Would there be BR issues with this design concept?

[^woody^]

A piece of timber as a rafter can't magically grow in length to push a wall out.

That is a ludicrous statement

The end is actually pulled inwards with any bending of the rafter. Do that little experiment with a ruler held against a wall and see what happens to the other end on a desk or surface

Trusses are completely different engineering principles to rafters

It is the load placed on the rafters and the ability of the wall to resist that load which are the governing factors, it has nothing to do with the presence or lack of any ceiling ties

 

[tony1851]

I didn't say that the rafter incrased in length.
What I said was that the underside of the rafter increased slightly in length (being in tension) while the top decreased slightly due to compression. It is this small increase in length on the underside which has to be acommodated somewher, hence the outward push.

Can't we just agree to differ on this?

 

[indus]

The technical discussion is interesting but a little beyond me I'm afraid.

So for a simpleton like me, what is the difference between the vaulted ceiling I am suggesting and what has happened when I did my loft conversion. The whole of my converted loft has no ceiling, just plasterboard over the rafters.

The whole roof then sits on these rafters, why isn't the roof then pushing out the four walls of the house?

Thanks

 

[tony1851]

The reason your roof has not pushed out on the walls is that the original
ceiling joists will still be in position. (if your loft has been correctly built, there will be new floor joists beside or above the old ceiling joists),

The ceiling joists will be nailed both to the sloping rafters, and the piece of timber which runs along the top of the external wall. In cross=section, this makes the whole roof a triangle shape, which is very stable.

(No-one who asks questions can be a simpleton. For my part, I'm sorry if the responses to your original post seemed to go off on a tangent but opinions often differ on matters such as this - as you have seen!)

 

[jeds]

The technical discussion is interesting but a little beyond me I'm afraid.

So for a simpleton like me, what is the difference between the vaulted ceiling I am suggesting and what has happened when I did my loft conversion. The whole of my converted loft has no ceiling, just plasterboard over the rafters.

The whole roof then sits on these rafters, why isn't the roof then pushing out the four walls of the house?

Thanks

A vaulted ceiling on a lean-to roof is very easy to achieve and very common. Not knowing your exact situation it's difficult to give a precise construction but the usual solution is to restrain the top of the rafter on a fixed wall plate or ridge beam. The rafter does have a tendancy to move laterally under deflection but if the rafters are sized correctly the actual movement is very small and can be ignored. The issue is creating athe fixed point. Easy if you have a wall to build off - less easy if you don't.

 

[tony1851]

Easy if you have a wall to build off - less easy if you don't.

Agreed; the point is that I think the OP was talking in terms of a two-storey extension, where presumably the top-ends of his rafters would end up somewhere in the roof.
Where can he put something rigid to anchor the rafters to? It could easily end up as a mechanism rather than a structure.

 

[indus]

Thanks. Sorry about the poor quality pics but this might give you guys a bit more to go on

 

[mysteryman]

The guy who is on about bending and increases in length is confusing his rafters, beams and trusses. The bottom of a beam will be in tension, as he says, it may increase in length, but this is very small, and if it happens, the beam will bend slightly, so the overall length between the end supports may be slightly less. This is all very theoretical, and the effects are insignificant.

A pitched roof without a base to the triangle requires something else to counteract the outward thrust of the rafters [the main sloping members]. You will need a structural engineer to specify a suitable method. It can be done.

 

[jeds]

Indus. A vaulted roof on that construction is simple. Include a ridge beam supported on the outer gable and existing inner gable. The loadings will need to be checked and the ridge designed accordingly but for a small span you can often get away with a timber ridge. Otherwise a small UB will do it easily.

 

[tony1851]

The guy who is on about bending and increases in length is confusing his rafters, beams and trusses. The bottom of a beam will be in tension, as he says, it may increase in length, but this is very small, and if it happens, the beam will bend slightly, so the overall length between the end supports may be slightly less. This is all very theoretical, and the effects are insignificant.

A pitched roof without a base to the triangle requires something else to counteract the outward thrust of the rafters [the main sloping members]. You will need a structural engineer to specify a suitable method. It can be done.

As you Know all about rafters, beams and trusses, perhaps you can tell us how it can be done.

 

[indus]

Thanks guys.

So it seems like it is not a big deal to design from a structural integrity point of view.

I shall ask for it to be incorporated into the BR drawings.

Many thanks