to replace our slate roof, three out of four roofers submitted half-page contracts without any of these specifications.

“That bothers me,” said Shaun Rowe of Brax Roofing in Gaithersburg, Md. “For a roof like this, that’s built to last a century, it’s better for both contractor and customer if the materials and workmanship are spelled out in detail.” Brax has adopted Slate Roof Central’s five-page model slate-roofing contract, something savvy homeowners can ask other roofers to do.

At the very least, ask contractors to include a clause in their contracts stating that they will build your roof according to the installation guidelines drafted by the Slate Roofing Contractors Association. If the house stays in the family, your children —and even grandchildren — will thank you.

Elisabeth Leamy is a 13-time Emmy winner and 25-year consumer advocate who has appeared on such programs as “Good Morning America.” Connect with her at Leamy.com.