From my research, it appears you DO need to replace the cracked
slate, although an experienced ?slater? may be able to repair the
slate. Generally, the cracked slate is removed and replaced, as
moisture that enters the crack and destroy the wood flooring
underneath. Slate is expected to last 60-125 years depending on the
type and maintenance, so you may want to check your warranty!
Most slate specialists recommend NOT using tar to repair slate roofs.
You may want to invest in ?The Slate Roof Bible? before you begin
repairs. I have placed a link below with information about the book;
it was recommended on various slate roof sites.
?Depending on the quality, a slate roof can last from 60 to well
over 100 years. Towards the end of a roof's serviceable life, however,
slate maintenance becomes increasingly necessary. At this point, the
slate shingles - or tiles - are usually still in good shape. It's the
fasteners, or nails, that corrode and fail from years of exposure to
the elements, causing the slate tiles to fall and break.?
?Slate roofs are susceptible to physical damage. Walking on or
throwing objects against a slate roof can and will probably break some
of the slates. Do not allow any foot traffic on a slate roof that is
not absolutely necessary. Replace broken and cracked slate regularly!
Do not allow debris to accumulate on the roof surfaces. If your slate
roof does not have snow and ice guards consider installing them. Do
not let anyone walk on your slate roof without taking special
I hope you have Power Point on your PC, so you can download this
very informative slide show. You can install a free viewer from
Microsoft if you don?t own Power Point. Click anywhere on each slide
to prompt the text to appear and to move to the next slide.
Here are some useful tips from the slide show:
? ?The tendency of old, weathered slates to absorb and hold moisture
can lead to rot in underlying area of woodsheathing.
? Delamination and flaking are just as bad or worse on the underside
of slate as on the exposed surface. This is why most slates cannot be
flipped over for reuse.
? Broken, cracked, and missing slates should be repaired promptly by
an experienced Slater in order to prevent water damage to interior
finishes, accelerated deterioration of the roof and roof sheathing,
and possible structural degradation to framing members.
? When many slates must be removed to effect a repair, the sheathing
should be checked for rotted areas and projecting nails.
?Roof sheathing itself can cause problems. Wood is hygroscopic. It
attracts and liberates moisture in an attempt to reach an equilibrium
with outdoor humidity. This property cause the wood to shrink and
swell. These movements can cause the nails that hold a slate shingle
to pinch a corner. This stress causes the shingle to fracture and
detach from the roof. Slate is not supposed to be nailed to a roof. It
is supposed to hang from the nails much like a picture hangs on a
?DEAR TIM: My older home has a slate roof. Some of the pieces of
slate have cracked and fallen. Entire pieces have come loose. The
slate is very brittle. Is it time for my roof to be replaced? If not,
why is the slate breaking and falling? In my attic you can see the
bottom of the slate. Half of my roof boards are missing. Is this the
cause of my problems? D. D.
DEAR D. D.: Brittle is good! If you had told me that your slate was
soft and flaky like my favorite dinner rolls, you would have a major
job ahead. Slate is one of the finest roofing materials available. It
is fireproof, resists hail damage, possesses unquestionable beauty,
and often has a service life of 100 years or more.?
?The wood decking may be to blame. Wide swings in temperature and
humidity cause your wood roof sheathing to expand and contract. This
creates stresses on the slate. If the nails are in a bind, a corner
may pop off and the slate will crack or fall.?
?I have purchased some reclaimed peach bottom recently ~60 yrs old,
10x18, and 12x18 from a classified on this site. After initial sorting
for thickness I noted breakage of ~50 slate/crate. I am able to use
these on the roof, but what I found after getting under way were
slates with a hairline crack visible from both sides running mostly
vertical on the slate, this was easier to spot on a damp slate as a
black line. At first I incorporated these slates, but as I became more
familiar with them I found I could snap these slates in two very
easily especially when cutting, always following the crack. The roof
is still in progress and after installing 15 sq. I estimate a 10-15%
breakage factor. As you can imagine this creates a lot of extra labor.
I am unable to use these slates because my roof is all hips. Is
anybody familiar with this slate? I purchased it from a classified on
this site and would be interested to know if anyone else who purchased
from this lot is having any problems, the seller says that I am the
?All slate shall be hard, dense, sound rock, machine punched for
two nails each. No cracked slate shall be used. All exposed corners
shall be practically full. No broken corners on covered ends which
sacrifice nailing strength or the laying of a watertight roof will be
allowed. No broken or cracked slates shall be used.?
?Generally speaking, Mr. Jenkins said, colored slates are almost
always hard slate and black slates are almost always soft. (There are
some black slates that are hard, and there are some colored slates
that are soft because they're really black slates that have been faded
by the sun.) In any case, he said, a slate expert -- known as a
''slater'' -- will be able to tell hard slate from soft simply by
tapping on it.?
?'If it's hard slate, you probably should restore it,'' he said,
explaining that even a 100-year-old hard slate roof could have another
100 years of useful life. ''But if it's soft, it may not be worth
Mr. Jenkins said that in most cases, leaks in slate roofs are caused
either by broken or missing tiles or faulty flashing. If the problem
is with the tiles themselves, he said, repairs can easily be made by
an experienced slater. ''All you have to do is pull out the bad slates
and replace them with good ones,'' he said, explaining that a mistake
many homeowners make is to wait until there are several broken or
missing slate tiles before calling in a contractor.
''That's like having a cavity in your tooth and waiting to get more
cavities before getting the first one fixed,'' Mr. Jenkins said,
adding that even one cracked or missing tile can allow water
penetration that can damage the roof deck.
Leaks caused by faulty flashing, he said, may require a full restoration.?
This Slate Roof forum has some good information:
?I have a slate roof, about 60 years old in need of some repairs.
There was a leak behind the chimney (flashing pulling away) that I had
repaired last year by a guy who wasn't exactly professional. He
applied tar and a membrane, instead of removing and replacing the
flashing. It looks pretty bad from the street. So I've had some
estimates from others, and I'm getting conflicting stories.
Should I forget about the flashing behind the chimney as long as it
doesn't leak? Or should I have it re-flashed with copper like the
There are some loose slates. One roofer wants to just slide new slates
in under the existing, nail them (copper nails), and caulk over the
exposed nail heads. Another has a galvanized bracket that slides under
the existing slate and is pointed so that it pounds into the roof. It
has a hook on the bottom to hold the new slate. Which method do
Should I consider ridge rolls? This is a flashing that would cover the
seams in the roof. I don't have a problem with leakage now, and I
think this shiny flashing will detract from the roof's appearance. But
some of these guys insist that this is what I need.
What about flashing around vent pipes? The original is still up there
and not giving me any problems. Is this likely to corrode and cause
problems in the future?
Sorry this list is so long, but I think there is a lot of bad advice
out there. I will appreciate any advice you can give me.?
Answer:? Hi, Yes your flashing should be replaced for protection from
leakage in the future..Copper would be a good choice for durability
and looks..The slate hooks are a good choice for repairing the slates
that need replaced,You really should avoid face nailing the slate if
possible..Your roofer shuold use a slate ripper to remove the nails
and damaged slate,install the slate hook,then install the replacement
slate,size and color to try to match your existing slate as close as
possible..Ridge iron on your peaks is okay,(once ridge irons are
installed unless they are aluminum(they are to light a gauge to use
,do not recommend)they must be painted every 8-10 years ect.)but it
sounds like you prefer the way your roof looks without it, If the
peaks of your roof are not leaking then I would leave them be. If your
roofer is up there they can inspect them to see if they are any voids
that might cause a problem..--Or if you wanted to you could have your
roofer point or caulk your hips with a good quality roof cement or
urethane caulk, this does take a little bit of time and patience to do
a nice neat job, you do not want the bead to wide or wavy.?
?Under General Exemption (G1), repairs to slate and terracotta
tiled roofs must be based on the principle of doing as little as
possible and only as much as necessary to retain and protect the
element. Repairs must match existing in material, form, dimensions and
profile and there must be no damage to significant fabric. Where
original roofing elements have been replaced with modern substitutes,
reinstate the original if this is known and can be matched exactly.
Repairs to slate and terracotta roofs approved under General Exemption
? refixing gutters, downpipes and drains;
? temporary fixing of slipped slate tiles with zinc or copper clips;
? re-nailing loose battens;
? partial replacement of tiles;
? patching flashings;
? re-pointing cappings;
? partial replacement of flashings and cappings;
? patching roof ventilators;
? patching rainwater heads; and
? reinstating roofing elements where original is known and is matched exactly.
?Inspect the roof on a regular basis and look for any slates that
appear to have slipped or are slipping. If the bottom edges of the
slates are in uneven lines across the roof, the slate could be
slipping from the battens due to breakage or faulty nails. Look for
noticeable cracks or breaks. Inspect flashings and valley gutters for
deterioration and keep them free of debris that may trap water.?
The Slate Roof Bible
About sloppy repairs
?As part of any slate roof repair project, be aware that unsound
wooden decking may be uncovered during the project and will need to be
replaced; the wood decking actually supports and holds the slates in
place. New wood should match adjacent wood and be back primed before
There you go! Please request an Answer Clarification, and allow me to
respond, before you rate this answer.
Cracked slate roof
reparing slate roof
slate roof care
Replace cracked + slate roof