Metal Fascia and Drip Edge: How the Roof Edge Comes Together
The eave is where roofing, fascia, drip edge and eavestrough all meet in about 150 millimetres. Get the layering wrong and water runs behind the trough, into the fascia and down the wall.
The eave is the busiest 150 millimetres on a house. Rafter tails, subfascia, fascia cover, drip edge, underlayment, roofing, eavestrough hangers and the trough itself all converge there, and every drop of water off the roof passes through the assembly. It is also the detail most often built by three different trades on three different days, which is exactly why it goes wrong.
The Parts, From the Inside Out
Understanding the layering starts with knowing what each piece is for.
- ✓Subfascia — the structural board across the rafter tails. It carries the eavestrough load and provides the straight line everything else follows.
- ✓Fascia cover — pre-finished aluminum or steel formed over the subfascia. It is a weather cover and a finish, not a structural element.
- ✓Drip edge — the formed metal at the roof edge that carries water off the deck and directs it away from the fascia.
- ✓Underlayment and eave protection — the membrane layers on the deck, whose relationship to the drip edge determines where water on the membrane ends up.
- ✓Eavestrough and hangers — collects the runoff and carries it to the downspouts. Hung from the fascia, so its load passes through the fascia cover into the subfascia.
Why the Drip Kick Is Not Decorative
A drip edge has a bend at its lower edge that turns out and down. That small kick is the whole point of the part, and the mechanism is surface tension.
Water leaving a flat edge does not simply fall. It clings to the underside of the material and runs back along it — the same effect that makes a teapot dribble down its own spout. On a roof edge, that means water running back under the drip edge, behind the eavestrough, down the face of the fascia and into the joint between fascia and soffit. The kick breaks that path: water reaches the bend, loses contact with the surface, and falls clear into the trough.
This is why a drip edge that is too short to reach into the trough, or one installed with the kick flattened against the fascia, causes staining and rot on a fascia that appears to be well covered. The dark streaks people notice on a fascia below a roof edge are almost always this.
If the back of your eavestrough is stained, or there is rot behind an aluminum fascia cover that looks perfect from the ground, the drip edge is the first place to look. Water is getting behind the trough instead of into it.
Layering the Drip Edge with the Underlayment
The relationship between drip edge and underlayment differs between the eave and the rake, and the reasoning is worth understanding rather than memorizing.
At the eave, the concern is water that has made it onto the underlayment — from a leak, from ice damming, from wind-driven rain. That water runs down the underlayment to the edge and needs to be delivered onto the drip edge and into the trough. So the common practice is drip edge first, underlayment lapped over it, so water on the membrane drains out onto the metal.
At the rake, the concern is water running sideways off the roof surface under wind. Here the drip edge typically goes over the underlayment so that water on the roof surface is carried out over the metal rather than getting under it. Follow the roofing manufacturer's published instructions and the applicable code requirements for the specific system, since eave protection membranes and metal roof systems each have their own detailing.
The Metal Roof Version of the Eave
A standing seam or other metal roof handles the eave differently. Instead of a simple drip edge under the roofing, there is usually an eave trim or offset cleat fastened to the deck, and the panel's leading edge is hemmed and locked onto that cleat.
That hem-and-cleat connection does two things at once: it gives the panel a stiff, straight edge that will not flutter, and it mechanically restrains the eave against wind uplift — which at the eave is the highest uplift zone on the roof. It also has to allow the panel to move along its length as it expands. A metal roof eave that is face-fastened through the panel instead of hemmed onto a cleat gives up both the movement allowance and the uplift restraint.
Fascia Cover: Do Not Wrap Wet Wood
Aluminum fascia cover is one of the most common exterior products in Ontario, and it is often applied over subfascia that is already damp or partly deteriorated, on the reasoning that it will keep the weather off. It will not. Metal cover over wet wood traps the moisture — the wood cannot dry outward through aluminum any more than a wall can dry through metal siding — and the wood continues to rot invisibly while the exterior looks perfect.
- ✓Assess and replace deteriorated subfascia before covering it, not after.
- ✓Fix the reason it got wet — usually a short drip edge, an overflowing trough, or a missing kickout flashing — or it will happen again under the new cover.
- ✓Fasten fascia cover with colour-matched fasteners at the intended locations, not face-nailed randomly, and allow for expansion in long runs.
- ✓Lap fascia cover pieces with a slip joint rather than butting and caulking them; a long run of fascia moves noticeably in summer sun.
- ✓Do not rely on fascia cover to carry eavestrough load. Hangers must land in the subfascia or into rafter tails.
Hanging the Eavestrough
Hidden hangers screwed through the trough into the subfascia have largely replaced spikes and ferrules, and for good reason: spikes work loose over time as the fascia takes freeze-thaw cycles and seasonal load, and every loosened spike is a hole in the fascia. Hidden hangers with screws hold better and distribute load more evenly.
Hanger spacing is a load question, and in Ontario the load is not just water. A trough full of water is heavy; a trough full of ice is heavier and stays loaded for months. Tighter hanger spacing than a mild-climate default is sensible here, particularly on north-facing eaves and long runs.
Sliding Snow: The Metal Roof Problem
This is the detail specific to metal roofs and it is the one most often discovered the hard way. A metal roof sheds snow, and it sheds it as a slab, all at once, when the sun warms the panel enough to release the bond. That mass hits the eavestrough on its way off.
- ✓Snow guards or snow retention systems hold the snow on the roof so it melts and leaves gradually instead of releasing in one slab. They are engineered for the roof's slope, snow load and panel type, not selected by eye.
- ✓Where snow is allowed to release, the trough position relative to the roof edge matters — a trough hung so the sliding slab passes over rather than into it survives, one directly in the path does not.
- ✓Reinforced hangers and tighter spacing help but will not save a trough from a full slab release on a steep roof.
- ✓Consider where the snow lands. A slab of snow off a metal roof onto a walkway, a deck, a gas meter or a parked car is a genuine hazard, and that is the main reason to specify snow retention over entrances and above anything that matters.
Kickout Flashing: Small Part, Large Consequences
Where a roof edge runs down and dies into a wall — at a dormer, an offset, or where a lower roof meets an upper wall — the water running down that roof-to-wall intersection has to be diverted out into the trough. A kickout flashing is the piece that does it: a formed flashing at the bottom of the step flashing run that turns water away from the wall.
Leave it out, and the entire roof plane's runoff is directed at one point behind the wall cladding. Behind metal siding — which cannot dry outward — that is a concentrated, continuous wetting of the sheathing at a single spot. It is one of the highest-consequence missing parts in residential construction, and it costs almost nothing to install when the roof is being done.
Frequently Asked Questions
Why is my fascia rotting behind an aluminum cover?
Almost always water getting behind the cover rather than through it — a short or flattened drip edge letting water run back onto the fascia, an overflowing trough, or moisture that was already in the wood when it was covered. Aluminum keeps rain off the face and also keeps the wood from drying, so an existing moisture problem accelerates rather than stops.
Should the eavestrough be replaced when the siding is done?
It is the sensible time to do it. The fascia is accessible, the trough has to come off to fit the fascia cover and terminate the soffit properly, and any drip edge correction is easiest with the trough down. Reinstalling an old trough onto new fascia usually means old fastener holes in new material.
Do I need snow guards on a metal roof?
It depends on slope, panel type, snow load and what is below the eave. They are strongly worth considering above entrances, walkways, decks, driveways, gas meters and lower roofs. On a rural building where snow can drop harmlessly to open ground, they may not be needed. Snow retention systems are engineered for the specific roof rather than chosen generically.
How far should the drip edge extend into the eavestrough?
Far enough that water leaving the kick falls inside the trough rather than behind it, with allowance for how the trough sits relative to the roof edge. If you can look up from the ground and see the back of the trough getting wet, the drip edge is not reaching far enough or is not kicked out.
Can fascia and soffit be replaced without touching the roof?
Usually yes for fascia cover and soffit panels. What often cannot be corrected without lifting the roof edge is the drip edge, since it sits under the roofing at the eave. If the drip edge is the underlying problem, that is worth knowing before scheduling the work rather than after.
Get a Free Metal Roofing Quote.
We measure your roof remotely by satellite and deliver a written, itemized quote — same day, no obligation.