How to Install Step Flashing Under Existing Siding

Retrofitting Step Flashing

By Robert Robillard on Roofing

Moss and paint failure seen on clapboards touching a second layer of roof over shingles

Roofing Over Existing Shingles

This article discusses retrofitting step flashing on a re-roof where the contractor left the old flashing in place and did not install new sidewall, step flashing.    Before we get into the retrofitting we first must discuss roofing over existing shingles.

It is possible to install new shingles over old shingles.  While I do not advocate doing this there are a few reasons why people do this, they are:

• It's a lot easier
• Less mess
• Less cost

A roof over is easier than for a tear-off job because you can use the existing shingles as guides for laying the new ones. But you must take care to install the shingles correctly so they lie flat.   A proper job involves retrofitting step flashing into the side wall as opposed to relying on the flashing.

Tear off All Shingles for Best Practice:

In my opinion the advantages for tearing off the old shingles far out weigh the three reasons to roof over.   The most important advantages to a complete tear off are:

• Manufacturer warranty
• Proper sealing of shingles
• Proper flashing and ice and water installation
• Proper fastening into roof deck
• Roof deck sheathing inspection and repair
• Reduced weight from your roof rafters

Manufacturer warranties do not usually cover a new shingles being layered on top of old ones.  The reasoning being that it is unlikely that the new shingles will not install flat enough to sealing to each other.   Proper sealing is necessary to protect the shingles from wind damage.

Re-roofing does not allow the contractor to inspect the roof deck for rot or damage and often times the contractor fails to  use longer nails on a re-roof and the new roof fasteners have not penetrated deep enough into the wood deck.

Many times  a roof job is done without replacing the step flashing and that is where additional problems can occur.

Step flashing is needed whether a contractor installs new roofing or re-roofs over existing shingles.   On re-roofs, the sad case is that contractors do not always take the time to retrofit step flashing because it slows them down, adds cost and additional carpentry work to repair or replace siding.  Frankly these contractors, and many homeowners, do not want to deal with the added work scope or the increased cost.

What is Step Flashing?

Step flashing,  L-shape flashing pieces  woven into the shingles as you work your way up the roof protects the interface between a roof and a vertical side wall from water leaks.   The step flashing transitions a vertical wall drainage plane onto the roof  drainage plane.

Step flashing comes in different materials but the most common seen on residential homes is aluminum and copper.

Re-Roofing Causes Problems:

In the photos on this post you see that the house siding is touching the second layer of shingles as evidenced by the growing moss and peeling paint.  siding rot, carpenter ants and side wall leaks occur next.

At one time this siding was most likely spaced 1/2″ to 3/4″ off the shingles but the re-roof installation raised the shingles, eliminating the spacing.

The main reason to install this siding spacing is to provide an an air space and drainage plane between the siding and the roof shingles.  The air gap allows water to drain from the roof and the siding, end grain, to dry out.

To make matters worse the roofing contractor re-roofed this house and did not install new step flashing.  This allowed water to get under the new shingles and into the new nail holes.

Because the siding is now buried into the new roof, water can also work its way along the top edges of  the clapboard siding and penetrate the building envelope, rotting the side wall plywood.

Retrofitting Step Flashing Without Removing the Siding

We needed to cut back the siding to create a new air gap but we knew that 3/4″ was not enough to slip flashing under without damaging the siding.  Our solution was to cut a 6-1/4″ of the siding away, creating a space to allow the installation of step flashing.   We would then cover this space  with a removable trim board.

We chose 6-1/4″ because it allowed us to install a 1×6 [5-1/2 wide] without having to cut the trim board and it also provided a 3/4 air space over the roofing.

We measured up off the roofing 6-1/4″ on both the top and bottom of the  roof,  snapped lines and then removed any siding nails that were on the chalk line.

We then set our circular saw to cut through the siding and not the plywood behind.  We removed the cut clapboards next and installed step flashing.

Once the step flashing was installed we primed all of the cut clapboard ends and installed a drip cap flashing under the clapboards.

The flashing covers and protects the joint on top of the trim board.   The trim board was then installed under and tight up to the drip flashing.   We made sure to keep out fasteners high on the trim board and away from the step flashing.  When finished we had a 3/4″ air gap and drainage plane.

Installing Side Wall Step Flashing

1.  Purchase pre-cut step flashing of cut your own step flashing pieces to be 10 in. wide and 2 in. longer than the shingle's exposure. This allows each step-flashing piece to overlap the piece below.   Bend the 10 in. length to extend 5 in. up the wall and 5 in. over the roof deck.   Store purchased step flashing usually comes  5″ x 7″ ad in 100 packs.

2.   Bend the 5″ sides in half by hand until you have a right angle.

3.  Install a piece of step flashing over each shingle in a weave fashion.  Each piece of step flashing laps over the shingle below and under the shingle above. The bottom edge of the flashing should extend just below the nail line.

When retrofitting step flashing you will need to slide the step flashing up and under the existing shingles.

Tip:  Try to work install the step flashing when the roof shingles are cool and not baking ins the sun.  The cooler shingles will separate easier from each other.  A putty knife or flat pry bar is useful for separating the seal on the shingles and a  cat's paw or nail puller is useful to remove any roofing nails in the way of the step flashing.

4.  Fasten each piece with a single nail on the side wall, high enough to be covered by the next course of flashing, the building wrap, and the siding.

Alternative Step Flashing Installation:

Some contractors prefer to nail flashing to roof deck only because this allows the flashing to be replaced later when re-roofing.

Kick-out Flashing or Diverters

Not covered in this post but very important is the use of kick-out flashing.  Kick out flashing, or diverter flashing, is a special type of flashing that diverts rainwater away from the house siding and into the gutter.   This flashing is installed at the roofs edge and does an excellent job at keeping roof water off  the house wall and from penetrating the building envelope.

On this project we wrapped our flashing around the house corner wall.

How to Install Step Flashing Under Existing Siding

Source: https://www.aconcordcarpenter.com/retrofitting-step-flashing.html

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