Fixing joists for suspended timber floor to sill plate in Victorian cellar

[RFw3Duwrd7Yu]

I've got a small cellar under our sitting room (about 8' by 20'), floor boards are all still intact, but a number of joist ends are rotted, sill plate is seriously deteriorated with dry rot in several places and there are signs of localised woodworm infestation on a few beams. I'm going to pull up the floorboards (it's just 12 of them) and replace the joists which are only bearing the load of the floorboards (not walls) and the sill plate. The rot has probably come from presence of damp and lack of ventilation on the wall (which is all underground), so I'll be taking care to treat joist ends with damp-proof paint etc. (have ruled out caps as there isn't adequate clearance for these, unfortunately).

Here's my question - how was the sill plate traditionally fixed in place (this is all 1880-1910 construction)? It's all relatively (surprisingly, really) structurally stable, so I'm planning to simply replace like for like as much as possible here. Josts fit into the brick wall into 4.5" pocket holes at the top of the wall atop a sill plate that is abouit 11' wide (some sections of the plate have long since been cut out and replaced with masonry, from what I can tell). Is it nailed into masonry? And same for joists? Were these fixed with nails?

Very happy to hear any suggestions that folks have.

 

[Deleted member 174758]

Very often in Victorian buildings the joists are just spiked into the sill plate with girt great wrought or cut nails. The sill are may or may not be nailed into the mortar joints, but are often just held in place by the weight on top of them - any nails into the mortar joists would only be there to temporarily hold the sill plate in position until all the joists are fixed

May I make a couple of suggestions about timbers? I'd recommend using treated timbers and end sealing them after cutting. You may also be better off protecting the sill plate from the dry rot ridden masonry by laying it on a piece of DPM and wrapping that up the back. Where your timbers contact the masonry you need to en elope wrap them in somethi like roofing felt. Finally when we do this sort of repair we invariably space the joists out with solid strutting beneath them if they are over about 1.5 metres long. This is to ensure a stiff floor

 

[RFw3Duwrd7Yu]

Thanks @JobAndKnock for this really helpful input!

Re: fixtures - I was originally assuming that the sill plate might just rest in place by gravity and pressure, but had a wobble and started wondering about nails etc. Based on what you've said, I might stick with my plans for just resting it in place (and making sure the cavity in the masonry is clean and relatively square. But, I've got a 16ga nail gun, would it be worth shooting in a couple nails for each joist into the sill plate? Or should I just let gravity and careful milling take care of things? Or better to do screws / bolts from above?

I'm planning to get pressure-treated timber all around, definitely. For places where there is contact with masonry/earth, I was considering using damp-proof paint to completely coat surfaces and ends of sill plate, and then seal cuts as well as surfaces on joists which will be resting on masonry - should I double up with membrane alongside those painted surfaces as well? May I ask what the roofing felt does? Is this about preventing moisture ingress or providing some kind of vapour-permeable barrier? I'd been hoping to use plastic caps for joists, but it seems like there aren't any that are made for less than 225mm height, so wrapping or painting joist ends seems like my best plan.

Should I have something special underneath the sill plate? Or just membrane ok there?

I'd also been thinking about adding struts to support floor. We'll be pouring a concrete pad for the floor of the cellar in a couple weeks, so this will be a bit more feasible. Would you suggest leaving slots for timbers below? Or just pressure fitting on the bottom and using steel hangers on top?

The last detail I need to sort out is how to add ventilation to the side of the cellar which is currently closed off (e.g. earth against the brick wall on the outside, so no way to do air bricks). I've been thinking about ventillation, and am wondering now if I should dig a trench on the outside of the exterior wall, fill with gravel and perhaps even add in an air brick or two to allow for a bit more circulation. Alternatively, I could try to draw air away from that side using a humidistat extractor...

 

[Deleted member 174758]

...I've got a 16ga nail gun, would it be worth shooting in a couple nails for each joist into the sill plate? Or should I just let gravity and careful milling take care of things? Or better to do screws / bolts from above?

16ga isn't worth it - far too small. Just skew in a couple of 3in ovals, one each side of each joist to hold them in place, but you do need something to hold it all together and stop anything moving whilst you get the floor boards on

¿For places where there is contact with masonry/earth, I was considering using damp-proof paint to completely coat surfaces and ends of sill plate, and then seal cuts as well as surfaces on joists which will be resting on masonry - should I double up with membrane alongside those painted surfaces as well? May I ask what the roofing felt does? Is this about preventing moisture ingress or providing some kind of vapour-permeable barrier?

Paint has a tendency to crack as the timber moves (and all structural timber moves). End wrapping not only prevents moisture wicking, but also inhibits dry rot incursion. And yes, we use cheap roofing felt (tar paper) and either twist nails or clouts to hold it. I'll do some sketches when I get home and post them so you see how it is done

Should I have something special underneath the sill plate? Or just membrane ok there?

We normally use either a purpose made plastic membrane, the sort that brickies sometimes use beneath permeable coping stones, such as sandstone, to reduce water ingress into the masonry. An alternative is good old Visqueen black polythene membrane folded double or treble. Note that ideally you do need to lap the membrane up the back of the sill as well

I'd also been thinking about adding struts to support floor.

It's a fairly common misconception that solid strutting supports or strengthens a floor. It doesn't. What supports a floor is the joists, which need to be correctly sized to carry the load in the first place. What solid strutting does do is minimise the tendency of joists under load or overload to twist - something you experience in a bouncy floor. So strutting actually stiffens a floor and makes it less bouncy

Maybe I'm being a bit dense here, but I'm not sure what you are asking about the concrete pad. Could you clarify, please?

The last detail I need to sort out is how to add ventilation to the side of the cellar which is currently closed off (e.g. earth against the brick wall on the outside, so no way to do air bricks). I've been thinking about ventillation, and am wondering now if I should dig a trench on the outside of the exterior wall, fill with gravel and perhaps even add in an air brick or two to allow for a bit more circulation. Alternatively, I could try to draw air away from that side using a humidistat extractor...

To keep a cellar dry you certainly need some form of airflow. When adding air bricks be careful to ensure that they cannot be a identically blocked and that they cannot get water in them. I have seen vents which look like a dort of plastic periscope and are designed to allow air in from above ground level to cellars. They stop water ingress and are far less likely to be blocked by leaves, etc. Can't recall the name of them, however