The Cathedral of St. Andrew in Little Rock, Ark., got its money's worth, even after adjustments for inflation. In 1881, for less than $2,000, it received a 120-year roof — without the benefit of a warranty or even an underlayment. The performance of slate roofing is legendary, with roofs in Europe still functioning after 400 years.
Slate, along with quarry tile, is also one of the most environmentally friendly roofing materials available today, and there's a mystique to the product that is undeniable. For roofers already at the top of their game, the professional challenge of installing a slate roof is often difficult to resist.
Besides training, the primary prerequisites for a successful slate installation are a commitment to quality and attention to detail — two qualities that the majority of our readers already possess.
The actual installation isn't rocket science, either. One manufacturer uses battens and hooks to hang its slate. On paper, the installation instructions seem a lot more straightforward than for certain high-end architectural shingles. One false move on a high-definition shingle often results in an unsightly pattern across the roof. All slaters need to do is remember to pull the slate from different crates to avoid color matching problems.
Contractor Joseph Jenkins is one of the country's leading slate roofing experts. Here's a partial list of his top slate roofing mistakes:
1. Lack of information on choosing and installing slate, despite the plethora of instructions on the Internet
2. Insufficient contract documents
3. Lack of headlap and poor flashing work
4. Inappropriate sheathing/decking materials
Jenkins is admittedly hardcore when it comes to slate installations, and he finds the current emphasis on underlayments disturbing:
"If a slate roof leaks, it's because it was installed improperly, not because of the underlayment or lack of it," he says. "Properly installed slate roofs need no underlayment. The main purpose of the underlayment is to keep the water out of the building until the slate and flashings are installed."
Underlayments do provide a good surface for chalk lines during installation and a cushion for when the slates are being slapped down during installation. Jenkins just worries that lazy or unscrupulous installers will use a high-end synthetic underlayment as a crutch to give them time to "get out of Dodge" before the roof starts leaking.
"Many older homes in the U.S. were installed with No. 30 felt under the slate," Jenkins says. "These homes are so old that the felt has deteriorated to dust, but the roofs are fine. If the slates and flashings are intact, the roof will not leak. This is a proven fact, not a theory."
Other slaters tell RSI of the following mistakes:
- The installer cuts the slate with a diamond saw rather than a slate cutter or hammer and stack.
- A drip edge is not installed and/or two hangers are not used for each piece of slate on batten systems.
- Synthetic slate requires gaps to expand; natural slate does not require these gaps and should butt up close to adjacent slates.
- When using HDPE (high density polyethylene) interlayment, run lengths no longer than 10 feet to 15 feet and overlap 1 foot to avoid buckling.
- Chimney corner flashings must be folded correctly (i.e., to prevent water from entering the corner from the roof or chimney surface), or the corners must be soldered. Do not use "negative" overlap on flashings.
- Roof slates are not walkable on steep-slope applications. The roof must be properly staged, with installers working off roof ladders and scaffolds.
The first place to go for more information on slate is www.traditionalroofing.com. Even if you never plan to install slate, the site's articles are fascinating reading. After all, there's something awe-inspiring about installing a roof with the potential to outlive your great-grandchildren. RSI
