Standard thickness roofing slates (3/16 inch TO 1/4 inch) that have drilled HOLES without counter-sinking are sub-standard slates. After installation, the nail heads will rub against the overlying slates
and eventually wear holes in them. Thicker slates are OK to be drilled as there is usually room between them for a nail head.
SLATE ROOF INSTALLATION GUIDE
1) Construct the roof properly. Assume all elements of the roof assembly must last and function for at least 150 years. This means using a roof deck material with a known and proven longevity of a century and a half, which includes solid lumber at least 3/4 inch thick and some other decking materials such as nailable concrete and gypsum. Avoid any laminated or glued roof decking products.
2) Select the correct slates. This means you need to do your homework and understand the differences between slates. Price and color are NOT good criteria to go by. Size, thickness, type and quality of manufacture are very important. Some slates leach pyrites and stain the roof. Others fade badly. Thick slates are harder to cut and may require more experience. Small slates take longer to install. Some manufacturers have poor quality control. Some drill their slates, some punch them, some cull out bad slates, some don't. Foreign slates can be a real gamble. There is a lot to think about. Standard thickness domestic or Canadian slates with punched nail holes from a reputable manufacturer are usually a good bet. There are a lot of good foreign slates too, but finding a reliable source of them can be difficult.
5) Don't rely on underlayment to keep your roof water tight. The underlayment is temporary - NOT permanent. It will wear out. If your slate roof depends on the underlayment to keep out the water, it is a faulty roof, incorrectly installed. Slate roofs do not require underlayment. Use the underlayment to keep out the water until the roof is installed. 30# felt is traditional, but a double layer (half-lap) is better today as the older felts were a better quality than today's felts. The underlayment will keep out the water until the slates and flashings are installed and it makes a good surface on which to chalk lines. Underlayment should always be installed UNDER the slates, NEVER on top. If you're concerned about ice-damming, increase the headlap along the eaves.
6) Get the starter course right. Read this important article about starter courses. Install a cant - either a wooden one, a copper one, or use another method, but get it in there. Make sure your starter course has enough headlap.
7) Blend the slates. If using slates from several pallets, blend them on the roof by taking them from all pallets at once.
9) Do not walk on the slates during installation. Use roof jacks and planks. Stage the roof properly.
10) Chalk your courses on the roof deck. Install every course of slates along a permanent (red) chalk line chalked on the surface of the roof that marks the top edge of the slates. Do not chalk on the slates themselves. Do not "eyeball" the courses. You will lose your headlap if you do not follow this bit of advice.
11) Lay your starter slates back side up. The starter course should be laid back side facing up to allow the edge bevel to merge flush with the edge bevel of the first course (which is laid back side facing down, as are all other slates on the roof, except the starter course).
12) Do not over-nail or under-nail the slates. Slating nails shall not be driven in so far as to produce an excessive strain on the slates and shall instead be driven to a depth such that the nail heads lie within the counter-sunk nail hole and do not rub excessively against the overlying slates. This is why a counter-sunk nail hole is important, rather than a drilled nail hole. Read more about nail holes.
13) Use good flashing material.Copper flashings or stainless steel are best. Use minimum 20 ounce copper on valleys and built-in-gutters. You can use 16 ounce copper on ridges, step flashings and chimney flashings, although 20 ounce is better. Sheet lead is also a good flashing material.
14) Use a good ridge and hip system.Saddle ridges, Boston hips, mitered hips and copper hips or ridges, are all good. The worst ridge is when you just run your field slates to the top and leave them with exposed nail heads and sealant along the apex and nothing else.
15) Use good nails. Do not use electro-galvanized nails. Use copper or stainless steel roofing nails. You can use hot-dipped galvanized roofing nails too, especially when installing recycled salvaged slates.
16) If you're hiring a contractor - get a good one. I guarantee that if the contractor is not familiar with everything on this page, he should not be installing a slate roof.
17) If you're using a contract document, make sure it is thorough and detailed. Do not leave any details to guesswork. Spell out everything - headlap; type of slates including size, thickness, origin, color, and shape; gauge and length of nails; type and gauge of flashings; type of cant; etc., etc. Here is a sample contractas a PDF - as a Word file.
18) Know how to repair the roof. You will probably break some slates during the installation and you could lose a slate or two after the roof has been installed due to damage to the slates during installation. You should have a few slate hooks available to use for repairing the roof.
above illustration shows basic installation details for a
traditional, standard-thickness slate roof. The cant strip
is variable in thickness according to the thickness of the
slates being installed (thicker slates require a thicker
cant strip). Gable edge projection and drip edge projection
can also vary somewhat. Headlap is also variable. Greater
headlap (for example, 4 or 5 inches) may be preferred when the roof slope
is lower (4:12 to 6:12) or along eaves when ice
damming conditions are prevalent.