Loft Floor Joist & Pole Plate Junction Detail

[t2023]

Hi All,

I'm converting my loft space into a habitable room and would like to hear your thoughts on the floor joist construction.

CONTEXT: As shown in Sketch 01, the existing ceiling joists are fixed to the existing wall plate on the inner leaf of the external cavity wall, and the existing rafters are fixed to the existing pole plate directly above the ceiling joist.

PROPOSAL: The new floor joists (8x2") for the loft are designed to be installed on top of the existing wall plate. The plan is to cut the pole plate to fit the new floor joists. As shown in sketch 02, the pole plate will be bolted and braced to the wall plate.

Question 01 - My builder and SE have approved this strategy, but I'd like to hear your thoughts on it. Is this a common occurrence?

Question 02- Depending on the results of the site survey, we may have to pack the floor joist with 75x100 timber as shown in sketch 03 to ensure the floor joists are levelled and to create enough void zone for the WC waste pipe. Do you think this will cause any problems?

Thank you for your time.

 

[jks7492]

01
Looks fine, if its been approved/designed by your SE and builder is happy with it then not much more to it?
A more common solution for loft conversions is to run a beam between gable walls (above the ceiling joists) and hang the new floor joists from it - the existing ceiling joists and rafters etc can all be left as they are currently.

02
No impact structurally but raising your joists might mean the top of your joist clashes with the sarking board?

 

[t2023]

01
Looks fine, if its been approved/designed by your SE and builder is happy with it then not much more to it?
A more common solution for loft conversions is to run a beam between gable walls (above the ceiling joists) and hang the new floor joists from it - the existing ceiling joists and rafters etc can all be left as they are currently.

02
No impact structurally but raising your joists might mean the top of your joist clashes with the sarking board?

Thank you for responding.

Yes, SE has approved it. The calculations have been approved by the local authority, but not the specific detail. Maybe I should run the details by them? Or am I going overboard? I don't want to cause any additional delays.

Because my property has a load-bearing wall in the centre, there was an opportunity to avoid having steels, which was advantageous logistically as I live on a busy road.

If we do decide to raise the floor joists, I imagine the ends will be cut to match the roof profile.

 

[gonch69]

If you're joining the joists on the load bearing wall should be okay but if you're doing lengths the full span you would only get them in by removing tile and sliding in from a scaffold

 

[t2023]

If you're joining the joists on the load bearing wall should be okay but if you're doing lengths the full span you would only get them in by removing tile and sliding in from a scaffold

Hi Gonch69

Thanks for your response.

Yes, the floor joists from opposite ends join centrally on the load bearing in a sistering connection with a maximum overhang of 100mm.

 

[catlad]

From what I can see! you are proposing to notch your new joists around your pole plate leaving it resting partially on the pole & wall plate? if that's the case then I would recommend adding say 4x1" timber to the wall plate to allow for discrepancies in the latt & plaster. I would overlap the joists on the spine/central wall at least 3' personally, not 100mm

 

[t2023]

From what I can see! you are proposing to notch your new joists around your pole plate leaving it resting partially on the pole & wall plate? if that's the case then I would recommend adding say 4x1" timber to the wall plate to allow for discrepancies in the latt & plaster. I would overlap the joists on the spine/central wall at least 3' personally, not 100mm

Hi Catlad
Thank you for responding.
The plan was to cut the pole plate to allow the floor joist to pass through (if that make sense). Although your notching sounds intriguing. However, wouldn't this potentially weaken the floor joist? The pole plate measures 4" x 2".
I agree that the wall plate will need to be packed to prevent the floor joist from resting on lath and plaster.

 

[kp00110]

I think the blocking between the wall plate and pole plate will need to be positioned directly under the rafter locations ofcourse, to keep everything stable - unless the photo is your exact house and the joists are directly beneath each rafter. I had the same situation in my house but the rafters were a different spacing to the ceiling joists, which means everything is thrown off and you get an incosistent spacing of new floor joists

 

[catlad]

Hi Catlad
Thank you for responding.
The plan was to cut the pole plate to allow the floor joist to pass through (if that make sense). Although your notching sounds intriguing. However, wouldn't this potentially weaken the floor joist? The pole plate measures 4" x 2".
I agree that the wall plate will need to be packed to prevent the floor joist from resting on lath and plaster.

You can't cut the pole plate without weakening your roof!