THE ROOF. SLATES, AND OTHER ROOF COVERINGS.
We might, perhaps, under the designation of “Slates and Slating,”
have included the operations usually understood to appertain to the
construction of a roof. But modern improvements have rendered such a
designation incomplete. We cannot now properly understand the mode of
roofing houses without referring to many other substances besides slate.
Slate-Quarries.
Slate is the popular name for a variety of rocks which are sufficiently
stratified in their structure to allow of their being cleaved into
thin plates, a property which renders them valuable for a variety of
purposes. Slate has superseded the use of lead for covering roofs, even
of the largest buildings: from its lightness it is preferable to tile,
but the latter being cheaper, in flat countries which do not contain
rocks, but which yield brick-clay, slate in such localities is only
used on the better class of houses. In mountainous countries, a slaty
rock, which admits of being split thin, though not so much as clay
slate, is used under the name of _shingle_.
Besides being employed for roofing, slate is used in large slabs to
form cisterns, for shelves in dairies, for pavement, and similar
purposes, for which its great strength and durability, coolness, and
the ease with which it can be cleaned, owing to its non-absorbing
property, adapt it. The latter quality renders it also of great value
as a cheap substitute for paper, in the business of education; the
system of teaching in large classes in National and Sunday-schools
would be greatly fettered but for the use of slates.
The principal slate-quarries in Britain are in Wales, Cumberland, and
various parts of Scotland; the mode of working them is generally the
same. The rock is got out in tabular masses by means of large wedges,
and is then subdivided by smaller to the requisite thinness; the pieces
are roughly squared by a _pick_, or axe, and sorted, according to their
sizes, for roofing. The largest called _imperial_, are about three and
a half feet long, and two and a half wide; the smallest average half
those dimensions. When wanted for paving, &c., the large blocks are
_sawn_ into thinner slabs, in the same manner as stone or marble is.
A few words respecting the position and working of some of the
slate-quarries may be appropriate, as illustrating the nature of this
remarkable geological formation.
[Illustration: A Slate-Quarry.]
The most extensive slate-quarries in Great Britain are those near
Bangor, in Wales, from which slate is shipped to all parts of the
world. The slate occupies the greater part of the distance from Snowdon
to the Menai Straits. Upwards of two thousand men are employed in these
quarries; and the proprietor is said to gain from thirty to forty
thousand pounds per annum by them. Although this one is the largest,
yet there is one in Cumberland in which the slate is found more
remarkably situated. This is Hourston Crag, a mountain near Buttermere
Lake, about two thousand feet above the level of the lake, and nearly
perpendicular. On account of the difficulty of access, the workmen
take their provisions for the week, and sleep in temporary huts on the
summit. During the winter months they are generally involved in clouds,
and not unfrequently blocked up by the snow. The slate is conveyed
on sledges down a zigzag path cut in the rock, one man attending to
prevent the acceleration of the descent. When the slate is emptied at
the bottom the sledge is carried back on the man’s shoulders to the
summit.
Notwithstanding the value of slate, few quarries are worked to a very
great depth, or have subterranean galleries like mines. There is one,
however, near Charleville, in France, which is an exception to this
rule. The mouth of the mine is near the summit of a hill; the bed
inclines forty degrees to the horizon, and is about sixty feet in
thickness, but the extent and depth are unknown. It has been worked by
a principal gallery to the depth of four hundred feet, and many lateral
galleries have also been driven, extending about two hundred feet on
the side of the main gallery. Twenty-six ladders are so placed as to
give passage to the workmen and carriage for the slate. Of the sixty
feet which constitutes the thickness of the bed of slate, about forty
are good slate, the rest being mixed with quartz. The slate is cut into
blocks of about two hundred pounds each, called _faix_; each workman,
in his turn, carrying them on his back to the very mouth of the pit,
mounting all or part of the twenty-six ladders, according to the depth
of the bed where he may be working. When brought to the surface, these
blocks are split into thick tables called _repartons_, by means of a
chisel and mallet; and these repartons are divided by similar means
into roofing-slates.
Another remarkable slate-quarry in France, is situated near Angers.
The bed of slate extends for a space of two leagues, passing under the
town of Angers, which is in great part built of slate; those blocks
which are the least divisible being employed in masonry. The quarries
actually explored are all in the same line, from west to east, as well
as the ancient pits, the bed of the best roof-slate rising to the
surface in this direction. Immediately under the vegetable earth is
found a brittle kind of slate, which, to a depth of four or five feet,
splits into rhomboidal fragments. A little lower is the building-stone,
which is a finer but scarcely divisible slate, and is employed in the
construction of houses, after it has been sufficiently hardened by
exposure to the air. At fourteen or fifteen feet from the surface is
found the good slate, which has been quarried to the perpendicular
depth of three hundred feet, without its lower limit being attained.
The interior structure of the slaty mass is divided by many veins or
seams of calcareous spar and quartz, fifteen or sixteen feet in length,
by two feet thick; these veins are parallel, and proceed regularly
from west to east in a position rising seventy degrees to the south;
they are intersected by other veins at intervals of a similar kind,
but whose rise is seventy degrees north; so that when the two series
meet, they form rhombs or half-rhombs. All the layers or laminæ of
slate have a direction similar to those of the veins of quartz, so
that the whole mass becomes divided into immense parallel rhomboids.
The slate is extracted in blocks of a determinate size, which are then
divided into leaves for roof-slates. When the blocks have been drawn
from the quarry, if they are left exposed to the sun or the open air,
they lose what is called the _quarry-water_, and then become hard and
untractable, and can only be employed as building-stone. Frost produces
a singular effect on these blocks; while frozen, they may be broken
with more ease than before; but if thawed rather quickly, they become
no longer divisible; yet this quality may be restored by exposing them
once more to the frost.
The Process of Slating.
When the blocks of slate for roofing have been split, and the laminæ
roughly squared, they are sorted, according to their size and quality,
and are brought to market under the quaint names of _Imperial slates_,
_Duchesses_, _Countesses_, &c., the first variety being the largest.
The best roofing-slates come from the celebrated vale of Festiniog.
Slates are laid on _battens_, or thin narrow deal boards, which are
nailed horizontally on the common rafters of the roof, at equal
distances apart, which distance is governed by the size of the slate
to be employed. An entire board is nailed along the lowest edge of the
roof to receive the lead of the gutters, which are first laid, and
then the lowest _course_ of slates are nailed and pinned down to the
lowermost batten; so that two-thirds the length of the slate, at least,
shall lie over the lead. The next course of slates is then fixed, so
that every slate shall overlap two-thirds the depth of the course below
it, every slate being also laid over the joint, between two slates of
that undercourse. By this construction the rain that runs through the
joint between any two slates is kept from penetrating into the roof by
being received on the surface of the slate beneath that joint; and the
bottom course of slates is double, to continue the same principle down
to the lead gutter.
[Illustration]
The slates are fixed to the battens by two copper nails and a wooden
pin when the work is well executed; holes being picked through each
slate for the nails to pass through.
Paper Roofs.
Although, as intimated in a former page, in covering our imaginary
dwelling with tiles or slates, we may seem to have done all that is
necessary in respect to “roofing,” yet we should leave our subject only
half treated if we were to omit mention of other contrivances which
have been partially acted on; such as the use of paper, of asphaltum,
and various other substances.
About thirty years ago, Mr. Loudon published a pamphlet, in which
he described the mode of preparing paper for roofs, and discussed
the various arguments for and against its adoption. His description
had immediate relation to a series of paper roofs in a large farm at
Tew Lodge, in Oxfordshire, and comprised the following among other
particulars.
Paper roofs may be made very flat, being raised no higher than just
sufficient for throwing off the water. Instead of tile, slate, or
thatch, they are covered with paper, prepared by immersion in a mixture
of tar and pitch. In the first place, pieces of wood called “couples,”
are laid across the walls of the building, rising two inches and a half
to the foot to obtain a drainage obliquity; these couples vary from two
or three to six inches square, according to the size of the roof. On
the couples are placed horizontal rafters, about two inches square; the
distance between the couples being from five to eight feet, and between
the rafters about eighteen inches; the couples are nailed to the wall
plate, and the rafters to the couples. At Tew Lodge, the rafters used
were young larch-trees, sawn up the middle, cut to the proper lengths,
and prepared so that the upper surface should be level. On the rafters
are placed thin boards, from a half to five-eighths of an inch in
thickness; these boards are nailed to the rafters, not horizontally
as for slating, but in a direction from the eaves to the ridge of the
roof. In some cases substitutes for thin boards may be used; such as
close copse-wood hurdles, plastered over; or common plaster-laths.
The paper employed may be any common, coarse, strong kind; that kind
used by button-makers being favourable for the purpose. It is prepared
as follows: a boiler or cauldron, three feet wide by two deep, placed
over a fire, is filled to within six inches of the top with tar and
pitch, in the proportion of three parts of the former to one of the
latter; the fire being applied and the mixture made to boil, the paper
is immersed in it one sheet at a time, and then laid in a stack or pile
with such a slope as to allow it to drain, a little grease of any kind
being placed between the sheets to prevent their adhering; and when dry
the paper is similarly treated a second time. The paper thus prepared
is then nailed down to the roof. The workman begins at the eaves, and
allows three inches for being turned down and nailed underneath the
end of the board, which boards project an inch over the first rafter.
If the paper be common, coarse, wrapping paper, it is laid on much the
same as slate, so that when finished it will remain in double thickness
all over the roof; but if thicker paper be employed, it is only made
to overlap about three inches in each layer. Every sheet is fixed down
with four nails about an inch in length, having broad flat heads.
On the paper thus fixed is laid a composition consisting of two parts
of tar to one of pitch, thickened to the consistence of paste, with
equal parts of whiting and powdered charcoal. The composition being
well boiled and kept constantly stirred, it is spread over the roof
with a hempen mop as quickly as possible on account of the speedy
cooling. When properly laid on and dried, the composition totally
conceals the joints of the paper, and forms a smooth and glossy
black covering an eighth of an inch in thickness. Sometimes, while
the composition is yet wet, sand, dust, or ashes are strewed on, to
increase the substance, and shield the composition from the action of
the sun.
Mr. Loudon enumerates as the advantages of this roof--economy,
durability, and elegance. The economy is shown by the circumstance
that, on account of the lightness of the paper, less massive walls
and timbers are required than for other kinds of roof. The expense at
Tew Lodge was from fourpence to tenpence per square foot, everything
included. It is one result of the flatness of the roof, that ten square
feet will cover as much as fourteen feet at the usual pitch of slated
roofs. As to the durability, many proofs are adduced to support it.
A paper roof to a church at Dunfermline remained forty years without
requiring any repairs; and several warehouses at Greenock, Deal, Dover,
and Canterbury, had paper roofs, which were known to stand from ten
to twenty years. Mr. Loudon considered that, from the flatness of the
roofs, and from other circumstances connected with the appearance
of the prepared sheets, the paper roofs were more fitted to join
harmoniously with certain styles of architecture than slated roofs.
Objections have been made to this kind of roof, on the ground that it
is liable to be blown off by high wind, and still more that it is very
inflammable. With regard to the former, Mr. Loudon states that if the
roof be properly made there is little danger of its being removed by
high wind. In reference to the second objection, he states:--“They seem
to me not so liable to set fire to as thatch. Pitch (especially if
coated over with sand or smithy ashes) will not be lighted by a spark,
nor even by the application of a slender flame, as will that material;
though, on the other hand, when lighted, it will unquestionably
burn with greater velocity than any species of thatching.... In the
steward’s house and men’s lodge wood is constantly used as fuel, which,
though more dangerous for emitting sparks than coal, yet no accident
has or is ever likely to happen to the roof. In my house, where coals
were chiefly used, the chimneys have been repeatedly set on fire to
clean them, without the least accident happening to the roof.”
Many years afterwards, when Mr. Loudon published his elaborate
_Encyclopædia of Cottage, Farm, and Villa Architecture_, he briefly
sketched some of the forms of roof which have more or less recently
come into use. These we must here notice.
Terrace Roofs.
_Terrace roofs_ have been much used in and about London. They are
formed of thin arches of tiles and cement, supported on cast-iron
bearers or ribs, which are placed about three feet apart. The arch is
composed of three courses of common plain tiles, bedded in fine cement
without sand. In laying the tiles, laths or small slips of wood are
used, resting on temporary bearers between the iron ribs; the laths
being shifted as the work advances, in the course of about half an hour
after the tiles are laid. Particular attention is required in bonding
the tiles both ways; and they are rubbed down closely upon each other,
much in the same manner as a joiner glues a joint. Sometimes these
terrace roofs are coated with a layer of coarse gravel, and then with
nine inches of good soil, so as to form a terrace garden. The roofs of
two taverns at Hungerford Market are formed of these cemented tiles.
Asphalte Roofs.
_Asphalte_ or _bitumen_ has come into use as a material for roofs. It
had been employed for various purposes in France for many years, but
did not attract much attention till within the last eight or ten years.
It is now in very general use in that country for foot pavements, flat
roofs, and water-cistern linings; and in England it has also been
a good deal used for the same purposes, and for barn-flooring. The
particular modes in which it is employed for floors and pavements we
need not here consider, but it has been used for roofs in the following
manner. Mr. Pocock has patented a “flexible Asphaltic roofing,”
intended to supersede the use of slates, tiles, zinc, thatch, &c.,
in the covering and lining of farm-buildings, sheds, cottages, and
other erections; and it is approved for its durability, lightness, and
economy. The weight of this material being only sixty pounds to the
square of one hundred feet, the walls and timbers to support it need to
be but half the usual substance; it is also a non-conductor of heat,
impervious to damp, and will bear a heat of two hundred and twenty
degrees without injury. This peculiar material is said to be formed of
asphalte mixed with the refuse felt of hat manufactories, compressed
into thin plates.
Scotch Fir Roofs.
_Scotch fir roofs_ are occasionally made. The method of giving
durability to the timber for this purpose consists in first cutting
the wood to the required size, and then steeping it for a fortnight
in a pond of lime-water; it is found that the acid contained in the
wood becomes crystallized by combining with the alkali of the lime. Sir
Charles Menteath is said to have some farm buildings which, although
roofed with Scotch fir forty years ago, are as well protected now as
when the roofs were first laid on; the wood having been previously
steeped in lime-water. The sulphate of copper, the chloride of zinc,
the corrosive sublimate, and the various other chemical substances
which have been recommended of late years as means for preventing the
decay of timber, will possibly render the use of timber roofs more
practicable than it has been hitherto considered.
Iron Roofs.
_Roofs of iron_ are in great request at the present time. One of these
sorts of roofs may be formed of three kinds of cast-iron plates. The
first, called the “roof-plate,” is shaped with three of its sides
turned up and one turned down, and is made tapering narrower towards
one end; the second, called the “low-ridge plate,” has two of its
sides turned up and the other two turned down; the third, called the
“high-ridge plate,” has all its sides turned down, and is formed with
an angle in the middle, so as to slope each way of the roof. Such a
roof may be made very flat, inasmuch, that for a house twenty feet
wide, the height of the roof in the middle need not exceed two feet;
no boarding is required, the plates resting without either cement or
nails on the rafters. From the manner in which the edges of the plates
overlap, there is no risk of contraction or expansion.
Some of the iron roofs recently made are on the principle of those
used in Russia, of which the following description has been given in
the _Repertory of Patent Inventions_:--“Sheet-iron coverings are now
universally made use of in all new buildings at Petersburgh, Moscow,
&c. In the case of a fire, no harm can come to a house from sparks
falling on a roof of this description. The sheets of this iron covering
measure two feet four inches by four feet eight inches, and weigh
twelve pounds and a half avoirdupois per sheet, or one pound five
ounces each superficial square foot. When the sheets are on the roof,
they measure only two feet wide by four feet in length: this is owing
to the overlapping. They are first painted on both sides once, and,
when fixed on the roof, a second coat is given. The common colour is
red, but green paint, it is said, will stand twice the time. Small
bits or ears are introduced into the laps, for nailing the plates to
the two-inch square laths on which they are secured. It takes twelve
sheets and a half to cover one hundred feet, the weight of which is one
hundred and fifty pounds--the cost only £1 15_s._, or about threepence
per foot.”
Iron roofs are now often made of _corrugated_ or _furrowed_ sheet-iron.
In this form the iron is impressed so as to present a surface of
semi-circular ridges with intervening furrows lengthwise of the
sheets. By this means, a piece of sheet-iron, which, as a plain flat
surface, has no strength but in its tenacity, becomes a series of
continued arches abutting against each other; and the metal, by this
new position, acquires increased strength. Iron so furrowed is deemed
preferable to common sheet-iron for covering a flat-roof, because the
furrows will collect the water and carry it more rapidly to the eaves.
But there are greater advantages than this. If the furrowed sheets be
bent into a curved surface, convex above and concave below, they will
form an arch of great strength, capable of serving as a roof without
rafters or any other support, except at the eaves or abutments. Iron
roofs measuring two hundred and twenty-five feet by forty have been
constructed in this manner. To increase their durability the iron
sheets are coated with paint or tar.
Zinc and other Metallic Roofs.
Additions are made every year to the number of contrivances for forming
metallic roofs, among which is one now the subject of a patent, for the
use of _galvanized_ iron. In this case the aid of the electric agent is
employed to give iron sheets an amount of durability which they do not
possess in their natural state.
Zinc has been much employed within the last few years as a material for
roofs. Its availability for this purpose rests partly on its superior
lightness as compared with lead, and its superior condition under the
action of the atmosphere as compared with iron. The latter quality
arises thus; after the zinc has been covered with a thin film of oxide
by the action of the atmosphere, it suffers no further change from long
exposure; so that the evil of rust checks itself. At the temperature
of boiling water, zinc sheets, which are brittle when cold, become
malleable, and their availability for roofs is thereby increased. The
property which zinc has, however, of taking fire at a temperature of
about 700° Fahr., rather detracts from its value as a material for
roofs.
Thatch Roofs.
The most common material employed as thatch is either the straw of
wheat, rye, or other grain, or reed, stubble, or heather. The straw
of wheat and rye, when well prepared and laid, forms the neatest and
most secure thatching; the former being preferable to the latter in
smoothness, suppleness, and durability. Barley-straw is placed next
to rye in fitness for thatch, and oat-straw the lowest of the four.
The reed is a very durable material for thatch, but is generally too
expensive. It has been stated that, in Norfolk, where the reed is a
favourite material for thatch, a reed roof will lie fifty years without
wanting repair, and that, with very slight attention, it will last for
a whole century. Viewed in this light, a reed roof may probably be
considered economical.
The method of thatching with reed, (which is one of the best and most
difficult specimens of the thatcher’s art,) has been thus described.
No laths being made use of as a support to the thatch, a few of the
longest and stoutest reeds are scattered irregularly across the naked
spars as a foundation whereon to lay the main coat; and thus a partial
gauze-like covering is formed, called the _fleaking_. On this fleaking
the main covering is laid, and fastened down to the spars by means of
long rods called _sways_, laid across the middle of the reed, and tied
to the spars with rope-yarn or with brambles. In laying on the reed,
the workman begins at the lower corner of the roof on his right hand,
and keeps an irregular diagonal line until he reaches the upper corner
on his left; a narrow eaves-board being nailed across the feet of the
spars, and some fleaking scattered on. The thatcher begins to “set his
eaves” by laying a coat of reed, eight or ten inches thick, with the
heads resting upon the fleaking and the butts upon the eaves-board.
He then lays on his sway, or rod, about six or eight inches from the
lowest point of the reed, whilst his assistant, on the inside, runs
a needle threaded with rope-yarn close to the spar and to the upper
edge of the eaves-board. The thatcher draws it through on one side of
the sway and enters it again on the contrary side both of the sway and
of the spar. The assistant, in his turn, draws it through, unthreads
it, and, with the two ends of the yarn, makes a knot round the spar,
thereby drawing both the sway and the reed tight down to the roof;
whilst the thatcher above, beating and pressing the sway, assists in
consolidating the work. The assistant, having made good the knot below,
proceeds with another length of thread to the next spar, and so on till
the sway is bound down the whole length, that is, about eight or ten
feet. This being done, another stratum of reed is laid upon the first,
so as to make the entire coat eighteen or twenty inches thick at the
butts; and another sway is laid on and bound down about twelve inches
above the first.
When the eaves are completely set they are adjusted and made even
by an instrument called a _legget_. This is made of a board eight
or nine inches square, with a handle two feet long adjusted to its
upper surface in an oblique position. The face of the legget is set
with large-headed nails, and these enable the workman, by using the
instrument somewhat as if it were a turf-beating tool, to lay hold of
the butts of the reed and to adjust them in their places. When the
eaves are thus shaped, the thatcher lays on another stratum of reeds,
and binds it down by another sway somewhat shorter than the last, and
placed eighteen or twenty inches above it; and above this, others, in
successive rows, continuing to shorten the sways until they diminish to
nothing, and a triangular corner of thatching be formed. After this the
remaining surface of the roof is similarly done.
In order to finish the ridge of the roof, a _cap_ of straw is adjusted
to it in a very careful manner. In this operation the workman begins by
bringing the ridge to a sharp angle, by laying straw lengthwise upon
it: and to keep this straw in its place, he pegs it down slightly with
“double-broaches,” which are cleft twigs about two feet long and half
an inch thick, sharpened at both ends, bent double and notched, so as
to clasp the straw on the ridge. This done, the thatcher lays a coat of
straight straw six or eight inches thick across the ridge, beginning
on either side at the uppermost butts of the reeds, and finishing with
straight handsful evenly across the top of the ridge. Having laid a
length of about four feet in this manner, he proceeds to fasten it
firmly down, so as to render it proof against wind and rain; this is
done by laying a “broachen-ligger” (a quarter-cleft rod, half an inch
thick and four feet long) along the middle of the ridge, pegging it
down at every four inches with a double-broach, which is first thrust
down with the hands, and afterwards driven with the legget or with
a mallet. The middle ligger being firmly laid, the thatcher smooths
down the straw with a rake and his hands, about eight or nine inches
on one side; and at six inches from the first, he lays down another
ligger, and pegs it down with a similar number of double-broaches, thus
proceeding to smooth the straw and to fasten on liggers at every six
inches, until he reaches the bottom of the cap. One side being thus
finished, the other is similarly treated; and the first length being
completed, others are done in like manner, till the farther end of the
ridge is reached. He then cuts off the tails of the straw neatly with a
pair of shears, level with the uppermost butts of the reed.
When straw or heather is used for thatching, the material is laid on in
parallel rows, much the same as the reeds, but the mode of fastening is
generally somewhat different.