It can be tricky to keep the area around the chimney waterproof but by using two layers of flashing, this can normally be achieved. These two layers are called base flashing and cap/counter flashing. It is important to clean the entire area around the chimney before you start work on the chimney flashing. You do this by using a wire brush to clean all four sides of the chimney. Once this is done, apply a layer of asphalt primer to the cleaned area. Continue by applying plastic cement over it and you should be ready for the flashing. However, when it comes to cutting the pattern of your chimney flashing to fit around the chimney, the different sides of the chimney have to be dealt with separately. I will now tell you a bit more about the base flashing on the four sides of the chimney.
The Low Side
Start by cutting a strip of material, 12 inches wide and about 12 inches longer than this side of the chimney. Take care in completing bends and cuts. Apply plastic cement to the deck and lay down the cut-out on top of it before nailing it down using roofing nails (1 ¼ inch nails should do it). After this, take your mortar nails and nail the top of the cut-out material to the chimney – taking care not to crack the mortar. When it comes to bending the cut-out against the chimney, I would make sure to bend the angle a bit larger than the real angle between the roof and the chimney. As a result, the spring tension will make create a good and tight fight.
The Sloping Sides
Start by making metal steps on the chimney’s two sloping sides, also known as step flashing. You will need to cut out galvanised metal squares that are twice the dimension of the weather exposure of the shingles you are using. The numbers of squares that you will need are equal to the number of courses running up against the chimney – on both of the sloping sides. Bend the squares into L-shapes, taking care to make the angle just over 90 degrees in order to create a spring tension. The spring tension will make sure that the metal is tight against the chimney. The steps should be installed over every end shingle and it is a good idea to use plenty of plastic cement for this. The cement goes under the metal steps and also between the steps and the chimney. Use a 1 ½ inches galvanised roofing nail to fasten the flashing to the deck. The nail should be about 1 inch from the top edge of the step.
The High Side
The highest side of the chimney should be installed last and the reason for this is that this side of the chimney will have the highest level of water flow. This means that the flashing should be 32 inches wide and about 12 inches longer than the width of this side of the chimney. By making the flashing 32 inches wide, you will cover about 8 inches of the actual chimney and about 24 inches of the roof deck – above the chimney. Take care when you fold and cut the flashing. If there is a cricket or a saddle at the back of the chimney, you are best to cover it with asphalt primer and cement before installing the flashing. A cricket or a saddle is there to deflect water and they prevent ice and snow from collecting.