2 x 75mm Rafters bolted together loft conversion?

[gasbusters]

Hi, after some advice.

My loft currently has 2x3 afters (75mm x 50mm). Can these be doubled up with another 2x3 bolted and glued to it to give the required 150mm depth to allow for the 120mm rigid insulation?

Im in the process of re-doing my loft plans as the structural engineer has been a nightmare to get original plans done so trying to find solutions myself which he can then calculate for me.

 

[blup]

Don't see why not in principle, if appropriately fixed, but I'm not a structural engineer. Maybe placing 6 x 2 or whatever alongside the existing or even running 3 x 2 at right angles which might also spread the loading.

Blup.

 

[foxhole]

If you did that I would also add a sister 6x1 to the side to tie it all together.Though best to simply replace rafters .

 

[gasbusters]

If i was to bolt and glue the two peoces together, whats a strong wood glue you would recommend thats low toxicity?

 

[foxhole]

https://www.toolstation.com/d4-wood-glue/p61864

 

[Deleted member 174758]

Maybe placing 6 x 2 or whatever alongside the existing or even running 3 x 2 at right angles which might also spread the loading.

It's quite normal to cross batten in this sort of situation, i.e. go at right angles to the existing ceiling joists.

Glueing/screwing and extra 3 x 2 on to of the existing in an attempt to get a 6 x 2 won't give anywhere near the strength of a 6 x 2. This is because the glue joints will be far from imperfect. Also there's an issue with the time of year. PVA glue needs a minimum temperature of 4° C to cure properly (as does PU glue) - below that point it often chalks out and fails - and it's the cold timber in the rafters which will give the most problems at this time of year. PVA glues are not gap filling so if there are any issues with the existing joists (e.g. wind, bow, surface imperfections including one or more surfaces which are not planed) it won't work, or if it does it won't work well, and any dirt or contamination will also reduce the glue bond. 2in width (probably 44mm in reality) is really too thin to bolt together vertically, and in any case the bolts would have to come through the roof coverings. If you are lucky you'll get the strength of a 4 x 2 or a 4-1/2 x2. BTW wood glues aren't toxic - if they were I'd have snuffed it years ago

 

[gasbusters]

Ive decided to get a couple of 2x3 and actually add them to the ceiling rafters to show what i mean. I have given a description on the pics of what im referring to, same as i sent to my structural engineer who advised when he first visited this was a possible solution.

Btw The large purlings will stay in place. The diagonal props willbe removed the the purling will be supported by verticle studs which will sit on top of the new steels that will sit directly below the purlins.

 

[gasbusters]

 

[Ian H]

I added a 2x1 onto mine so I could have 100mm insulation and 50mm gap.

The 2x1 was slightly narrower than the rafters so I got a good amount of foam in the gap.

 

[Leofric]

If the structural engineer has been a nightmare why don't you get a different SE who can design the structure for you instead of having to guess what is suitable ?

 

[gasbusters]

If the structural engineer has been a nightmare why don't you get a different SE who can design the structure for you instead of having to guess what is suitable ?

Unfortunately hes a relative, wifes uncle, and gong elswhere wud cause a bit of a family fallout although wifes starting to realise this guys taking the mick!

 

[gasbusters]

I added a 2x1 onto mine so I could have 100mm insulation and 50mm gap.

The 2x1 was slightly narrower than the rafters so I got a good amount of foam in the gap.

View attachment 158529

Did you add the 2x1 only upto the ceiling of the new loft room or did you have to take it all the way upto the ridge of the roof?

 

[Ian H]

I took it upto the ceiling (which was very close to the ridge anyway) and down to the dwarf wall.

Although, thinking back, my approved drawing said I only needed 75mm between the rafters and 50mm over them. I only used 100mm because I got a load cheap off e-bay.

There’s a few more pictures here:

https://www.diynot.com/diy/threads/100mm-insulation-between-5x2-rafters-too-much.492730/page-2

 

[gasbusters]

I took it upto the ceiling (which was very close to the ridge anyway) and down to the dwarf wall.

Although, thinking back, my approved drawing said I only needed 75mm between the rafters and 50mm over them. I only used 100mm because I got a load cheap off e-bay.

There’s a few more pictures here:

https://www.diynot.com/diy/threads/100mm-insulation-between-5x2-rafters-too-much.492730/page-2

Thanks for the info.

Werebuilding control ok with 100mm of insulation?

Both my architect and structural engineer have advised 120mm rigid insulation between the rafters which i thought seems ott

 

[Ian H]

Building control approved it with 75mm between and 50mm over.

They never actually checked it, just looked and said it was OK. I could have used nothing in between and 25mm over and they would have never known.

Both my architect and structural engineer have advised 120mm rigid insulation between the rafters which i thought seems ott

Are they recommending a full layer of insulation under the rafters as well?