Dormer construction questions.

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I have planning permission to add 2 dormers (rendered, timber construction) to the front of my property in order to create 2 extra rooms upstairs, a Bathroom and Ensuite.
The picture shows a cross section of the house. Black existing, red dormer.

crosssection.jpg

All walls A, B & C are 9" brick. In the dim distant past (1850s) the roof line for the front section (A to B) was shallower, it was raised in the 70s.

Rafters are only 3x2 but there are lots of them!

When it comes to adding dormers I understand the normal practise is to double up the rafters up to the ridge and build the cheeks off them. This is going to be tricky as they would have to be a bigger section, and they cannot be sunk into the roof as it would affect the headroom in places on the top floor, particularly part of the stairs. Obviously they cannot sick upwards as that would mean raising the whole roof, and cottage is attached.
Would it be possible to fit bigger rafters from A to B (rather than right to the ridge) given they are both load bearing walls, any extra height would then be within the dormer walls?

Alternatively I have to put new joists in to create the floors of the dormers as what is there is currently only for holding up the ceiling. Could I fit joists (steels?) of suitable size at either side (A to B) and build the dormer walls off them?

One of the dormers would actually have it's cheeks over existing brick walls, is there any reason in theory I can't build off them rather than bigger rafters?
Both 'dormers' will have their front walls on the outside wall below.

I think the confusion comes when they get described as dormers when more accurately they are probably 1st floor extensions.

Yes I know a SE would be required, but I would like to have some options in my mind when the time comes to talk to him.
 
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You can easily deepen the rafters between support points A and B, ie just under the dormer cheek - no need to go all the way up.
You can also build the cheek off the brick wall.
 
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Dragging this up again, more questions.
Being unable to find a builder to do the work, I'm planning on taking it on myself as much as possible.

First job is to replace the ceilings in the two downstairs rooms, as they are currently only ceilings on 3x2 timber.

The plans show 75x150 C16 joists at 400 centres for the new floor/ceiling. The span A to B on my diagram above is approx. 2.6m. They would be supported on joist hangers attached to timber bolted to the wall either end, in order to lower the ceiling a bit, again for headroom.

I'm struggling to find 75x150 locally and considered I might be able to go with 45x145 at a closer spacing or doubled up. Rooms above will be bedroom and bathroom. Can't go higher than 150 really in order to maintain headroom.
I've looked at span tables and believe they would be fine, unless I'm reading something wrong.

Thanks in advance.
 
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45 x 145 C16 @400 centres would span 3m comfortably. Can't see why your plan-drawer specified 75 x 150?

Just double them up under the bath if they run parallel to it.
 
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Can't see why your plan-drawer specified 75 x 150?

This appears to be the second time he's over specified. I'm getting the impression he arrives at a figure then adds on a bit extra just in case ...
Thanks.
 
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Back again with more questions.
In order to gain a bit more headroom in these new rooms (currently the floor would be about 320mm above the existing floor level B to C) I'm thinking I could steal a bit of space from the room below which has quite a high ceiling. This would mean rather than sitting the new joists on top of the wall A (pic in first post) they would be level with it. Is it acceptable to hang joist hangers from the wall plate? This is further complicated in that there is a window in Wall A and without ripping stuff out I do not know what there is in the way of a lintel. Given it's only just below the wall plate I'm guessing not much.

The new front wall of the dormer is going to sit on the inner of wall A, so I guess the wall plate in that section could be replaced with something beefier and long joist hangers used to drop the new floor lower, maybe I'm overthinking this.
Would have been so much easier if I had not insisted on gabled dormers rather than flat rooved ones, but I wanted something more sympathetic.
 
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Just in case anyone searches, comes across this thread, and wonders what the answer was

The new front wall of the dormer is going to sit on the inner of wall A, so I guess the wall plate in that section could be replaced with something beefier and long joist hangers used to drop the new floor lower

Building Inspector says this is an acceptable solution.
 
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Another query if I may.
Attached is a snippet of the plans. Rafters are specced at tripled C24, 75x125. I have to say I had not noticed the tripled aspect before and it rather messes up with my plans of hiding the extra depth inside the dormer cheeks, as they are only 100mm thick. I could thicken them to match, but that cuts down on space, increases weight etc..

Capture.PNG


Before I suggest it to the Building inspector does the following sound reasonable?
If the spec of the tripled rafters is good enough to support the dormer cheeks and roof, if I installed the same spec (or bigger) as joists at the cheek side of the new floor could I not build the cheek right down to the floor, and leave the rafters alone? The floor joist span is less than the rafter span down the slope, the only difference is the joists would be on hangers rather than sitting on top of the 2 walls.
The tops of the cheeks walls (top plate?) will also be supported at either end on the 2 existing walls, which would (should?) help spread the load.

I really should stop calling these dormers. That is what the 'plans' refer to but as I see it, in my situation, the more normal dormer construction is not necessarily appropriate, or even desirable.
 
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Get someone to properly calculate the loads not just guess at a treble rafter.

For your 'building off the floor idea', the same principle applies and you need to know if the floor joists can bear the load.

Without seeing the rear elevation and plan view it is difficult to know what's going on
 

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