IINSTALLIING A SLATE ROOF — THE BASIICS
Adapted and excerpted from the Slate Roof Bible
Written for the Timberframers Quarterly
By Joseph Jenkins
Copyright, 2004: Joseph Jenkins, Inc.
143 Forest Lane, Grove City, PA 16127 USA; Ph: 814-786-9085
Portions of this publication may be reproduced subject to the following conditions: 1. The information must not be changed or altered; 2. Joseph Jenkins, Inc., 143 Forest
Lane, Grove City, PA 16127 USA, ph: 866-641-7141 is credited as the source; 3. The reproduction is not done for profit without express written permission from the publisher.
A slate roof is a perfect roof. It’s beautiful, natural, durable, recyclable, environmentally friendly, easy to
maintain, and costs less than just about any other serious roof when the life of the roof is taken into consideration.
Although there are many myths and misconceptions about installing slate roofs, there’s really nothing mysterious
about building a roof of stone. The stone is dug or quarried from the earth, brought to the surface, then hand split
with a hammer and chisel into thin sheets about a quarter of an inch thick. These slate shingles are then trimmed
into particular shapes, usually punched for nail holes and are then ready to be installed on a roof. They typically
come from American quarries already trimmed and punched, so the roof installer only needs to know how to put
them on the roof. Let’s start with roof construction.
ROOF CONSTRUCTION
Standard traditional roof framing is sufficient to handle the weight of standard thickness slates, which are
3/16” to 1/4 inch thick. Traditional roof construction includes “stick built” or timber-framed styles with board sheathing
(decking). I have seen 130 year old slate roofs in good condition built on 2×4 or 2×5 rafters. Of course, the
rafters were oak, on 16” centers, with only 8’ unsupported spans, and these are the sorts of things a builder needs
to take into consideration when using a material like slate, which is three times as heavy as common asphalt shingles.
What type of wood are you building with? How strong is it? What are the spans? What are the distances on
center? If you want to build a roof that will easily last a century or two, follow the tried and proven traditional building
styles. Use full framing members, such as 2×8 or 2×10 rafters, rough cut is ok, green lumber (undried) is fine.
Local building materials are ideal as they stimulate the local economy, utilize local renewable resources, minimize
transportation and fuel costs, and require the least amount of technology, especially when green or air dried lumber
is used. Any older carpentry book will explain the spans and sizes for standard roof framing.
One consideration regarding weight: the smaller the slate size, the heavier the roof. Standard roof slate
sizes range from 6”x10” to 14”x24” and everything in between. The 6×10 slates require 686 slates per square (a
square is one hundred square feet of roof coverage), while the 14×24 only require 98 slates per square.
The thickness of the slates also affects the weight. Standard thickness (3/16”) slates weigh 600-700
pounds per square (the smaller slates make a heavier roof). Half inch thick slates weigh double that. The smaller
the slates, the more nails, more nailing, and more labor costs. A 6×10 slate roof will require the nailing of 686
slates to cover one hundred square feet; a 14×24 slate roof will only require the nailing of 98 slates. Obviously, the
larger slates will cover a roof relatively quickly, but the smaller slates can sometimes be bought quite inexpensively,
and some people like the look of smaller slates.
The steeper the roof slope the better, although a 10:12 slope (ten feet of rise in twelve feet of run) is common.
Do not go below a 4:12 slope. When you do put roof slates on a lower slope, you must increase the overlap
(headlap), which is discussed under “installation,” below.
SHEATHING (DECKING)
Use natural boards for the roof deck. This sheathing can be one inch rough sawn lumber, green, or air
dried. It can be 3/4” kiln dried lumber, preferably a softwood like spruce, pine, or fir, as kiln dried hardwoods will
not take a nail very readily, or it can be inch and a half tongue in grooved lumber, also typically softwood. The inch
and a half tongue in grooved lumber is more often used on larger institutional buildings such as churches. One
inch thick or 3/4” thick boards are perfectly adequate for residential construction, and can be either standard or
tongue in grooved. My personal preference is one inch local rough sawn lumber, either green or air dried, and not
tongue in grooved. It’s the most environmentally friendly roof decking, it lasts the longest, and it’s the least expensive.
Do not use plywood, particle board, or any other laminated wood product for your roof deck. Plywood
became popular as a roof sheathing material when asphalt shingles became popular. It didn’t take many years for
roofers to realize that plywood delaminates, especially along the drip edges or anywhere where it becomes
exposed to moisture. Rather than go back to solid lumber roof sheathing, the roofing industry invented a contact
paper to protect plywood, now known as ice and water shield. Avoid plywood and you will find that ice and water
shield is not needed anywhere on a slate roof. You want to build a minimum 100 year roof when making a roof of
stone, so do it right —use natural wood and leave the unnecessary roofing products on the shelf where they
belong.
Sheath the roof deck solidly, butting the boards against each other on the sides and ends, leaving no gaps
other than toe holds every few feet on steep roofs (although gaps won’t hurt anything either). Almost all wood will
shrink once installed on a slate roof, due to the heat and dryness. When using green or air dried lumber, you do
not have to leave an airspace between the boards. Many older slate roofs were constructed of slating lath, which is
simply 1×2 or 1×4 strips of wood spaced apart to allow for nailing the slates. Although this traditional system does
work and conserves wood, it is inferior to a solid wood deck because it makes repair, maintenance, and restoration
in the later years more difficult.
Cover the sheathing, no matter how green, with one layer of 30 lb. roofing felt paper, overlapped about
three inches at the top edges, and nailed to the roof with 1”galvanized roofing nails (EG is OK here – see below).
Felt paper isn’t absolutely necessary for the roof to function; many slate roofs (primarily barn roofs) don’t have any
felt paper at all. Felt paper does, however, provide a temporary cover in the event of rain during installation, and it
helps insulate and waterproof the roof, so it is recommended to use 30 lb. felt in order to do the best job.
NAILS
Many years ago, roof slates were hung on slating lath with a single wooden peg driven through a hole in
the top center of the slate. No nails were used. Today we “hang” the slates onto the roof deck with two nails. When
nailing new slates, use 1.5” copper roofing nails. When re-using old slates, a 1.5 inch hot-dipped galvanized nail
will do. Make sure though, that it’s “hot-dipped” and not electrogalvanized (EG). Do not nail the slates tightly
against the deck or you will break them. It’s called “overnailing.” Do not “undernail” them either, or the protruding
nail head will rub against the overlying slate and wear a hole in it. Nailing slates takes some practice, but it’s not as
difficult as it sounds. The nail holes come pre-punched and they’re naturally countersunk to allow the nail head a
place to hide. In Europe, it has become common to hang all slates on “slate hooks.” This practice developed
because the Europeans, especially the French and Germans, decided to use slates that were split very thin. The
slates were so thin that a nail head could not hide in the slate, so hooks are used instead. This is another example
of traditional methods being replaced for inferior, but modern (like plywood) substitutions. The best slate roof, however,
is still the traditional one — nailed onto a solid wood deck.
TOOLS
Many people think that it’s difficult to work with slate. Wrong. Slate is a very nice material to work with,
especially old roofing slates. It cuts readily, you can punch a hole in it easily, and you don’t need electrical tools,
only simple hand tools. A good slate cutter looks somewhat like a paper cutter and will cut straight cuts, and even
convex and concave curves. A slate hammer not only nails slates, but will punch holes in them, and even cut
them. A slate ripper is a long sword-like tool that removes a slate from the roof without having to remove overlying
slates even after the slate has been nailed in place. A ladder hook attaches to a ladder section and hooks over the
ridge of the roof to allow for a way to get up and down on a steep roof. Roof jacks nail to the roof deck to create a
platform to work from. You should have all of these tools, plus a nail belt, chalk line, utility knife, and a collection of
ladders.
INSTALLATION
When laying out a roof in preparation for slating it, chalk lines across the entire roof area marking the top
edge of every row of slate. No aluminum drip edges (as are commonly used on asphalt shingle roofs) are needed
on slate roofs. When measuring for the starter slate and the first row, allow for the slate to hang beyond the drip
edge of the fascia (or trim moulding) one and a half inches. The starter slates are usually made of the same size
slates as those on the main roof, turned sideways and upside down (back facing out), and usually 1/4 of the length
of the first one is trimmed off to allow the joints to be properly staggered in relation to the overlapping row. The rule
of thumb is that all butt joints between slates should
have a minimum of three inches of lateral clearance in
relation to the butt joints of overlapping slates. On
many old roofs the starter slates are not laid sideways,
but are simply the same slates as the rest of
the roof — cut short — and again the joints are staggered.
In all cases, the starter slate must be laid over
a shim or cant strip about 1/2 inch thick, which cants
the slate at an angle comparable to the angle of the
slates on the rest of the roof.
The slates that run up the side of the roof should
extend beyond the gable ends one full inch. Run a
string up the edges of the roof to give yourself a
straight edge to follow when laying the slate (tie the
string to temporary nails), or chalk lines up the roof
edge to align the inside edge of the slate in order to
leave a one inch overhang on the outside.
When you reach the top of the roof, the top rows of
slates must be cut shorter in length to fit the roof.
Frequently the top row, the “cap” slates, must be
shimmed underneath so they’ll remain flat when the
ridge iron or copper is installed; otherwise they’ll cock
crookedly and look bad. They can be shimmed with
pieces of slate, usually the pieces that are cut off the
top rows when the slate is laid.
Headlap is critical. Every slate overlaps TWO rows beneath it. The only exception is the starter row and the
first row. The overlap on the second row beneath the slate is called the “headlap,” and it is typically three inches. If
you do not allow for sufficient headlap you may as well not put the roof on. On lower slopes such as 4:12, you
must increase the headlap to four inches. On very steep roofs (12:12 or greater) a two inch headlap may be sufficient.
Rule of thumb — don’t put slate on low slopes and do use a three inch headlap when installing on steep
slopes.
NEVER walk on slate during the installation, or any other time. This is very important. Run the rows of slate
up the roof at an angle and work from the side. Slate is not asphalt — it cannot be walked on. Don’t sit on it either.
Hook ladders will keep your weight off the slate. Also, roof jacks and planks with ladders sitting on them lying on
the roof are a good way to go. The ladders CAN lie on the slate.
VALLEYS
Be sure to use a non-corrodable metal in the valley. Twenty ounce copper or terne coated stainless steel
are recommended. Valley metal flashing is installed over the felted sheathing before any slating begins. The felt
paper need not overlap the valley flashing (the valley metal can be laid right on top of the felt). Remember that the
felt is only a temporary covering which will become full of holes when you install the slate. It’s the SLATE that
makes the roof waterproof, NOT the felt underlayment! This is why it’s ridiculous to rely on ice and water shield to
waterproof a slate roof.
Before the valley metal is installed, strike a chalk line up the edge of one side of the valley, on the felt
paper, to indicate where the edge of the metal valley flashing should be. In most situations, a valley that is overlapped
by 5” of slate will suffice, providing there is enough slope and the roof planes are roughly equal in size and
slope. Therefore, a standard 6” exposed valley will require 16” valley flashing material. Wider material can be
used, although you’ll find that you just nail holes in the outer edges anyway, which is a waste of material. Larger
roofs that drain more water, such as on churches, should have wider exposed valleys (like 8”) with wider valley
material. There are, of course many variations in valley styles, from open valleys to closed valleys to rounded valleys,
creased valleys, inverted V-groove valleys, etc. The inverted V-grooves are necessary when two unequal roof
planes are draining into each other.
The metal is then nailed in place with a nail of a compatible material (i.e. copper flashing with copper or
brass nails, etc.), and the nails are kept to within one inch of the edge of the valley metal. The valley metal is then
carefully forced into the valley with the pressure of a knee as the other side is nailed (or pre-break a line down the
center of the valley for especially steep roofs). Some schools insist on folding the outer edges of valleys and cleating
them into place rather than nailing them directly to the roof
sheathing. Having replaced literally miles of old leaking valleys
myself, both nailed and cleated, I have never seen an instance
of a nailed valley leaking because it was nailed. Cleated valleys
leak for the same reason as nailed valleys — corroded
metal. Cleating is a practice recommended when solder joints
are used in the flashing to prevent strain on the joint. Valleys
sections do not need to be soldered; they are instead overlapped
by 6 inches — therefore cleating is an unnecessary
step that can be readily avoided.
Valleys should be laid in sections not to exceed twelve feet
in length, although a ten foot maximum length is recommended
due to the adverse effect of expansion and contraction that
can cause long pieces of metal to buckle and develop a leak
over time. The valley sections should simply be overlapped by
six inches — no soldering is necessary or recommended, as
it’s the old solder joints on the old valleys that tend to leak,
once again, due to expansion and contraction. Do not use roof
cement or other adhesives along the edges of a valley (except
as an emergency seal in the event of rain during installation), as adhesives make later repairs of the roof unpleasant
and difficult while adding no advantage to the functioning of the roof.
Valleys are typically laid “open,” with approximately six inches of metal exposed. The overall width of the
valley metal can vary from 14” to 20”, although a minimum of 16” is recommended for a 6” exposure (providing no
nails penetrate the valley more than an inch or so from the edge of the metal when the roof is slated). Open valleys
typically have parallel sides running from bottom to top, although some roofers prefer open valleys that gradually
widen toward the bottom. When laying slate into a valley, chalk a line the length of the valley on both sides to
indicate the edges of the slate, then draw over the chalk lines with a permanent ink felt-tipped pen, as the chalk
lines will wear off the metal almost immediately. When nailing slate over the valley metal, be careful to nail only
along the edge of the metal, and not anywhere near the center. If a small, triangular piece of slate cannot be nailed
over the valley at the end of a slate row without nailing too close to the center of the valley, eliminate that piece of
slate — you won’t need it.
RIDGES
Ridges, like valleys, are typically made of
metal, but are often slate, and sometimes ceramic
tile. When finishing slating along a ridge, it’s important
that the roof sheathing does not have any
appreciable gap at the peak. If a gap exists (as is
typically left when a carpenter sheaths a roof for ventilated
ridge) the slates may not lay properly, and
ridge metal will not have a sufficient base in which to
nail. Ridge ventilation became common on houses
as more and more people had problems with plywood
delaminating and walls sweating moisture.
Slate roofs, when properly built, naturally breathe.
The gaps between the sheathing boards and
between the slates allow for some air flow. This is
contrary to asphalt shingle roofs with plywood decks,
which do not breathe and must be ventilated. If you
want to ventilate your slate roof, ventilate out the
gable ends, or through roof top vents. If you insist on
venting through your ridge, buy or build a ridge vent
system specially designed for a slate roof — do not
use the cheap aluminum vented ridge sold for
asphalt shingle roofs.
ROOF JACKS NAIL IN THE SLOT OVER STANDARD THICKNESS
SLATES WITH 16P COMMON NAILS, AND UNDER
THICK SLATES (AS SHOWN IN THE CENTER) TO HOLD A
2X10 PLANK. A LADDER CAN BE SET ON THE PLANK AND
LAID ON THE ROOF. ALTERNATIVELY, REMOVE A SLATE
AND NAIL THE ROOF JACK BETWEEN THE TWO UNDERLYING
SLATES.
DO NOT
1. Do not use laminated wood roof decking — use solid lumber.
2. Do not use insufficient headlap — use three inches of headlap — more on lower slopes.
3. Do not walk on the slates or sit on them during installation — if possible, work from the side or from hook ladders
or roof ladders on planks.
4. Do not use “electrogalvanized” nails — use hot dipped galvanized nails on recycled roofs and copper or stainless
steel nails on new slate.
5. Do not rely on the underlayment (felt or ice and water shield) to waterproof the roof — you will puncture it profusely
when you install the slate. A properly installed slate roof will not leak, underlayment or no underlayment. Ice
and water shield is a product designed to protect plywood from delamination — you will not need it on a slate roof.
6. Do not use aluminum drip edges — they’re made for asphalt shingle roofs.
7. Do not use ventilated ridges unless they’re specifically designed for slate roofs — ventilate through gable ends
or through individual roof vents.
When Starting to Slate a Roof — Ten Quick-Reference Steps
1) Make sure that the fascia is completely installed beforehand and that the ends of the sheathing boards are firmly nailed.
2) Felt over the board sheathing with 30 lb. roofing felt, lapped at least three inches at the top and sides.
3) Nail a wooden starter shim at the bottom edge of the lowest sheathing board – it should be about 1/2” thick, and at least an
inch wide (eight foot lengths are convenient). Cedar or redwood is ideal (cedar shim shingles will work), but the same local
lumber as the sheathing will do just fine.
4) Chalk a horizontal line on the felt paper for the starter slates, measuring the width of the slate up the roof from the bottom
edge of the wood shim, deducting 1+1/2 inches for the slate overhang. Next, chalk a line for the first full row, now measuring
up the roof the length of the slate and deducting an inch and a half for the overhang.
5) Now measure up the remainder of the roof equal distances equivalent to the exposure of the slate, and chalk lines accordingly.
But first, make sure your second full row of slates will overlap the starter row by three inches based on your measurements
— if not, drop that second row down an inch or so to where you need it to be, then chalk the rest of the roof with the
exposure measurement. [Exposure is determined by subtracting the headlap from the total length of the slate, then dividing
the remainder in half. For example, a 20” slate with a 3” headlap will have a 8.5” exposure (20 - 3 = 17, divided in half =
8.5).]
6) Do not bed the starter slates or any slates in roof cement or caulk, except for very small pieces on edges in unusual circumstances.
Adhesives make it very difficult to repair the roof in the future. Instead, two 1+1/2” hot dipped galvanized or copper
nails per slate is a good rule of thumb which will ensure the secure attachment of all slates to the roof. Don’t nail the slates too
tightly, let them hang on the roof. Do make sure the nailheads are set into the slate however, as nails that stick up will eventually
wear a hole in the overlying slate, and cause a leak.
7) Tap a couple of temporary nails into the side of the fascia on the gable end, one at the top and one at the bottom, and run
a string up the edge of the roof positioned one inch out from the fascia. Use the string as a guide to align the edge of the slate
as you nail them into place. Remove the string when you’re done. Or chalk vertical lines on the roof for edge slate alignment.
Make sure the slots between the slates on the first full row are staggered at least 3” laterally from the butted ends of the
starter slates. If not, reinforce the joint by sliding a piece of flashing over the starter slate and under the first row at the joint.
9) You can work the first half dozen rows from a ground ladder or ground scaffold, then nail roof jacks and planks along the
bottom of the roof and work up from there. Use more jacks, planks and roof ladders as needed.
10) Have fun!
Exposure is determined by subtracting the headlap from the total length of the slate, then dividing the
remainder in half. A 20” slate with a 3” headlap will have a 8.5” exposure (20 – 3 = 17, divided in half = 8.5).
SLATE ROOFING — TOOLS OF THE TRADE
THREE TYPES OF SLATE
CUTTERS:
PEARSON (ABOVE)
STORTZ (LEFT)
FREUND (RIGHT)
STORTZ
SLATE RIPPER
(RIGHT)
LADDER HOOK
(RIGHT)
MISCELLANEOUS
SLATE HAMMERS
(LEFT)
