Thread: ductwork through a corro roof

This is a "step-by-step" to how I do it... which doesn't necessarily mean it's the right way. But AFAIK it's not the "wrong" way.

When making the 'ole, use a cutting tool such as snips or nibbler. Don't use an angle-grinder, especially if it's colourbond. Overheating is a good way to start a rust problem further down the track..

Then install the duct.

To help fit the seal over the duct, the rubber is normally marked in concentric circles as a guide. Choose one slightly smaller than the duct and cut it out with a Stanley knife. Slip it over the duct for a test fit... it should be tight and need to stretch a bit to go over, but not so tight that it'll tear from over-stretching. Wetting the rubber with a bit of detergent in water first will make it slide down into position easier.

Slide the seal down into position, mould it to shape and mark around it with a pencil both on the corro and where the rubber sits on the duct. Slide the seal back up the duct a foot or two to get it out of the way and clean the corro where it'll sit. Use steel wool if necessary, in preference to a wire brush, but make sure it's clean. Then I run a continuous bead of silicone just inside the pencil line that marks the outside edge of the seal and another continuous bead about an 1" inside that, to seal the inside edge of the strip.

I also run a bead of silicone around the duct, about 1/2" lower than the pencil line where the rubber sat for the test-fit, slide the seal back down into position and then drill pilot holes for a couple of opposite screws. Preferably on the "peaks" of the corro, but if the strip on the seal is predrilled you don't have much choice about positions. A dab of silicone on each hole (to seal the screws) and drive those two screws home.

That'll hold it all solidly in position, so now 'tis just a case of drilling pilot holes for the rest of the screws, more dabs of silicone in each and doing up the screws. I then slide the rubber down another 1/2" or so on the duct, to remove any tension on it and to ensure it beds into the bead of silicone I put there earlier. I know some blokes use a hose-clamp around the top of the rubber, but I never have.

Use a cloth dipped in turps to clean out the excess silicone squeeze-out if you're fastidious. I don't. All the above can be done in less time than it's taken to write this post... except for cutting the hole in the first place. (I don't care how good anyone reckons they are, that's always a long, fiddly bloody job! )

"Wetting the rubber with a bit of detergent in water first will make it slide down into position easier. "

Great suggestion - I've being doing it all wrong for all these years.

G'day.

Here is how I did it for our new wood heater.

I did it about the same as Skew but I used long pop-rivets instead of screws & stuck a daub of silicon on the head once it was in.
Because we live in a very wet area, I also had a flashing made to go over the corro & turned up the edges of the hole to help keep out any water that manages to leak in under the decktite (rubber witch hat).

One extra thing to note, when you buy your silicon, make sure you get a good one for exterior use & make sure it is 'neutral cure' otherwise it will cause corrosion.

I'm almost finished the flue for the exhaust fan over the hob. I'll post a thread with pics of that when I'm finished.