Do I require a purlin on my kitchen extension

[headboard99]

Hello. I've just had our plans accepted for a single storey extension on the rear of our property. The extension runs 9m across the rear of the house and out by 3.4m Max. This will give a pitch of around 17deg. The tiles can be used between 15 and 20 deg. I'm planning to put a lean to type roof, with 2 velux windows, just to make the extension feel bigger inside. I just can't work out whether i need a purlin to span the 9m or would I not need one if I used approx 3.6m 150x50 rafters with a 450mm spread. Any advice would be gladly appreciated.
Thanks.

 

[theoldun]

Answer question below and will give you information required
Is your run 3400 to outside of brickwork or to outside of wall plate?
oldun

 

[headboard99]

Hello oldun thanks for the reply. The 3400 will be to the wall plate. We have not built the extension yet so the measurements can be reduced down to 3000 but will still be 9000 across the rear of the house. Thanks.

 

[tony1851]

It would be cheaper and easier to use 175 x 50s @400 c/s, rather than use 150 x 50s and have a whacking big (steel) purlin sticking down below your ceiling.

 

[headboard99]

Thanks Tony. We certainly don't want a purlin so good advice. Cheers

 

[theoldun]

The run on roofs is measured over all the wall plates less the ridge thickness, divided by two to give you half run from which you determine rafter size and rafter length.

In your case it will be slightly different as your ridge plate will be 47mm in width and will only serve one half run.

Therefore your half run will be 3350. This will give a rise of.1027 and a slope of 3512 plus overhang as required.

NOTE. We would add that if you are pitching of a 300mm cavity wall this will make your outside brickwork width 3800

You do not need any purlin of any description.

Rafters including overhang to be 4200 C24 47x150 @ 400 centres

Depending on size and weight of Velux, you may need to double up trimmer rafters each side.
If vaulted ceiling suggest you birds beak rafters over ridge plate

Edit Rafter size is based on 1KN/m2 imposed load
Regards oldun.

 

[headboard99]

Thats brill advice. Thanks very much for your time. Cheers.