Field report Coquitlam, BC Concrete tile

Moulded into the vent flange USE NO TAR

So they used tar.

A leaking plastic roof vent, sealed with the one product its own maker casts a warning against. Here is what that does, and what we put on instead.

Called for: Active leak
Roof: Concrete tile
Failed part: Plastic gooseneck vent
Replaced with: Hot welded metal vent

A leak call in Coquitlam. On a tile roof we check the vents first, and this one gave itself away the moment the ridge tiles came off.

The vent was plastic, bedded to the tiles in black roof mastic, with more mastic forced into the joint. That is how these go in. The trouble is what mastic sticks to. It grabs concrete and holds. It does stick to the plastic, just nowhere near as well, and not for as long.

A plastic gooseneck roof vent on a Coquitlam concrete tile roof with its flange sealed to the field tiles with mastic, ridge tiles removed. — As foundRidge tiles off. The flange of the plastic vent is sealed to the field tiles with mastic, or tar as most people call it.

The tar lets go

Lift the tile beside it and you can see what happened. Most of the mastic stayed on the tile. A bit of it is still clinging to the vent. But the two are no longer joined to each other, and that is all it takes.

Fresh and soft, mastic fills the gap and everything looks sealed. Then it dries out, shrinks and goes hard. When it pulls, something has to give, and it is always the weaker of the two bonds. On a tile roof that is the plastic. Now there is an open path where everybody assumed there was a seal.

The tile beside the roof vent lifted away, showing where the roof mastic has let go of the plastic vent flange with some still stuck to the vent. — Lifting the tileThe mastic has let go of the plastic, with a bit of it still stuck to the vent.

Roof mastic left stuck to a concrete Columbia roof tile after the bond to the plastic vent gave out. — The proofMost of it stayed on the tile. The bond to the plastic is the one that gave out.

You do not have to take our word for it. The vent says so itself.

The words USE NO TAR moulded into the flange of a plastic roof vent that had been sealed with roof mastic. — Cast into the flangeUSE NO TAR, moulded into the plastic. The maker knew. They put it on the one part nobody ever looks at.

Then it sags

The seal is only half of it. Look at how the vent is held up. Nailed to the battens at the back, resting on tile at the front, and nothing at all under the middle.

So the middle drops. The flange stops sitting flat and tips back toward the roof. Any water that does get past the seal is now running the wrong way, under the tiles instead of out and down.

Side view of a plastic roof vent nailed to the batten at the back and resting on tile at the front, with the middle of the flange unsupported. — The supportNailed at the back, resting at the front, unsupported through the middle.

And the plastic gets old

There is a third way these fail, and it has nothing to do with tar. Plastic sitting in the sun goes chalky and brittle. Give it enough summers and it cracks, usually at the flange where it is thinnest. At that point the seal does not matter, because the vent itself has a hole in it.

Brittle plastic. Dried mastic. A sagging flange. Three separate problems on one part, and all three end the same way.

What the photos cannot show you

The same detail in section, cut across the slope so you are looking straight up the roof. Left is what came off. Right is what went on.

Removed

Plastic, bedded in mastic

01Water runs down the roof, lands on the flange, heads for the edge.
02The mastic in the lap has dried and let go, so the edge is open.
03Nothing stops it, so it carries on under the tile and into the roof.

Installed

Metal, upturned lip

01The same water lands on metal now, not on sealant.
02A half inch lip is bent up inside the lap, parallel with the tile edge.
03It stops there, stays on top, and runs off the bottom onto the tile below.

■ Tile ■ 1x4 batten ■ Underlayment ■ Flange ■ Water
Cut across the slope. Down-slope is toward you.

Two upgrades, not one

Metal is only half the answer. Plenty of metal vents have their seams held together with caulking, and you see them on shingle roofs everywhere. That puts you straight back to waiting on a sealant to dry out and let go.

Ours are hot welded. The seam is made in the metal itself, so there is nothing in it left to fail.

Came off

Sealant doing structural work

A plastic flange that sags where nothing holds it, relying on a mastic that grips plastic far more weakly than concrete, on a part cast with a warning against that exact mastic.

Went on

Metal doing the work instead

A rigid flange with a half inch upturn that diverts water mechanically, and hot welded seams. Nothing in the water path depends on a sealant.

The replacement metal gooseneck roof vent going in, with a half inch lip bent up along the flange to divert water mechanically. — Going inThe replacement, with the half inch lip bent up along the flange.

A finished painted metal hot welded gooseneck vent on a Coquitlam concrete tile roof with the tiles laid back down over the flange. — FinishedPainted metal hot welded gooseneck, tiles back down over the flange.

Worth a look from the ground

Black mastic smeared around a vent. On a tile roof that is a repair, not how it was built.
A vent that has dropped in the middle or no longer sits level with the tiles.
Plastic gone chalky and grey. Brittle plastic cracks.
Any stain on a ceiling below a vent, even a faint dried one.

Aldergrove Roofing Serving the Lower Mainland

Leak you cannot trace on a tile roof?

Vents are the first thing we check, and most get sorted in one visit. We work across Coquitlam, Port Coquitlam, Port Moody and the rest of the Lower Mainland.

Book a roof inspection

Related: vents on wavy S-tile roofs need a lead flange instead, and here is our concrete tile roof repair work.