Rafter span for lean to extension

Hi,

First post here. Must say thanks to you all for helping me to renovate my house so far. I've tried looking through previous posts on this matter but struggled to get much clarity.

The house is a small victorian cottage with tall sash type windows so height of extension is limited. So thinking a lean to type extension is best, so that a roof can go under the window sill. Hoping to be able to get an extension at 3.82m long so that is big enough for a lounge.

I've worked out it's around 3365mm on the finished extension floor level to the top of the ridge plate. So with bi-folds at 2.1m tall, a cavity lintel and block or rsj over these at 200mm ish in height plus wall plate. Is right that it's about 1m rise giving around 15 degree roof pitch?

Is it feasible for something like 2x7/2x8 to span 3600mm off the ridge plate on to the wall plate with a vaulted ceiling at this pitch? Tiles will be low pitch pan tiles.

Any help would be great, cheers!

 

Google rafter span tables and work it out, but I think you’re looking at 6x2 or 7x2 from memory at 400 centres

 

Struggling to find a span table to match what I'm after. Is it acceptable to work of a c16 flat roof joist table which states 50 x 175mm spans 3.81m on 400mm centres? Assuming this would be similar to a flat roof with a pitch of only 15 degrees

 

Looks like 7x2 as you’re not that much more than the 6x2 max span at that pitch

 

Span is measured bearing-to-bearing up slope so your 15° span would be a little longer than 3.81m. I suspect your options would be 50 x 175 C24 @ 400 c/s or 50 x 200 C16 @ 400c/s.
If you wanted to use cheaper 50 x 175 C16, consider bringing them in a little to 350 or 375 centres.
If the cill/roof pitch is a problem, you can take a good 3-4" out at the lower birdsmouth, which might enable you to get another inch or two below the cill.

 

Thanks both for your help! Just wanted to make sure it's possible for a lean to extension at that span/pitch as most I see are around 3 metres

 

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