Flat roof construction

[Fractal]

Hello
I'm hoping someone can help me with a couple of queries about flat roof construction.
I've read a lot about how they are constructed and am confident I can manage it, but I can't see to get one thing straight in my mind.

At the walls running parallel with the joists, where is the last joist positioned? It seems to me it should be flush with the outer skin of bricks with the fascia nailed on to it, overlapping the bricks.
If I do it like that, how do I fit the last joist hanger onto the header plate?
If it should be positioned over the inner leaf (or elsewhere) do I nail battens to the joist to the level of the outer leaf and then nail the fascia on?

It will be a warm roof, so I am assuming because it doesn't need vented the ends of the joists should be cut flush with the outer skin of the adjacent wall and the fascia nailed onto them.
Is that correct?

Thanks for any help.

 

[noseall]

The end joist i.e. the one that cradles the fascia is fitted via what we call a 'ladder set'. This one is not particularly structural

The last primary joist is the one that sits internally and catches the plasterboard right against the wall. You then fit noggings or 'ladders' to the primary joist that span over the masonry. The end joist is then fixed into the ladders. It is not essential but it is practical. You could do away with this end joist and fix directly into the noggin ends. We however always fix an end joist as it locks the ladders together and allows for decent fixing of both the fascia and the ends of the deck and the cocking fillet.

Your cavity insulation should continue up to meet the underside of the warm deck.

The overhang or soffit should be determined by looks or design and will be decided when cutting your primary joists to length. We always allow for a timber to be fixed to the primary joist ends when determining soffit over-hang. Again, not essential but robust.

Your side ladder nogging lengths will then be cut according what ever overhang you have on the front so that you have a continuous parallel soffit.

 

[Fractal]

That's really helpful noseall thanks.

So when you mean the last primary joist catches the plasterboard, does that mean the inside face of the joist lines up approx with the inside of the block skin?

By my reckoning that makes the distance between the two joists on the end somewhere about 250mm instead of the 400mm I was planning, so do I just cut one of the decking boards to line up with the centre of the second joist in rather than starting with a whole sheet at the end?

 

[noseall]

So when you mean the last primary joist catches the plasterboard, does that mean the inside face of the joist lines up approx with the inside of the block skin?

The two end primary joists i.e. the ones that will be catching the ends of the plasterboard can be located to within say 100mm of the wall and as close as say 25mm (thickness of wall plaster) of the wall.

In other words if you were looking up at your roof from inside you will either see a 25mm gap or a 100mm gap, or something in between.

By my reckoning that makes the distance between the two joists on the end somewhere about 250mm instead of the 400mm I was planning, so do I just cut one of the decking boards to line up with the centre of the second joist in rather than starting with a whole sheet at the end?

Cut your deck to whatever is practical or economical. In a perfect world there would be no waste, however this is rarely the case. We often use the ply sheets to 'square-up' a roof whereby we try and locate the starter sheet on an outside corner. Other times we may start by butting against the wall and work away from that.

By my reckoning that makes the distance between the two joists on the end somewhere about 250mm instead of the 400mm I was planning, so do I just cut one of the decking boards to line up with the centre of the second joist in rather than starting with a whole sheet at the end?

You need to plan the joist layout to benefit the plasterboards, the deck and the soffit overhang (if any).

It is unlikely you will be able to satisfy all three but strive to get somewhere near or acceptable.