Creating a gable roof out of a mansard roof

Posted by Scout on July 20, 2006, 5:03 pm

Hi Everyone,

I've just purchased a new home with a mansard roof which is pretty
ugly. I'm looking for an economical way of creating a gable roof over
the top of the mansard roof without removing the old roofing. Can I
build a new rafter system over the top of this one. At the same time,
I will remove the side part(the mansard) and replace with siding. If
this is feasible, how do I tie the new ridge board and rafters securely
to the new roof.

Posted by marson on July 20, 2006, 11:19 pm

Scout wrote:

> Hi Everyone,
> I've just purchased a new home with a mansard roof which is pretty
> ugly. I'm looking for an economical way of creating a gable roof over
> the top of the mansard roof without removing the old roofing. Can I
> build a new rafter system over the top of this one. At the same time,
> I will remove the side part(the mansard) and replace with siding. If
> this is feasible, how do I tie the new ridge board and rafters securely
> to the new roof.

rafters and ridge board really don't meet code to my knowledge. where
rafters are used (rather than trusses) typically you will find a
structural ridge, which is a beam supported by posts. this would have
to be engineered (which could be done by the company that sells the
beam). this would be a lot of trouble to retrofit since you would need
to tear into your exterior walls down to the foundation as well as
likely find an interior point down to a new footing.. if the mansard
has a low slope, it might be possible to get a scissor truss to sit
over the whole works (i'm just shooting from the hip here). otherwise
you are looking at ripping the whole roof off and replacing it. no
small matter.

Posted by Scout on July 31, 2006, 4:19 pm

Thanks for your response. I will be looking into the idea of adding
scissors trusses over the top. There is a local truss building company
that's going to come in and let me know if it's feasible. Thanks for
pointing me in a new direction.

marson wrote:

> Scout wrote:

> > Hi Everyone,
> > I've just purchased a new home with a mansard roof which is pretty
> > ugly. I'm looking for an economical way of creating a gable roof over
> > the top of the mansard roof without removing the old roofing. Can I
> > build a new rafter system over the top of this one. At the same time,
> > I will remove the side part(the mansard) and replace with siding. If
> > this is feasible, how do I tie the new ridge board and rafters securely
> > to the new roof.

> rafters and ridge board really don't meet code to my knowledge. where
> rafters are used (rather than trusses) typically you will find a
> structural ridge, which is a beam supported by posts. this would have
> to be engineered (which could be done by the company that sells the
> beam). this would be a lot of trouble to retrofit since you would need
> to tear into your exterior walls down to the foundation as well as
> likely find an interior point down to a new footing.. if the mansard
> has a low slope, it might be possible to get a scissor truss to sit
> over the whole works (i'm just shooting from the hip here). otherwise
> you are looking at ripping the whole roof off and replacing it. no
> small matter.

Posted by Jonny on July 21, 2006, 6:30 am

Can't be done. There's no way of tying the rafters to the top plate with
the current roof in place.
The roof is tied to the rafters, not the other way around.
--
Jonny

Posted by marson on July 21, 2006, 8:04 am

Jonny wrote:

> Can't be done. There's no way of tying the rafters to the top plate with
> the current roof in place.
> The roof is tied to the rafters, not the other way around.
> --
> Jonny

that part is doable with a simpson ST strap tie or something similar.

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