What Architects Need To Know About Slate Roofs (Powerpoint)

What Home Inspectors Need To Know About Slate Roofs (Powerpoint)

Basic Slate Roof Installation Instructions

Top 10 Mistakes Made When Installing a Slate Roof

Here are some important things to consider:

Mr. Jones in a southern state wrote to me to describe the following ordeal:

“We built a new home here last year and put a slate roof on it. When we specified black slate our general contractor obtained samples from various suppliers and we chose the one that was the blackest. It was offered by [Company X] Slate, Inc., and was represented to be ASTM S1 and domestically mined, and priced to me at $130,000 installed for 110 squares. It was put on in June 1996, and by August it had large red rust spots all over it. It got much worse very rapidly and every time it rains it leaks rust down on my white stone entrance, walks, etc. When we investigated, it turns out the slate was shipped from overseas and had large amounts of pyrite. Independent testing revealed a modulus of rupture and an absorption rate that were both so poor as to not even rate this slate as an S3! The slate company is now expected to replace the entire roof, and the threat of litigation is coming up repeatedly.”

Mr. Jones could have saved himself a lot of grief (and probably a lot of money) if he had consulted with an impartial slate roofing professional not affiliated with a slate quarry or slate supplier, before he spent a small fortune on his roof. In situations like this, the General Contractor may avoid the services of a consultant and grope blindly through the darkness of his/her inexperience because he/she doesn’t want to spend the money on a consulting fee. This is a good case of “penny wise and pound foolish,” where everyone suffers in the end.

[STANDARD THICKNESS ROOF SLATES (3/16" TO 1/4") THAT HAVE DRILLED HOLES WITHOUT COUNTER-SINKING ARE SUB-STANDARD SLATES THAT SHOULD BE REJECTED, IF POSSIBLE. AFTER INSTALLATION, THE NAIL HEADS WILL RUB AGAINST THE OVERLYING SLATES AND EVENTUALLY WEAR HOLES IN THEM. ALWAYS CHECK BEFORE BUYING TO MAKE SURE THE STANDARD THICKNESS SLATES HAVE COUNTER-SUNK OR PUNCHED HOLES. IF NOT, DO NOT BUY THEM. THICKER SLATES DON'T MATTER SO MUCH IF THE SLATES ARE DRILLED AS THEY TEND TO NOT LAY SO FLAT AGAINST EACH OTHER AND THERE IS USUALLY ROOM IN BETWEEN THEM FOR A NAIL HEAD.]

You cannot rely on the opinion of your roofing contractor unless he specializes in slate. The vast majority of roofing contractors as well as general contractors, know very little about slate roofs, and will try to throw you the two “curveballs” of slate roofing:

Line #1: “Your slate roof is old and must be torn off and replaced (but not with slate).”

This is a very common line thrown at the roof-owner no matter how good the roof actually is. Beautiful roofs that will still last longer than any person currently alive are judged finished and then sentenced to death by ignorant roofing contractors as the duped roof-owner nods in agreement and forks out money to pay for the destruction. Get a second opinion from a slate roofing professional, even if it involves a small fee. That small fee may save you a small fortune.

Line #2: “Sure we can install a slate roof for you, and heck yes we can install it on a roof designed for asphalt shingles.”

This is where a home owner or roof owner can quickly get lost. The specifications of roof construction are foreign to most people, and if your roofing contractor tells you the roof must be constructed a certain way, you want to trust and believe him. Unfortunately, most roofing contractors don’t know how to construct a roof suitable for the longevity of slate and will try to stick slates on a roof designed for asphalt shingles, which is a waste of good slate (see below).

This issue has become a major point of contention in certain roofing circles. Plywood, which consists of thin layers of wood glued together into four foot by eight foot sheets, became popular as a roof decking material at about the same time that asphalt shingles became popular (late 1940’s early 1950’s). Plywood won the affection of asphalt shingle roofers for three main reasons: the big sheets will lay down faster than boards, the material is less expensive than kiln-died boards, and plywood is convenient to buy - every lumber yard has it and most will deliver it to the work site.

After nearly fifty years of plywood roof decking, most living roofers today don’t even remember back when roofs were built of solid wood. Modern roofers have convinced themselves that plywood is suitable as a decking material for any roof, mostly because 99.9% of all American roofs installed today are temporary, disposable roofs which have to be replaced every twenty years, or less, no matter what. If the plywood is bad after twenty years it’s simply replaced as well. This presents a grave problem for people who want a slate roof correctly installed. Plywood has not demonstrated the longevity of slate and should be avoided under any permanent roof (i.e. slate or tile). Unfortunately, many roofers will tell you:

a) that the use of any other roof decking material will raise the cost of the job sky-high, and

b) that the use of rough sawn lumber (the traditional material under slate) will not work and can’t be done, despite the fact that virtually every old slate roof in America is decked with rough-sawn lumber (a small percentage are decked with planed, kiln-dried lumber).

This leaves the roofing customer between a rock and a hard spot. On the one hand, you have your preferred roofing contractor, whom you want to trust, smooth talking like a Philadelphia lawyer, convincing you to use plywood under your slate roof. His influence is bolstered by some slate brokers and most architects who also erroneously recommend plywood under slate. On the other hand, when you look into the matter, you will find that plywood has a well-known proclivity for delamination (which is why it doesn’t have the longevity of solid wood) and you know that there’s no way you’ll have a 100 or 200 year slate roof if you deck the roof with plywood. The following quote describes a typical scenario:

“Though the building was only twelve years old, the roof was leaking at every conceivable point. The original shingles were guaranteed for twenty years, but poor ventilation had caused the plywood deck to delaminate in many places, and shingles to blow off during high winds. The roof was repaired on a consistent basis, but the efforts were in vain due to the delamination of the plywood" [emphasis mine]. - Quoted from Roofer Magazine, November, 1997: The Roof Doctor’s Prescription for Success, by Melinda North (page 27).

The above situation involved the complete replacement of the roof, all 32,000 square feet of it. Ironically, the roof was again sheathed in plywood, however an improved ventilation system presumably will enable the new plywood to last as long as the asphalt shingles (which is not very long when compared to slate).

What should you use under slate? There are several good options:

OPTION #1) The standard material, traditionally used for generations, is 1” thick rough sawn lumber, locally derived, if possible.

Rough sawn lumber is made when a log is sawn into boards, and it is not kiln dried. It can, in fact, be used green and full of sap (it will dry in place with no detriment to the roof or to the structure). It can also be air dried before use by stickering it up (layering it with air spaces and protecting it from rain) for a month or two before installing it. Any species of wood will work, including hardwoods such as oak, however, it’s easier to use the hardwoods when they’re green (which is why a lot of rough sawn lumber is used green and not dried). Mixed hardwoods can also be used.

The material should be a full inch thick (“four quarter”) and care should be used in selecting lumber that is accurately, and uniformly, sawn by a skilled sawyer. The carpenter or roofer installing this sheathing lumber should exercise some discretion and cull out any marginal boards.

Rough sawn lumber is purchased wholesale from saw mills, not from retail lumber yards, and usually must be ordered in advance. Look under “lumber, wholesale” in the yellow pages of the phone book to locate a local sawmill, or just ask around - many of the smaller sawmills aren’t listed in the phone book. The cost of rough sawn lumber (“number 2 grade”) has traditionally always been LESS than the cost of plywood, and rough-sawn will easily last a century or two. It’s also easily repaired if damaged. Make sure your rafters are spaced no more than two feet on center, and use 8 penny nails to fasten the 1" lumber to the rafters (see The Slate Roof Bible: Chapter 12).

OPTION #2) The rough sawn lumber can be purchased “five quarter” thick (one and a quarter inches), then planed on both sides down to one inch in thickness.

This technique is obviously more time-consuming and expensive but may be preferred by the die-hard perfectionist who wants boards that are all exactly the same thickness. Again, moisture content of the wood isn’t important so long as wood with high moisture content (“green wood”) is nailed solidly into place, or previously stickered up to air dry. The most important consideration regarding moisture content is that once some hardwoods are completely dry, you won’t be able to drive a nail through the boards (most of the nails will bend), so the lumber should be used green.

OPTION #3) In areas where no local lumber products are available, imported kiln-dried softwood sheathing lumber (no less than “number 4 grade”), 3/4” thick, can be used in place of rough sawn lumber. This option works fine.

Again, the workers installing this lumber must cull out the bad boards, or cut out the bad sections where large knots or other flaws will weaken the roof deck. Three-quarter inch thick, planed softwood boards will last a century or more under slate.

OPTION #4) Tongue in groove lumber, kiln dried, is suitable as a roof decking on slate roofs.

OPTION #5) Slating lath, or 1X2 to 1X4 strips of wood spaced for nailing the slate, made of at least 3/4" thick solid wood.

Institutional buildings, and any building with a heavy slate ot tile could use 2" thick (actually 1.5 inches) tongue in groove roof decking, such as 2x6. This was commonly used on churches, college buildings, and other institutions where a roof deck was needed to withstand the heavy weight of ceramic tiles or graduated slates.

Here are a few other mistakes commonly made when a slate roof is installed:

1) WRONG NAILS ARE USED

When installing new slate it's a good idea to use copper roofing nails (an inch and a half long). When installing used, recycled, vintage, antique, in other words OLD slate, hot dipped galvanized roofing nails will do fine. In fact, almost all old slate roofs in the US were installed with hot dipped galvanized nails. These nails easily last 100 years. NEVER use electro-galvanized nails. Electro-galvanized nails are fine for asphalt shingles, but those roofs are only expected to last 20 years, so a cheap nail will do. Slate roofs last much longer and are not cheap, so never use a cheap electro-galvanized nail. Stainless steel nails are also good.

2) INADEQUATE HEAD LAP IS USED

The head lap is the overlap each slate has with the slates TWO courses below. That lap should be between two inches and four inches depending on the slope of the roof (three inches is generally recommended). Read an article about headlap.

3) THE ROOFER IS NOT EQUIPPED WITH THE RIGHT TOOLS (Check out the tools in our store!)

Proper slate cutters, hammers, roof jacks, ladder hooks etc. make the job so much easier (see The Slate Roof Bible).

4) CHEAP FLASHING IS USED

Don't waste time constructing a nice slate roof and then use cheap aluminum flashing in the valleys, around the chimneys, etc. Copper, stainless steel, terne coated stainless, lead coated copper, lead sheet, and in some cases heavy aluminum are all good with slate. (We sell 20 ounce valley copper and 16 ounce ridge copper, and ridge iron in our store.)

 

Top 10 Mistakes made When Installing a Slate Roof

Basic Slate Roofing Installation Information