Designing my loft boarding and supporting joistwork

[danlightbulb]

I found a thread on DIYnot about someone who wanted to cut joists at an angle going into eaves. Seems to be ok to do. https://www.diynot.com/diy/threads/cutting-ends-of-new-loft-floor-joists.433570/

Still the issue of getting 270mm of height though. If I can use 6x2's into the eaves then I'd still need another 6" of height on top of this to insulate properly. Could be achieved by cross battening with another set of 6x2's but if I do that its twice the amount of joists.

I wonder if I could lower the amount of mineral wool insulation by insulating the rafters as well. So say 150mm of rockwool, and 50mm of celotex in the rafters. Or I could use celotex insulation instead of rockwool which I think would lower the required thickness?

 

[Hagar]

I found a thread on DIYnot about someone who wanted to cut joists at an angle going into eaves. Seems to be ok to do. https://www.diynot.com/diy/threads/cutting-ends-of-new-loft-floor-joists.433570/

Still the issue of getting 270mm of height though. If I can use 6x2's into the eaves then I'd still need another 6" of height on top of this to insulate properly. Could be achieved by cross battening with another set of 6x2's but if I do that its twice the amount of joists.

I wonder if I could lower the amount of mineral wool insulation by insulating the rafters as well. So say 150mm of rockwool, and 50mm of celotex in the rafters. Or I could use celotex insulation instead of rockwool which I think would lower the required thickness?

I was up in my loft today having taken a single wooden bed apart for storage. It is fully boarded and I looked at its construction. The roof is a traditional purlin rafter design of the 1960's. Timber hangers are nailed to the purlins at alternate rafter centres say 4ft or 1200mm. These support cross timbers which are also supported at internal wall positions by timber noggins. Onto these cross timbers lay 3" x 2" timbers nailed under fibre boards.

It was professionally constructed and has 300mm of insulation under. I do not have all the details as the work was carried out by a previous owner. It cost about a thousand pounds at the time.

 

[jpaul]

I found a thread on DIYnot about someone who wanted to cut joists at an angle going into eaves. Seems to be ok to do. https://www.diynot.com/diy/threads/cutting-ends-of-new-loft-floor-joists.433570/

yes - I had subsequently read it, wondering if calculation was legit, or whether you could get some stronger metal sleeve to encase the end of the cut joist,
but, also thought cross battening off of a ledger board or face bolted joist supports on the other wall might be better.

Onto these cross timbers lay 3" x 2" timbers nailed under fibre boards.

I think if you are moving nimbly around 9st/70kg - fine, but older relations who are more like 12st, could stumble I'd be more worried -
anyway, what is the actual span as most 60s houses have internal solid walls to take the load half way across - none of this paper mache modern stuff.

 

[Hagar]

I think if you are moving nimbly around 9st/70kg - fine, but older relations who are more like 12st, could stumble I'd be more worried -
anyway, what is the actual span as most 60s houses have internal solid walls to take the load half way across - none of this paper mache modern stuff.

Luckily I am a nimble 10st. I have not measured spans and yes we have solid brick internal walls I expect they are used in a support capacity.
If the timbers were used as a two span member the central support would attract 1.25WL and the end spans 0.375WL. However I suspect they lap over the wall. But still carry best part of 50% of the beam load.

I am not ripping it up to have a look though.