Roof Flashing

Flashings

The materials commonly used for flashings are copper, tin, lead, galvanised iron and zinc. Flashing materials should be selected with the same care as the other materials entering into the roof construction. The first consideration should be durability. When used with a material as permanent as slate, it is, of course, poor economy to use any but the most permanent and non-ferrous metals and the best workmanship.

Copper

Copper is peculiarly suitable for all flashings (including valleys) as it is easily worked and shaped and adjusts itself to temperature stresses. Copper sheet is made in a wide range of weights and thicknesses and in what is known as soft or "Roofing Temper" copper and hard or "Cornice Temper" copper. Soft copper only should be used for flashings. It is generally defined by the ounce weight per square foot; that is, " 16 ounce copper" means copper sheet weighing 16 ounces or one pound per square foot. All copper sheets used should be rolled from copper conforming to the Standard Specifications of the American Society for Testing Materials.

Sixteen ounce copper sheet is the minimum weight that should be used for flashings of any kind. Many architects will specify nothing lighter than 18-ounce material, and with the best work, where heavy sheets are used, 20 ounce metal is recommended as better practice.

Flashings lighter than 16-ounce are undesirable. All rain water carries with it off the roof dust and grit which have some erosive effect on the metal. It is well to use metal thick enough to do the work of carrying away the water for a period of time at least as long as the life of the building. Sixteen-ounce copper will meet such conditions; 14-ounce is too light.

The following table will be found of value in specifying the weight of sheet copper. Copper for flashings should always be specified by weight and never be gauge.

Copper sheets may be obtained in widths from 6" to 108" and in lengths from 6" to 200". Roll or strip copper is made in widths from 2" to 20" and in rolls of 75' or strips from 6' to 10' long. Sheets from 24" to 42" wide and from 60" to 96" long are in general use in the sheet metal trade and are carried in stock. All specifications and details should be drawn with these sizes in mind.

The edges of all copper flashings to be soldered must be tinned 1 1/2 " on both sides and the seams thoroughly sweated with solder. Proper care in making the seams tight is of utmost importance. Use pieces up to 96" in length, and except on steep slopes 15° or over), lock and solder all base flashings. Cap or counter flashings need not be soldered. The point is made by lapping the sheets in the direction of the flow . When there is any likelihood of deep, wet snow packing in or of wind lifting the cap flashings, the joints should be soldered, using either locked of lapped seams.

All exposed edges of flashings - such as the bottom edge of cap flashings - should have a 1/2" fold back under for stiffness against wind action. This is a practice that should be axiomatic with good flashing.

Copper requires no painting or other treatment unless it is desired to hasten the development of the natural green patina. In this case it is absolutely necessary that all the grease and oil used in the manufacturing process be removed from the copper. A strong soda solution (4 to 6 ounces per gallon of hot water) will do this. A uniform finish will not be obtained unless the copper is thoroughly cleaned.

Copper can be painted provided the surface be thoroughly cleaned and roughened. This can be done by washing the copper with a solution of 4 ounces of copper sulphate in 1/2 gallon of lukewarm water in a glass or earthen vessel, to which has been added 1/8 ounce of nitric acid. Before painting, the surface must be carefully washed with clean water to remove the last trace of the solution. For additional data regarding colouring copper, refer to Part 3 of "Copper Flashings," of the Copper and Brass Research Association, or write that association direct at 25 Broadway, New York City.

Flashings - General

Flashing should be used at all intersections of vertical or projecting surfaces through the roof or against which the roof abutts, such as walls, parapets, dormers, sides or chimneys, etc. Flashings used over or under the roof covering and turned up on the vertical surface and bent down over the base flashing is termed a "cap flashing" or "counter flashing."

Base Flashings

The base flashings should be extended under the uppermost row the full depth of the slate or at least 4" over the slate immediately below the metal. The vertical leg must be turned up not less than 4" and preferably 8" on the abutting surface. Where a vertical surface butts against the roof slope, it is necessary to build in the base flashing with each course of slate as laid. Turn out 4" on the slate and at least 8" above the roof. If the roof stops against a stuccoed wall, a wood strip 4" wide having a bevelled top edge should be secured to the wall. The base flashing is then turned out over the slate at least 4" and bent up vertically at least 3" on the board. Except in unusual cases, it will be found satisfactory to turn the base flashing out 4" on the roof surface and up on the vertical surface from 6" to 8" for either sloping or flat roofs. Posts, flagpoles, scuttles, etc., where projecting through the roof, should have base flashings. Vent pipes should have bas flashings in the form of special sleeves or one of the numerous patented roof flashing devices.

Cap Flashings

Where the base flashing is not covered by vertical slate, siding, etc., a cap flashing must be used. This member should be built into the masonry joints not less than 2", extend down over the base flashing 4", and the edge bent back and up 1/2". Reglets in stone or concrete are usually about 1" wide and 1" deep. The flashing should be formed and laid in the bottom of the cut and thoroughly caulked with molten lead on flat surfaces or lead wool on upright work. After caulking, the reglet is filled to the surface with elastic cement. Flashing hooks should be used to secure stepped flashings and the vertical legs be made tight with roofers' cement coloured the same as the masonry. One the best work these flashings should be soldered.

It will be noted on the drawings that all exposed and unfastened flashings have the edge of the strip turned under 1/2". This is done to give the strip stiffness against wind. Thus the sheet is held in place and the packing in of snow under the flashing is prevented.

Saddles or Crickets

Where a chimney or other vertical surface breaks through the roof at a right angle to the slope, a saddle or cricket must be built to throw the water away from the back of the vertical member. If the roof construction is of wood, use light rafter construction covered with sheathing boards, paper and sheet metal. If of very large area and exposed to prominent view, it should be slated the same as the other roof areas. Valleys will be formed with the main roof and it is recommended that they be of the open type. The size of the saddle is largely determined by the roof condition. It is usually sufficient to make the slope of the saddle the same as the roof.

It is most important that the saddle or cricket be of adequate size, of ample slope and well flashed as shown in Figure 34.

Estimating Flashings

In case it is desired to estimate the amount of metal required for flashing mitred hips (where metal is used), closed valleys, cheeks or side walls and base and cap flashings for walls, the following rules used by one roofing contractor may prove of value. This rule considers that flashings for hips and valleys are bent diagonally from corner to corner, while those for cheek or side walls are bent lengthways, allowing 4" to turn up on the side wall and 4" flashed under the slates.

Rule for Estimating Flashing:

for Mitred Hips, Closed Valleys, Cheeks or Side Walls and Walls, multiply the number of lineal feet of mitred hips and closed valleys by the following percentages, and the result will be the number of square feet of sheet metal for flashings, using slips.

for Hip and Valley Slip Flashings, multiply the number of lineal feet by 1.1, and it will give the number of square feet if flashings are 12" wide.

If courses in valleys do not line up, it will require double the number of slips to flash properly.

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