Make a 280mm hole in gypsum board at 45 degrees.

engdahl_86 · 21 Feb 2022 at 7:10 PM

Hey!
After searching the forum and trying to google without good results, I'm trying here now!
What is the magic formula for round holes in slanted ceilings?
I'm installing drywall in a ceiling that's 45 degrees. A 280mm chimney pipe for an open fireplace passes vertically through the ceiling.

How do I get a good cut?

pacman42 · 21 Feb 2022 at 7:17 PM

Divide the gypsum board into two parts, drill diagonally into both with plenty of gypsum boards underneath (including board two), cut both in the drill hole, mount around the pipe.

Zachtionman · 21 Feb 2022 at 7:18 PM

I would say that you make a reasonably okay cut and then place a cover ring around the pipe that hides your rough hole cutting. See the image below where you can glimpse what I mean,

Obygd · 21 Feb 2022 at 7:42 PM

You want to draw an oval or an ellipse, I think. But it sounds like advanced work to cut the plaster like that.
Move the pipe end towards the plasterboard at the desired angle and draw the outline of the pipe with a sufficiently long pencil.

Tomture61 · 21 Feb 2022 at 7:45 PM

E engdahl_86 said:
Hey!
After searching the forum and trying to Google without good results, I'm trying here now!
What's the magic formula for round holes in slanted ceilings?
I'm installing plasterboard on a ceiling that is 45 degrees. A 280mm flue pipe for a fireplace goes vertically through the ceiling.

How do I get a good cut?

Between the diffusion plastic and the pipe, you must have at least 50 mm to combustible material.
There is a rubber membrane for the purpose, to make it tight.

To draw an oval hole, take 2 screws/nails + a sufficiently long string, which you tie into a large loop/round ring. And a pencil.
It requires some experimenting with the right distance between screws/the right length of string.
But the result for a perfect oval is best with this method.

engdahl_86 · 21 Feb 2022 at 7:45 PM

Thanks for the quick responses!
The wife just came home and hears me grumbling upstairs about this and says, but we have cover frames for that in the room next door.
Then I got the template and everything as a bonus.

Sometimes it's a brain freeze.

Tomture61 · 21 Feb 2022 at 8:07 PM

E engdahl_86 said:
Thanks for the quick replies!
The wife just came home and hears me grunting upstairs about this and then says. But we have cover frames for that in the room next door
Then I got the template and everything as a bonus.

Sometimes.. crack in the brain

Hope you noticed my post about the distance to combustible materials + you must insulate with mineral wool next to the smoke pipe.
The chimney sweep might ask you to redo it when it's inspection time.
Or if you're really unlucky, you might end up outside a burned-down house.

engdahl_86 · 27 Feb 2022 at 5:38 PM

Tomture61 said:
Hope you noticed my post about distance to combustible material + you must insulate with stone wool closest to the flue.
The chimney sweep may ask you to redo it when it’s inspection time.
Or if you’re really unlucky, you might end up outside a burnt down house.

Hey! Yes, I saw it but missed replying. We had already been told that the combustible material was too close. It should be at least 50mm. And this has already been addressed even if it might not look so in the picture. The nearest wood is 70mm away. So it should be good.

MathiasS · 27 Feb 2022 at 5:42 PM

You have plastic against the pipe?

engdahl_86 · 27 Feb 2022 at 5:52 PM

MathiasS said:
You have plastic against the pipe?

Yes, it's from the house manufacturer hjältevadshus just like the studs that were flush against it before. The wooden studs can be moved, but the plastic is for the vapor barrier and probably looks like this in hundreds of their houses. How else would you make it airtight?

Tomture61 · 27 Feb 2022 at 6:03 PM

E engdahl_86 said:
Yes, it's from the house manufacturer hjältevadshus just like the studs that were flush against before. You can move the wooden studs yourself, but the plastic is for the vapor barrier and probably looks like that on hundreds of their houses. How would you make it airtight otherwise?

As I wrote earlier in the thread, there is a rubber sheet for that purpose. Combustible materials are wood - plastic - insulation material (except for stone wool) gypsum with paper on the outside.

engdahl_86 · 27 Feb 2022 at 6:38 PM

Tomture61 said:
As I wrote earlier in the thread, there is rubber sheeting for this purpose. Flammable materials are wood, plastic, insulation material (except for stone wool), and gypsum with paper on the outside.

Ah, I saw now that you wrote it. But it will be like this. As I said, they have done the same thing on hundreds of houses. This connection is also 6m from the "burning". The times we burn 3 times as recommended as the maximum, I haven't even felt any difference in temperature here upstairs when I've felt the pipe. I think it's probably worse if it isn't exposed.

MathiasS · 27 Feb 2022 at 6:45 PM

E engdahl_86 said:
Yes, it is from the house manufacturer Hjältevadshus, just like the studs that were flush against before. The wooden studs can be moved manually, but the plastic is for diffusion barriers and likely looks like that in hundreds of their houses. How else would it be sealed?

ok you do as you wish with your house, it's you and your family who will live and heat there and your discussion with the insurance company in case of problems. Personally, I don't understand how you dare to.

the house manufacturer doesn't seem to be much to rely on, they mounted wood against the pipe?

The shaft where the pipe goes will not be diffusion-tight via the plastic but through some rubber flange intended for the purpose. My tip is that you do EXACTLY as it says in the installation instructions for the chimney. It has a type-approved design for penetrations like these.

engdahl_86 · 27 Feb 2022 at 6:52 PM

MathiasS said:
ok you obviously do as you wish with your house, it's you and your family who will live and heat there, and your discussion with the insurance company in case of problems. For my own part, I don't understand how you dare.

the house manufacturer doesn't seem to be much to rely on, they mounted wood against the pipe?

The shaft where the pipe goes doesn't become diffusion-tight via plastic but via some rubber collar intended for the purpose. My tip is that you do EXACTLY as it says in the chimney installation instructions. It has an approved construction for penetrations like these.

Strange that it wasn't done right from the start anyway. The floor below should be the big hazard in that case. The house was built in 2007. But the upper floor is being done now.

Tomture61 · 27 Feb 2022 at 6:53 PM

E engdahl_86 said:
Ah saw now that you wrote it. But it will be this way. As I said, they have done the same with hundreds of houses.
This connection is also 6m from the "firing". The times we fire 3 times as recommended as max, I haven't even felt any difference in temperature here on the upper floor when I felt the pipe. I think it's worse if it's not open.

Here you have an installation guide for Premodul chimney.
My chimney sweep who inspected my own installation checked the dimensions to combustible materials also in the attic (where it is below freezing in the winter)
https://www.contura.eu/download/18....570093165/iav_Premodul_skorstenssystem_se.pdf
On page 7 you see the installation of the rubber sheet.
You tape with aluminum tape (against the flue pipe)
On page 10 you see how much insulation you can have when going through e.g., a roof.

If you sell the house, the new owner might want to fire more/harder, then the heat might rise in the flue pipe up by the roof.
If you get a chimney fire in the chimney, the temp rises to 1000 degrees in the flue pipe, what the temp becomes on the outside is hard to predict.