Flat Roof Joist layout

[tqgrant]

Hi all, great forum you've got here! Seem to always pop in here when i need to do some research on something, but thought i better join up (and in) as im currently doing an extension (mostly myself apart from bricky) and have a few question that i could do with some help with!!

This regarding a flat roof and the joist layout, wall plates etc. As i've never done roof joists before im a bit stuck on the layout and if wall plates are needed and where the outside joists are situated.

The extension is 10.6 (width of the detached bungalow) x 5.5 metres, back wall is the existing bungalow, im going to be attaching a wall plate to the back wall with joist hangers, using 6 metre lengths of 47x225.

The main thing i cant get my head around is the wall plates! Im pretty sure that i need one on the top of front inner skin wall to take the joists and have something for them to fix too? but do i need them on the two side walls aswell?

Also the outside joists, do they sit on the innerskin wall/wallplate or just inside the wall?

Getting really confused with it and can't seem to find much online to point me in the right direction. Anyone got any advice.?

 

[noseall]

You don't actually need a wall plate at all for a flattie but it does simplify the holding down headache.

You would typically fix a wall plate at each of the bearing ends of the joists. The plates are fitted to the inner skin. You then use holding down straps to secure the plates. Theses straps are fixed to face of the inner skin of blocks.

 

[tqgrant]

thanks for the response noseall..

So if i go for a plate on the front wall i pressume that i need them on the side walls aswell? or everything will be out of line.

I've just found a diagram showing a joist layout, showing the outer joists just on the insides of the inner skin and a ladder created to go over the two courses. Presume this is the correct way?

 

[noseall]

So if i go for a plate on the front wall i pressume that i need them on the side walls aswell? or everything will be out of line.

I've just found a diagram showing a joist layout, showing the outer joists just on the insides of the inner skin and a ladder created to go over the two courses. Presume this is the correct way?

You only need a plate where the rafters or joist bear onto the walls.

You make up the difference on the side walls with more masonry.

Any extension to the joists (or rafters) sideways is done with either noggins or a ladder set and fixed to the last joist or rafter. These are usually then built in with masonry.

 

[tqgrant]

thanks again..!

Think i've got all that! Timber is coming tomorrow so hopefully should go according to plan, still gots lots of planning to do for the joist layout as ive got 3 roof windows and a lob burner flue to add in there..

Now for the next step and more questions! Think i've been doing too much reading up online and have too much info swirling around in my head on how to do everything.

The insulation is 140mm (coming tomorrow aswell) and im going to use EPDM to finish it off.

Heres the Q's:

1. I wasnt too sure about the vapour control but from what ive read it looks like i can use a dpm (1000g/250mu), would that work ?

2. Decks: Was thinking 18mm OSB3 top and bottom, but see some people mention that you can lay the insulation on the joists then 18mm osb on top then use foil back boards. or perhaps i can use 18mm bottom and something thinner on top? I don't want to skimp but could also do with keeping costs down as much as possible as this roofs costs are much higher than i thought they would be! lol

3. Fixings: God those insofasts are expensive! Especially with the size of this roof.. Is it ok to use regular screws?

Sorry for all the questions, but my heads spinning with info! I havent even got onto the big fat fascia yet ! lol

 

[tony1851]

1. I wasnt too sure about the vapour control but from what ive read it looks like i can use a dpm (1000g/250mu), would that work ?

2. Decks: Was thinking 18mm OSB3 top and bottom, but see some people mention that you can lay the insulation on the joists then 18mm osb on top then use foil back boards. or perhaps i can use 18mm bottom and something thinner on top? I don't want to skimp but could also do with keeping costs down as much as possible as this roofs costs are much higher than i thought they would be! lol


my heads spinning with info!

Slow down a bit!

Can't quite follow what you're doing because there seem to be boards here, there and everywhere.

Are you contemplating a warm roof?

 

[tqgrant]

yes a warm roof with 140mm insulation and EPDM finish.

 

[tony1851]

Roof joists - decking (18 osb?) - 500 or 1000g polythene vcl - insulation - thin boarding (plywood) - EPDM.

The vcl should be turned up and sealed to the roof finish.

(And of course a deep fascia!)

 

[tqgrant]

thanks for the reply Tony.

Regarding the VCL, does it have to be a specific VCL or is it ok to use say a black DPM (1000g)?

When you say thin boarding, what sort of thickness ply do you think, 6 or 9mm or something like that? and would it need to be marine or would red faced hardwood be ok ?

 

[tony1851]

Either 500g or 1000g will do - what's more important is that all joints are taped.
For the top boarding (over the insulation) something like 9mm ply will do - it's not realy structural, just to spread the load a little over the insulation when working on the roof.

 

[tqgrant]

ok cool, so a red faced 9mm ply should be ok ? no need for Marine.

What about fixings? Are the insofast/suretwists the ones to use or would regular screws work (countersunk).

thanks for your advice so far!