Slate Roofs >
The following guideline is provided to assist in the repair/replace
decision making process:
- Consider the age and condition of the roof versus its expected
serviceable life given the type of slate employed.
- Calculate the number of damaged and missing slates. Is the
number less than about 20%? Is the roof generally in good condition?
If so, the roof should be evaluated for repair rather than replacement.
Also, keep in mind that the older a roof becomes, the more maintenance
it will likely require.
- Determine if there are active leaks and what their source
may be. Do not assume the slates are leaking. Gutters, valleys
and flashings are more likely candidates. "False leaks"
can be caused by moisture condensation in the attic due to improper
- Check the roof rafters and sheathing for moisture stains.
Poke an awl into the wood to determine if * is rotted. Remember
that very old, delaminating slates will hold moisture and cause
adjacent wood members to deteriorate even if there are no apparent
- Are many slates sliding out of position? If so, it may be
that ferrous metal fasteners were used and that these are corroding,
while the slates are still in good condition. Salvage the slates
and relay them on the roof. If the slates have worn around the
nails holes, it may be necessary to punch new holes before relaying
- Consider the condition of the roof's flashings. Because slate
is so durable, metal flashings often wear out before the slate
does. Examine the flashings carefully. Even the smallest pinhole
can permit large quantities of water to enter the building.
Is the deterioration of the slate uniform? Often this is not
the case. It may be that only one slope needs replacement and
the other slopes can be repaired. In this way, the cost of replacement
can be spread over many years.
- Press down hard on the slates with your hand. Sound slates
will be unaffected by the pressure. Deteriorated slates will
feel brittle and will crack. Tap on slates that have fallen
out or been removed. A full, deep sound indicates a slate in
good condition, while a dull thud suggests a slate in poor condition.
- Are new slates readily available? Even if replacement is determined
to be necessary, the existing roof may have to be repaired to
allow time for documentation and the ordering of appropriate
Note: measurements in this publication are given in both U.S.
Customary System and International (Metric) System for comparative
purposes. Metric conversions are in some cases approximate and
should not be relied upon in preparing technical specifications.