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NATURAL PHILOSOPHY, CHEMISTRY,
Description of a Method of Roofing Buildings securely with
To Mr. NICHOLSON.
I Had occasion some time ago to roof a large building in an uncommon manner. I send you an account of it; as it has succeeded; and, as I believe, it may be useful in many places where slates and tiles are not to be had.
Gaol of Long
ford roofed with Bags,
The gaol of Longford, in Ireland, which was built about twenty years ago, was covered with a circular arch of bricks; upon which broad flat stones, commonly called flags in this country, were laid with the best mortar that could be procured; these thin stones or flags were placed side by side; the lateral joints were filled up with mortar; and all the courses, as they descended, lapped over each other about two inches. After a short time the sun and frost cracked Penetrated by the mortar between the joints, and the rain found a way into every part of the building.
VOL. XXIX. No. 132.-JUNE, 1811.
Attempt to prevent it by a cement,
did not suc ceed.
Expense of a
One of those men of practice, as they are called, from having been employed practically in building, undertook for forty or fifty pounds to remedy the evil, and by a curious cement to render the roof impervious to water. He laid on his hot cement of resin, and wax, and brick-dust, &c. The first summer shower passed off without penetrating through the joints, and the undertaker received his money; but in a short time things were as bad as ever, and the miferable creatures under confinement were drenched with rain and snow in every part of the prison.
In the year 1809 the Grand Jury of the county of Longlead or copper ford desired, that I would endeavour to staunch this roof at covering, any expense, that might be required. I received proposals for covering it with lead, and with copper: this could not be done for less than seven hundred pounds.
and of slating.
Management of the flags themselves to keep out the wet.
I then proposed to belt down rafters upon the brick arch, so as to form a polygonous roof, upon which slates might be laid in the usual manner; but this I found would cost above four hundred pounds. It then occurred to me, that the flags on the roof might be so ordered, as to effect the intended purpose.
I took off a portion of the flags in fine weather, and without removing them from the top of the building I had them cut in the following manner; the flags (a a, Pl. III, fig. 3) were about three feet long, two feet or two feet six inches broad, and two inches and a half thick. The upper course was of fine even flags four feet broad, and each of considerable length, and under this course the roof was secure every where, except between the lateral joints. To prevent the rain or snow from penetrating between the upper and under courses or horizontal joints of the flags was the first object. For this purpose a groove was cut an inch deep in the surface of the upper part or top of those flags that were next the eaves; this groove was cut within one inch of the top of the flag. A similar groove was cut in the under side of the next course that lapped upon the lower course, and so on from the eave to the ridge; so that the upper flag or stone could hook upon the under one, as may be seen in the section, fig. 1. Pl. III.
The next object was to secure the lateral joints. To
effect this purpose, grooves were cut into the upper surface joints covered along each side of every flag three quarters of an inch deep with lead. at one inch from the edge, see fig. 2 and 3, where a section or profileis given. To cover these lateral joints caps of lead were laid from the ridge to the eaves, a cap for each flag, or rather for every pair of flags. These caps, which had the appearance of a bead, were fastened over the rabbets or grooves of the flags by copper nails, c, driven through the caps into the juncture between the flags. These nails were made fast by slips of sheetlead d d fig. 3, put between the stones. A representation of the full size of the grooves in the stones of the lead cap and mushroom nailsis given, fig. 3.
Where holes were made through the lead caps, the water might find a passage; but this was prevented by preparing the holes in the lead in such a manner, as to stop the water above the hole, and to turn it aside from the direction which might be hurtful. The caps before they were laid in their proper places were turned upside down, and where nails were to pass, a burr or button, b, b, was punched in the lead half an inch deep, and by a proper tool passing through the punch, a hole was made in the centre of the button. The cap, when put into its place, covered the ridges of the flags between the grooves, so that no water could find an entrance between the joints of the flags; nor could any water rise above the tops of these buttons, because the This effectual. descent of the roof would carry it off. Besides, the button or burr was covered by a mushroom-headed nail, the rim of which entered a little into the lead round the burr and prevented small particles of snow from gaining admittance.
The holes in these caps might have been closed by solder;
but whenever any work that is of difficult access is to Simplicity adbe performed, it is always advantageous to have it executed vantageous. by some one workman, in a manner that requires no difficult art or complicated apparatus. And I find that not one drop of water has penetrated through these joints during the two winters that have passed since they were covered according to this plan.
with respect to the scaffolding
At the commencement of the business many difficulties Difficulties with respect to the scaffolding were started. Very long ladders were requisite. Cripples hung on iron stauncheons
in the wall were deemed insecure; and the country work men trembled at the idea of being perched so high from surmounted. the ground without any apparent protection. I constructed eight light ladders, each six feet long; these were wider at one end than at the other, so as to permit them to be joined together by small bolts passing through the ends of both ladders. The ladders, thus joined, applied themselves commodiously to the circular roof, they were hung across the top and fastened by ropes, passing over the ridge of the roof to the iron bars of the windows of the upper cells on the opposite side of the gaol, which happened to be empty. On these ladders movable cripples were placed wherever a scaffold was wanting on these cripples, which extended six feet from the roof, strong planks were laid, with ledges to prevent their slipping sideways; round this scaffold a coarse substantial handrail was tied. The passage to this scaffold was through a large opening in the top of the roof whence the workmen. descended down the ladders to the lower platform, and thence to any part of the roof".
The scaffolding of this work cost but fifteen pounds, and the repair of the roof, exclusive of some other work that was carried on at the same time, came within one hundred and forty pounds.
As I may not have an opportunity of mentioning it in another place, I hope that you will excuse me for inserting a circumstance relative to this gaol, which is certainly not connected with the immediate subject of this letter; but as
The moment the scaffold was finished, I went upon it myself, and from that time no objections were made. Notwithstanding all the precautions that had been taken, a fatal accident threatened the lives of the workmen, It has been said already, that the ropes which held the lad. ders were tied to the bars of the upper cells of the gaol. One morning, towards the close of the business, the principal workman found the ladders, and the scaffold that was attached to them, giving way. He had sufficient presence of mind to throw himself off the scaffold on the roof; as he was near the top, the slope of the roof was not sudden. He could therefore stick there till his companions relieved him.
The cause of this sudden failure it was impossible to foresee. A mad woman had been accidentally put for a single night into one of the upper cells; there by moonlight, with that mischievous alacrity which is often the accompaniment of insanity, she untied the cords, and left the scaf fold without support.