please help with rafter sizes

[john25043405]

hi all

I'm currently building a new extension and I'm trying to work out the size of Rafters using the trada tables and I am being completely blind or stupid or possibly both.

The measurements are as follows:

Rafter Span = 4300 to external wall
Rafter spacing = ?
rise = 1100mm
Pitch =14.35 degrees
roof slope length =4438.4mm

Covering is with concrete tiles and the area is Wiltshire so believe the load to not be more than 0.75KN/m2

I think that the size required is 220x47mm,C16 graded with rafters spaced at 400mm, but could anyone confirm this please?

All posts gratefully recieved

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[jeds]

Sounds about right. But I'd use C24 47x195 which will be easier to get and probably cheaper and is suitable for spans up to 4600.

 

[tony1851]

Sounds about right. But I'd use C24 47x195 which will be easier to get and probably cheaper and is suitable for spans up to 4600.

Is C24 actually cheaper and easier to get than C16?
Here it's dearer, and most yards have to order it if you want it!
Just wondering.

 

[jeds]

I was referring to the 195mm size. It's more common than 220mm and more likely to be off the shelf.

 

[noseall]

14.35 degree roof pitch and concrete tiles?

Sounds like you should be opting for a flattie.

 

[john25043405]

14.35 degree roof pitch and concrete tiles?

Sounds like you should be opting for a flattie.

Noseall tanks for the reply

I have been advised to use a concrete tile as this matches the rest of the house by BC and been given a tile which can be laid to a pitch of 10 degrees (Centurion tile:
http://www.forticrete.co.uk/concrete-roof-tiles-and-slates/centurion®-low-pitch-roof-tile.aspx ).

Regards

 

[noseall]

Is this roof uncomplicated, i.e. no hips, valleys, Velux windows etc?

I have been advised to use a concrete tile as this matches the rest of the house by BC and been given a tile which can be laid to a pitch of 10 degrees (Centurion tile:

Building control are not interested in the appearance of a tile, so they would not advise using a tile that matches the house unless it is suitable for the proposed pitch.

Appearance is more of a planning issue.

Centurion tiles will deal with low pitches provided your roof is suitable and you pay attention to the installation guidelines.

 

[john25043405]

It is simple construction so yes.

 

[Static]

Sounds about right. But I'd use C24 47x195 which will be easier to get and probably cheaper and is suitable for spans up to 4600.

Seems a bit shallow for a rafter.. unless you plan to ply board and screw a deck over.. I would stick with 220 deep and avoid cracking of any finishes under..