Part 6 - Flashing
Flashings are a weather resistant barrier component used on a roof, often at a join between sections or around a window.
They are made from either a rigid or a flexible material and are designed to send water away from the join and towards the guttering.
Background
Flashing in one form or another has been around as long as roofs have been covered with roofing materials. Before sheet products were available carpenters had to make something in situ to do the job, such as angling shingles away from the join or building onto the sides of chimneys to redirect the water.
The creation of manufacturer flashings mean that waterproofing areas such as chimneys, vent pipes, windows and door opening became much easier. It also meant that there was a far less risk of mould getting into the property.
Types
There are three main categories of flashings; apron, step and chimney flashings. Of these, apron flashings are the most commonly used. This is because they are the most versatile and can be used in a variety of positions such as the front or back of a chimney, abutting brickwork on a flat roof or where a roof meets the brickwork of the house. On slate or tiled roofs, they are used where the wall meets a porch, conservatory, car port or a lean to as well as around windows.
Flashings can also be either concealed or exposed. Exposed flashings tend to make use of materials such as lead, aluminium, copper, galvanised or stainless steel, zinc alloys or lead-coated copper. They have joints in long runs to stop the material from deforming with expansion and contraction. These flashings should not be stained as this can cause a chemical reaction.
Of the metal flashings, copper is one of the most popular materials due to its malleability, strength and high resistance to the effects of mortar as well as the environment. It means that roofs are built without weak spots and also is more cost effective due to the length of time it lasts. Cold rolled copper is the most recommended version, as it is more resistant to expansion and contraction than soft copper. However soft copper can work where there are complicate shapes involved, as it is more moveable.
Soft zinc is another metal that is gaining in popularity for roof flashings. It is extremely malleable so is an excellent material for complicated flashings. It has soft soldering capabilities and can be easy folded. It is also an environmentally friendly replacement for lead flashing as it is totally recyclable and provides 100% clean runoff.
Flexible flashings are adhesive backed and can be used around window and door openings. Materials used in these include rubberised asphalt, butyl rubber and acrylic and this means that they are not designed to be exposed as metal flashings can be.
Installation
The first step towards fitting flashings on a new roof is to remove the mortar between the bricks where the flashing is going to fit, usually around 150mm above the roof. This can be reduced to 75mm in areas not facing the hardest of the weather. Use an angle grinder with a diamond tipped blade to remove the mortar from the area where the flashing will be placed. Aim to remove around 15-20mm worth of mortar. Thoroughly dust to remove any residual dust.
To measure the size of the flashing, you will use three measurements: the chase depth, wall flashing drop and the overall onto the roof underneath. This third measurement may not be needed depending on the location that you are installing the flashing. The first two are easy while the overlap, if required, was previously recommended at 75mm but is now recommended to be 150mm.
On the ground, cut a length of flashing 1.2 metres long and place it over a length of wood, such as a spare roof batten. Have the chase depth amount overhanging the piece of wood and gently tap along the length, bending it to shape. Next, fold the material by hand at the drop and overlap points to form the flashing to the location it will be fitting.
Before you begin to install the flashing, it is important to fit some straps of the flashing material to the nails on the roof, two per 1.2 metre. This eases wind lift and gravity effect on the flashing and allow adjusting of the flashing without it slipping.
When putting the flashings in place, overlap each piece by 100mm and begin to hammer fixings into the chase to start installing the flashing. Some people use a piece of the material rolled up for form a chock that can be inserted between the flashing and the brickwork to keep it in place. This is hammered into place then pushed fully into the gap using the side of a chisel. Add one of these chocks around every 450mm. Alternatively, you can use hall clips or flashing clips, depending on what the manufacturer recommends.
Now the top of the flashing is fixed, hammer gently with a piece of wood onto the rest of the material into place so that it moulds against the wall and the roof. Fold the strips that you installed at the beginning over the edge of the flashing and cut so that only a small amount shows then gently tap this flat. At the end of the flashing, the overlap should be gently hammered into the side of the roof tile.
The final step is to add the mortar into the chase. Before doing this, tap a small groove into the top of the flashing that creates a small indentation. This flattens the flashing against the chase and also gives the mortar something to adhere to. When putting the mortar into the chase, make sure you get it all the way to the back of the chase, as this is the top reason why roofs leak. Wipe away any mortar that gets onto the flashing once you have finished to give a clean line.
Depending on the material used, you may want to use a finishing product such as a patina oil used on lead to help maintain the look of the flashing long-term. The manufacturer will advise of what may work and if this is needed.
Conclusion
This a basic guide to a flashing being attached from a wall to roof tiles but the basic principles remain the same, no matter what you are applying the flashing to. Remember basic safety principles when doing this work and don't work alone.
