Advice on wall plate for garden room/office

[RashDIYMan]

Hi, looking for some advice on the roof installation, for a garden office/outbuilding.

The building is constructed from brickwork, double skin, with cavity wall insulation (50mm).

It will have a flat roof, span of 3metres. The joists will be 6 x 2, large roof, there will be over 20 joists in total, with an overhang of about 20mm all the way round. The fall will be around 50mm for every metre, 18mm OSB3 boards on top. Then EPDM rubber roofing on top of the OSB3 boards.

Question 1: Should I use C24 joists or will C16 suffice? I’m within permitted planning rights with the area and roof height, so no requirement in terms of building regs.

Question 2: Do I need to fit a wall plate on top of the inside skin?

Option 1 would be to fit a 4 x 2 wall plate on the inside skin, on top of a bed of mortar, and then secure it down with restrainer strips every 2 metres. The joists then sit on top of the wall plate. I’m a bit confused with what to put on the outside skin. Another brick could work, but the brick is 65mm, whereas the wall plate is 45/47mm. So, the wall plate will need quite a bit of mortar to raise it to brick level.

Option 2, don’t use a wall plate, secure the 6 x 2 joists directly to the inside skin of blocks using twisted restrainer strips, not sure if I need these on both sides or just one. This will also save me 2 inches on the height.

Can anyone advise please on the above

 

[Deleted member 292770]

yes to the wallplate. and why so much pitch? 150mm over 3m?

 

[^woody^]

Flat roofs don't have wall plates

 

[RashDIYMan]

yes to the wallplate. and why so much pitch? 150mm over 3m?

Hi, i thought the pitch was less! It was based on various (loads) YT videos to be honest, I'm no expert, it's been a steep learning curve from the start of the project. I would have gone with a builder, but the quotes were 100% more than what I worked out as the total cost, and times are a bit tight atm.
But height was also a factor, I have to keep the max height at 2.5m. I want some pitch for the rainwater to fall down.
You definitely advise flat roofs need a wall plate? What would you advise on the outside skin, a line of bricks?

 

[RashDIYMan]

Flat roofs don't have wall plates

Thanks for the response. That's what I have seen on various forums, and hence the question. Seems to be a 50% split/divided opinion. Would love to see an example of flat roof with no wall plate, but nothing on the internet.

 

[^woody^]

flat roof joists uk - Google Search

 

[RashDIYMan]

I've googled quite a lot on this subject, and the information/advice is very sparse. If you can send me links pertinent to my question I would really appreciate it.
That's why I have ended up asking the question here.

 

[Deleted member 292770]

its not a flat roof with a pitch of 150mm in 3m. your timbers will be sitting on the edge of the brickwork.
you could sit your timbers on the brickwork and install furrings to get adequate pitch.

 

[RashDIYMan]

Don't see how installing furrings will give better/more pitch than 150mm in 3m? Can you explain please?
So, If I sit the timbers on the brickwork, can you advise how you would secure these to the blockwork?

 

[noseall]

Flat roofs don't have wall plates

Yours don't perhaps. Twisted straps are gash.

What would you advise on the outside skin, a line of bricks?

Splitters or use a conventional 75mm tanolised wall plate.

 

[^woody^]

Yours don't perhaps

Neither do my first floor joists. Odd that.

Anyway what does the detail from any flat roof material manufacturer look like. And what does LABC or NHBC say?

Or even your favourite posi and easi joist makers?

 

[noseall]

Neither do my first floor joists. Odd that.

Anyway what does the detail from any flat roof material manufacturer look like. And what does LABC or NHBC say?

Or even your favourite posi and easi joist makers?

They all go very quiet when a roof lifts and breaks the mortar.
Twisted straps however are still shoite.

 

[^woody^]

A wall plate is not an anti gravity plate.

 

[noseall]

A wall plate is not an anti gravity plate.

Mine are all pro-grav