Rafter Issue

[SteveF]

I need to fit some pitched rafters (not load bearing...no roof on top) to go between 2 walls
is there a way to calculate rafter lengths? i assume I will need to do birdsmouth cuts but, I would like to also cut them into the wall plates partly
I thought about hangars...can these be notched in so they dont sit below the rafters? I know it is only a few mm but I need to avoid
does this make any sense?
is there some easy guide for this?

Steve

 

[skipdiver]

Not quite getting what you are after.

This site any use?

https://www.blocklayer.com/

 

[MikeG.]

Can you spell this out a little more. What sort of span between the walls? What are the rafters for if they aren't carrying a roof? What is preventing them pushing the walls over?

 

[SteveF]

so i dont get into the planning permission argument i was vague
imagine you was going to fit a false roof inside a conservatory and leave the original clear perspex above (not that anyone would do that)
and just put some celotex between the rafters and some form of plastic type cladding ceiling below
span is 2.8m approx

Steve

 

[MikeG.]

Ooooh....hang on a second. Have you really thought this through? There's so much wrong with that idea that I don't know where to start.

 

[SteveF]

Ooooh....hang on a second. Have you really thought this through? There's so much wrong with that idea that I don't know where to start.

please start Mike
theoretically speaking

Steve

 

[skipdiver]

This is now very common practice and a mate of mine makes a good living doing it. The issues are with condensation and how to prevent it.

 

[MikeG.]

Ooooh....hang on a second. Have you really thought this through? There's so much wrong with that idea that I don't know where to start.

please start Mike
theoretically speaking

Steve

Uh uh. No way am I getting involved in this. Sorry.

 

[Lons]

Ooooh....hang on a second. Have you really thought this through? There's so much wrong with that idea that I don't know where to start.

please start Mike
theoretically speaking

Steve

Uh uh. No way am I getting involved in this. Sorry.

I'm with you!

 

[Sgian Dubh]

... is there a way to calculate rafter lengths? Steve

You should be able to use the basic trigonometric properties of right-angle triangles. The two short sides of a triangle are labelled a or b and the hypotenuse labelled c. Therefore a² + b² = c², e.g. 3²(a) + 4²(b) = c²(hypotenuse), so to find the length find the square root of 25 = 5. Then you tinker about a bit with the result to allow for things like a ridge board, bird's mouth and any overhang.

As to the wisdom of what you're doing, I'll leave that to others because it's not something I know anything about, and doesn't really tweak my interest buttons anyway, ha, ha. Slainte.

 

[sunnybob]

In a purely theoretical mode, do both or either of the walls involved have the foundations and load bearing strength to support these rafters, and ceilings?

Rafters are not lightweight, and even insulation and false ceilings have mass and bulk. And what if there were a gale force wind applied to the side of the structure now that it has so much more mass?

I have a fair amount of indifference to planning rules, but building regs are pretty damn important purely to stop things falling down on folk while watching TV in the evenings.

 

[SteveF]

this is quite common these days
I calculated total weight to 108 kg
nothing will be attached to existing roof
the conservatory would have been built to cope with snow loading and the fact it would take my weight spread across just 2 beams
I think the foundations will cope

Steve

 

[Lons]

the conservatory would have been built to cope with snow loading and the fact it would take my weight spread across just 2 beams
I think the foundations will cope

Steve

That's not entirely my experience Steve.

I've built a lot of conservatories over the years and demolished quite a few as well and have found some huge difference in quality. From the greenhouse type sold by the big sheds to top quality with oversize and reinforced extrusions. There were ( not sure these days), very few regulations to stop cowboy companies churning out rubbish and I've seen expensive units with thin extrusions and no internal reinforcement which were hardly strong enough to support the poly roof.
There are of course many quality conservatiories out there and conversions to either heavier glass roofs or lightweight solid are common but suitability must be determined before this work is undertaken.
I bought trade kits from a number of companies who I trusted but avoided others like the plague as it was my reputation at stake.

I vividly remember getting an urgent call to a lean to about 4m x 3m on which the roof was slowly collapsing under a heavy rain storm. I had to prop the rof internally as a temporary measure to stop the thing falling down. Another just a couple of years ago went down like a pack of cards during strong winds.

Of course as the OP is asking only theoretical questions, it doesn't matter does it. :wink:

Bob

 

[skipdiver]

Most people doing these just attach timbers to the roof spars of the conny, then insulate, add a membrane, and finally airboard. The roof should easily cope with the weight of that and i've seen it done dozens of times. Only problem as i said earlier is possible condensation issues, depending on how you tackle it. If you wanted to put in a lowered ceiling at window frame height, you could attach timbers to the ringbeam with brackets. 4" CLS would span that distance with airboard attached.

 

[SteveF]

Most people doing these just attach timbers to the roof spars of the conny, then insulate, add a membrane, and finally airboard. The roof should easily cope with the weight of that and i've seen it done dozens of times. Only problem as i said earlier is possible condensation issues, depending on how you tackle it. If you wanted to put in a lowered ceiling at window frame height, you could attach timbers to the ringbeam with brackets. 4" CLS would span that distance with airboard attached.

thankyou for positive response
the condensation mentioned would be an issue
I need to come up with a way to vent between roof and insulation

Steve

 

[Lons]

the condensation mentioned would be an issue
I need to come up with a way to vent between roof and insulation

Steve

Hypothetically of course :wink: :lol: