Joist pockets, joist ends and securing joists

[nickheatley]

Dear All

I am restoring a 300yr old Georgian house with solid brick and lime mortar walls, and suspended wooden floors with joists that sit in wall pockets. While repairing floorboards (1st floor) I have exposed the joists and joist pockets along an external wall. I have been clearing out debris from these joist pockets to allow air circulation. Some joist ends are rotten and are being repaired, and some joists (specially in the window bays) were completely rotten and needed replacing with similar wood (listed property, most seem to be Douglas fir or oak). Most of the wall is dry and most joist ends are in good condition. I think most of the damp issues seem to have been in the past but I am keen to get this right now in case any damp returns, even just periodically during the winter or rainy periods.

I have a few questions on which I would be very grateful for people's thoughts:

-What is the best way of securing joists within joist pockets, especially if the pockets are larger than the joists? This includes setting the correct height as well as preventing lateral movement or rotation/twisting. My thoughts so far included slate or wooden wedges, or packing with lime mortar. If the latter, I assume you would just bed on lime and pack the sides. Although breathable, would lime packing impede airflow and promote rot? Or perhaps installing noggins away from the wall so that the joist ends are free in the pockets, thereby allowing air circulation?

-For external walls, is any treatment of joist ends advised? I have seen videos of wrapping joists in DPM but I think this will trap moisture and promote rot.

-Would you advise keeping a good gap between the ends of joists and the end of a joist pocket?

Many thanks

 

[Cubist]

To keep it brief:

• Don't bother trying to 'fix' the joists in the pockets. Leave them to rest inside but clear of as much direct contact with the masonry as possible. Installing noggins to to create a skeletal structure and some stiffness between the joists will be perfectly adequate.
• Yes, use noggins to to provide some stiffness between the joist ends and to prevent rotation or limit twist. However, most such will be overcome prevented when the floorboards are fastened down.
• Yes, use slate as a means to isolate the the foot of the joists from the masonry of the pockets. Also as shims to aid 'levelling' of the structure.
• Yes, leave some space between the end of the joist timbers and the masonry of the pockets. Moisture absorption is more rapid through the end grain.
• Try to use the same timber for any repairs to the timber if oak then stick with that.