Lean to extension roof

[Sproggy]

Hi I am looking at taking my lean-to poly carb roof off and replacing it with a timber and tile roof with two velux windows. My plan was to use 195 x 47mm C24 timbers at 400 centres. The room is 3600 wide and 3800 from the rear of the house to the front of the lean to. I have calculated that I need rafters 4000mm can any please confirm 195 mm x 47 mm C24 would be the right choice of timbers.

 

[foxhole]

Sounds good to me, are you insulating.?

 

[Sproggy]

I was going to use 100mm celotex insulation between each rafter and then 25 mm attached to 12mm plaster board over the full ceiling. I was just questioning myself over the rafter sizes I wasn't sure if I needed to go bigger

 

[23vc]

Check the rafter span and pitch against span tables (google).

 

[noseall]

Sounds OK to me. What is the pitch of the roof?

 

[23vc]

My slightly sketchy trig makes it just over 18 degrees if the 3.8 is the horizontal projection

 

[noseall]

My slightly sketchy trig makes it just over 18 degrees if the 3.8 is the horizontal projection

How did you assume/calculate the rise (height) of the roof?

 

[23vc]

rafter length is apparently 4m (hypothenuse)
horizontal run is apparently 3.8m (adjacent)
Cos=Adj/Hyp so we want the inverse cos of 3.8/4 = inverse cos of 0.95 = 18.2 degrees.
Agree?

 

[noseall]

rafter length is apparently 4m (hypothenuse)
horizontal run is apparently 3.8m (adjacent)
Cos=Adj/Hyp so we want the inverse cos of 3.8/4 = inverse cos of 0.95 = 18.2 degrees.
Agree?

With those figures yes (18.1948). Assuming that the 4m rafter bisects the joist at 3.8m.

 

[Sproggy]

Sorry I thought I put this on I can get 15* pitch.
3905 mm front to back external
4043 mm with the 15* pitch
There is no supports from front to back just back wall plate and front wall plate with two velux 780mm x 1140 mm.
Concrete roof tiles

Does using 195mm x 47mm C24 seem ok or do I need to go for something bigger

 

[noseall]

Does using 195mm x 47mm C24 seem ok

Seems ok at C24 @ 400mm c/c with a con' interlocking at 0.45kn/m.

 

[Sproggy]

Sorry Noseall what does a con' interlocking at 0.45kn/m. Mean?

 

[23vc]

Concrete interlocking tiles with that weight per area

 

[darreng1983]

If you’re doing it properly by building regs to get signed off, you’ll probably need to overboard with 40mm Celotex like I did.

 

[23vc]

Yeah, I thought 25mm was a bit light, I had to do 50. Go the foil board and separate plasterboard route rather than the plasterboard that comes with polystyrene stuck to it. Easier to fix.