Correct Roof Construction Method [Drawings]

[GateBeam]

Out of these three drawings, which is the best way to construct the roof of my planned rear kitchen extension?

This concerns the placement of the rafters, ceiling joists, and wall plate.

Figure 1.
Inner Leaf Method
29 Degree Pitch

Figure 2.
Inner and Outer Leaf Method
29 Degree Pitch

Figure 3.
Outer Leaf Method
25 Degree Pitch
Ceiling Joist 4m

Thank you.

 

[tony1851]

No. 1 is the conventional way; no. 2 is impractical because you will not get accurate cuts for both birdsmouths - once the weight of the tiles
goes on, the rafter will deflect downwards slightly, causing a slight lift off the outer wall plate.
No. 3 is done occasionally, but having the wall plate on the outer skin is more likely to expose it to dampness. It's also harder to tie the wall plate down if it's on the outer skin.

 

[GateBeam]

@tony1851
I see, very useful info, thanks for your help!
I'll proceed with Figure 1 as suggested.

 

[GateBeam]

[COLOR=rgb(13, 27, 48)]@tony1851[/COLOR]

I have been looking at some more construction techniques and even though Figure 1 is the conventional way, there are some variations within them. For example;

Figure 1
Conventional way you recommended.
Ceiling Joist on Inner Leaf Wall Plate
Rafters on Inner Leaf Wall Plate

Figure 2
Ceiling Joist on Inner Leaf Wall Plate
Rafters on Outer Leaf Block

Figure 3
Ceiling Joist on Inner Leaf Wall Plate
Rafters connected to Ceiling Joist

Which of these three would you recommend?

Thank you again.

 

[tony1851]

Fig. 2 - it's not usual to birdsmouth over a brick or block. It is better to b/mouth over the higher (timber) wall plate.
Closing the cavity in that way is structurally very little better than just letting the load go down the inner leaf;
Depending on how you insulate, you might get cold-bridging, with possible mould on the inner edge of the block.

Fig. 3 - only suitable for factory-made roof trusses (P.S. you can't make these with the metal gang-nail plates you can buy in the sheds).

Stick to Fig. 1 - safe, well-tried and trusted method.

 

[GateBeam]

@tony1851

Righto, thanks for that. I'll stick to fig 1.