Flat dormer roof to pitched

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Taz9126
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Flat dormer roof to pitched

Post by Taz9126 » Wed Nov 28, 2018 5:30 pm

Hi 
I'm looking at converting flat roof to pitched 
The Roof has kingspan on top of joists , so have a couple of questions .
Once roof has been removed will I have to run a joist at 90' along front edge on top of existing joists to act as wall plate to take the birdsmouth rafters ? Can this be nailed or screwed or does it have to be strapped down for BR ? 
Also how will I secure the other end of the new rafters where existing rafters of house roof meet the ridge board ? As these new rafters will have to be lower than existing ridge board as I will be looking at roof tiles 

Thanks in advance
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Re: Flat dormer roof to pitched

Post by moderator2 » Wed Nov 28, 2018 5:40 pm

Might get a reply in carpentry. I'll move it for now.
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Re: Flat dormer roof to pitched

Post by ayjay » Wed Nov 28, 2018 6:19 pm

Taz9126 wrote:Hi 
I'm looking at converting flat roof to pitched 
The Roof has kingspan on top of joists , so have a couple of questions .
Once roof has been removed will I have to run a joist at 90' along front edge on top of existing joists to act as wall plate to take the birdsmouth rafters ? Can this be nailed or screwed or does it have to be strapped down for BR ? 
Also how will I secure the other end of the new rafters where existing rafters of house roof meet the ridge board ? As these new rafters will have to be lower than existing ridge board as I will be looking at roof tiles 

Thanks in advance
It's always going to be easier to construct a roof up off of a wall plate, I can't see any reason not to do that here.

I'd like to see a bit more tail on the rafter past the birdsmouth. Is that just a quirk of the quick sketch or do you plan to have no soffit there? A solution to the lack of tail could be to move the plate inwards (you'd need to increase the thickness/height of the plate).

I don't remember ever strapping down a dormer wall plate.

I can see what you're getting at with the ridge tile - but your drawing doesn't show the actual roof tiles (or tile battens) - once you put those in the picture it should change things and allow the dormer rafters to finish at the same height as the common rafters on the roof.

A mix of 3" and 4" galvanised wire nails are sufficient for any normal roof.
One day it will all be firewood.
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Re: Flat dormer roof to pitched

Post by Taz9126 » Thu Nov 29, 2018 9:14 am

Hi Ayjay
Thank you for taking time out of your day to reply .
Yes sorry about the sketch lol it was just a very quick sketch!
I had been advised somewhere else that the wall plate would have to be strapped . The joists I would be securing it to are 6x2 so strap would at most be 6 inches long before touching ceiling below . So I thought could it be screwed/nailed down to the joists. I understand what you mean about the birds mouth and soffit it was just my sketch lol what sort of length is normally on the tail ?
Where the existing rafters meet the ridge board I think the the ridgenis a 6x1 and rafters 4x2(guesstimate) . The rafters I was think of using were maybe 6x2 because of the span and the new roof angle is probs around 19 to 20 degrees so didn't know if these would secure to existing ridge board.
If loaded a pic of the ideal roof I would be hoping to achieve and a pic just of inside of existing roof (rafters &ridge )
Thanks in advance :-)
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Re: Flat dormer roof to pitched

Post by ayjay » Fri Nov 30, 2018 12:28 pm

Taz9126 wrote:Hi Ayjay
Thank you for taking time out of your day to reply .
Yes sorry about the sketch lol it was just a very quick sketch!
I had been advised somewhere else that the wall plate would have to be strapped . The joists I would be securing it to are 6x2 so strap would at most be 6 inches long before touching ceiling below . So I thought could it be screwed/nailed down to the joists. I understand what you mean about the birds mouth and soffit it was just my sketch lol what sort of length is normally on the tail ?
Where the existing rafters meet the ridge board I think the the ridgenis a 6x1 and rafters 4x2(guesstimate) . The rafters I was think of using were maybe 6x2 because of the span and the new roof angle is probs around 19 to 20 degrees so didn't know if these would secure to existing ridge board.
If loaded a pic of the ideal roof I would be hoping to achieve and a pic just of inside of existing roof (rafters &ridge )
Thanks in advance :-)
The wall plate for the main roof is usually strapped down, the straps are at least a metre long and secured to the brickwork below the plate. I don't remember ever strapping down a plate for a dormer, the dormer rafters would usually sit on a double beam forming the top of the window opening and there's nowhere to strap that down.

If you're fitting a soffit the tail length will be fine, what you don't want is to trim it down to a couple of inches long (or less) as it'll be inclined to snap off below the birdsmouth.

Rafter size: at a 20° pitch a 4 X 2 is OK to span about 2.5m and a 6 X 2 will take you up to about 3.75m. They can be nailed direct to the ridge as per usual.
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Re: Flat dormer roof to pitched

Post by Taz9126 » Fri Nov 30, 2018 4:06 pm

Hi Ayjay
Thanks once again for your reply
I had seen that about straps usually been 1m long ,
What would be the best way to secure the wall plate knowing it's a dormer ? Is screwing or nailing acceptable ?
Or do you still use straps of some description ? I would assume there is a standard size to conform to BR ?
Regards
Taz
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