Rebuilding joists on Victorian suspended floor

[J_Ashley]

Since we moved into a Victorian end of terrace, the floorboards in the bay window half of the lounge have always been a bit bouncy. Our reluctance to raise them all has now been overcome due to being necessitated by central heating pipe work. It turns out the existing joist work in the area is a complete mess, consisting of multiple botched jobs, makeshift wedges, and previous damp (long since resolved). The biggest surprise is actually how little bounce this mess was creating!

Anyway, we are planning of replacing the lot and wanted to get a bit of advice. We’ve raised the boards up to a wall plate that runs across the room, from which afterwards everything seems solid. Our plan is then to install a series of 2-3 new wall plates, which sit on small columns of bricks every so often, and lay the new joists across them.

My main query is how do I go about securing everything together? At present there seems to be a lot of everything placed on top with a distinct lack of fastening. Also, are the wall plates meant to be fastened into the walls, as very little seems to be secured to the wall?

 

[MikeG.]

Do you have a photo or two?

 

[J_Ashley]

These two photos are all I have at present. As you can see, existing wall plates and joists have deteriorated in the past and never been properly replaced.

 

[MikeG.]

That's a bit of a mess!

Firstly, I don't think there is any need for a wholesale replacement. I would cut out all the areas which are rotten or showing insect damage and assess from there. The floor doesn't need to be attached to the house. Concrete floors generally aren't either. So long as they are stable, and bearing properly on some support, it doesn't much matter what they are attached to. Attach everything properly to each other and the floor will act as one.

I would be principally concerned with maintaining airflow under the floor, to all parts of the floor. That will mean clearing all airbricks, and removing all loose stuff under the floor (which impedes airflow). Be careful in siting and designing any new supports for the floor, because you must disrupt the air movement.

You might take this opportunity to get some insulation in between the joist, but again, don't obstruct airflow. Oh, and those pipes all require lagging.

Maybe if you cut all the bad stuff away, remove all the loose rubbish, then post another photo, we can work out a decent scheme for renewing this floor.

 

[sammy.se]

What kind of insulation would go between the joists and how would they be held in place? Wouldn't that also affect airflow?

Sent from my Nexus 5X using Tapatalk

 

[MikeG.]

Mineral wool is better in my view than board insulation in these circumstances, in that it does better gap-filling, and also allows moisture to wick away or run through if there is a spill on the floor. It does require thought to retain it in place, though. Batts, which are stiffer, are probably better than loft insulation rolls. They should ideally be the same depth as the joists, and held in place by battens underneath, if possible. Netting can work, a bit, but leaves sagging.

So long as the bottom edge of the joists is exposed, that's good enough to keep them dry. There should be 150mm of clear void below this, if possible, but certainly at least 100mm.

 

[Marineboy]

When I replaced the flooring in our Victorian hallway I used 25mm Celotex (should really have been thicker but I was a bit strapped for cash), cut to fit between the joists and resting on panel pins so that the top of the Celotex (foil side up) butted up against the underside of the floorboards. Don't know if that if accepted building practice but it seemed logical to me.

 

[MikeG.]

Yeah, it's fine. I just think that mineral wool is slightly better in the same situation.

 

[J_Ashley]

As per the attached photo, whilst it could do with more, we managed to get round to some clearing.

The joists left behind, that terminate randomly midway across the floor, all run from the area of the floor that is currently ok. We’re thinking of adding a new wall plate and then cutting these to end there.

With regards to sub floor insulation, I’m not yet convinced. The sub-floor void area shallows towards this end of the building where the majority of airbricks are located, and so I’m keen to keep maximum space for circulation. Additionally the remainder of the ground floor would remain largely un-insulated as we’re not taking up large quantities of flooring everywhere. The pipes will be insulated post central heating improvements that will be undertaken soon.

Furthermore, the floor in the hall (the other side of the lathe and plaster wall - to the right as you look at the computer screen) also needs similar renovation. Is it worth trying to run wall plates the whole width of both rooms, or just treat them separately?

 

[MikeG.]

OK, that's much better.

I'm guessing it's about 2.0 or 2.1m between the brick pads in the main part of the room, near the ends of the cut joists. If so, and if the joists are 4" thick (and the pads are to the underside), then I think a pair of 4x2s nailed together spanning between those pads is your best answer, with the cut joists held in jiffy hangers. Insert DPC under every piece of timber wherever it touches masonry. Wrap it up past the end of the timber if the timber is adjacent to the outside wall.

I think you might have to take away the board under the radiator to enable you to get some proper substructure in there. Again, make a 4x2 trimmer spanning between the 2 brick pads in the bay, and hang some intermediate joists off that back to the longer trimmer you've made that is parallel to it.

How does that sound?

ETA.

Looking at it again, some of the other pads seem to have room above them to fit a timber in under the joists, so it might be that the joist sitting on the left hand pad has just sunk down there as you've removed debris. If that is the case, then I wouldn't try to replicate that because it doesn't look to me that there is enough room to get a strong enough timber in there under the joists to span that gap. In which case, the easiest thing to do would be to use 6x2s or whatever works height -wise, instead of 4x2s for trimmers.

 

[J_Ashley]

Thanks for all the advice. Good point about the DPC for contact with the masonry – I hadn’t considered that.

Existing joists are 3” x 2” and wall plates are 3” x 4”, so I assume we’re best off maintaining those dimensions?

 

[MikeG.]

You keep confusing me with terminology. Wall plates are the top of a timber frame, on which the roof sits.

To correctly size the trimmers, measure from the top of the brick pads to the top of the floor joists when they are in their correct position. I'm guessing from what you've just said that this will be 3" + either 4" or 3", so 6 or 7 inches. If that is the case, then I would use 6x2's, as I said, and hang the joists from this trimmer with jiffy hangers. You can pack the trimmer up off the brick pads using pieces of broken slate to get it to the level you want. The short trimmer under the radiator will only need to be 4x2, but use 6x2 if it is more convenient height-wise. Don't try to replicate the original by sitting the joists on top of a piece of timber. There simply isn't enough depth to get a strong enough floor.

 

[mr rusty]

I'd repost your query here https://community.screwfix.com/forums/b ... talk.2004/

Personally, that looks like a very shallow underfloor space which is always going to be difficult to ventilate. If you insulate, the joists underneath will be colder and less likely to dry from roomheat.

In my own house project where only the front room was suspended (mid and rear flags on sand) the whole ground floor is now dpm + insulation + concrete slab. However, opinions from other builders would be helpful. Not sure a woodwork forum is the place for the best advice.

 

[MikeG.]

I'm not saying that it isn't worth seeking the advice of builders, but the guy is getting free advice from an architect at the moment. An architect who specialises in ancient buildings. And furthermore, he has already said that he won't be insulating.

 

[screwpainting]

That is an over polished turd my friend.

Stop the farce now, listen to Mike, rip out everything that's not solid and start again, doing the job properly, using new materials, that, to be honest, cost bugger all in the grand scheme of things.

 

[mr rusty]

but the guy is getting free advice from an architect at the moment. An architect who specialises in ancient buildings

Fair 'nuff, but I am now interested why it is preferable to rebuild a suspended floor when the void is obviously shallow and below current spec for suspended timber floor. rather than put in an insulated concrete slab - I expect one good reason might be because ventilation to other areas might be compromised, but with that shallow a void it doesn't look so clever anyway......

 

[MikeG.]

Because a solid concrete slab is seldom a good answer in an old building where one can't be certain of DPCs, and where only one room is being dealt with. For a start, damp migrates sideways from under DPMs , leaving adjacent walls and floor vulnerable. Secondly, you'd be cutting off the airflow from the airbricks in the front wall, so parts of the remaining sub-floor void which rely on that airflow would be left unventilated. Thirdly, the owner expressed a preference for his timber floor. Fourthly, inappropriate modern insertions are already destroying our built heritage. Fifthly, to take out all the current floor, (most of which is sound) then barrow in 2 or 3 tons of concrete is an almighty amount of mess and disruption, and inordinately more expensive than simply replacing parts of a few joists and digging out the loose stuff. Finally the services under there would need re-routing.

I think that's enough, don't you?

As for complying with regulations, with a repair you only have to make it no worse than it is now. He will be improving it, so this work will be compliant.

 

[heimlaga]

When you pour a concrete floor inside an old building the moisture rising from the ground is held down by the concrete floor and goes up the walls instead.
New buildings (if well bult) have neither timber nor brick and lime masonry below concrete floor level and a modern foundation is usually drained and filled with coarse gravel or macadam which breaks off the capillary action so the moisture from the ground cannot rise. Old buildings have none of that.
Therefore a concrete floor usually works well in a modern building but not in an old one. It is completely pointless to solve one problem by creating another.

I have worked quite a bit as restoration carpenter and I have a Batchelors degree in construction engineering.

Coming from Finland and not from Britain I would either:
- Build an insulated wooden floor with cellulose insulation between the joists and proper ventialation openings to the outside. Up here we have built insulated floors for a long time and it is a proven fact that you really need big ventialation holes so the wind can blow under the houseif it is going to work in the long term. The wind transports the moisture away.
Cellulose insulation dries faster than mineral or glass wood during good drying weather so it is prefered as a way of minimizing damage in case the ventilation happened to become a little underdimensioned.
-Or just dig out some more muck and fill the void and all the space between the joists up to the floor boards with foamglas gravel and put a wooden floor on top. No plastic carpet just wood. Then the little moisture that finds it's way through the foamglas will evaporate into the room and disapear.

I don't know if any of those solutions are acceptable by British standards......... but I have seen enough moisture problems to know the risks of concrete floors in old buildings.

well....while I was typing Mike typed much of the same.

 

[mr rusty]

Mike/Heimlaga - not trying to pick an argument - genuinely interested in your opinions. My own house project had a rotten suspended wood floor very similar to this in the front room and flags on sand in the back 2 rooms. I discussed the options with both my builder who was doing a small extension and our local BC officer who was on site at the time, and the consensus was it was better to take the new concrete floor in the back room right through the front as well, rather than rebuild the suspended part.

This is a 1902 Edwardian and I totally get the heritage angle - we are trying hard to retain as much of the original as we can, and I have remade the sliding sash windows to the same proportions as the original 1902 architect drawings that we still have.

 

[MikeG.]

.......New buildings (if well bult) have neither timber nor brick and lime masonry below concrete floor level and a modern foundation is usually drained and filled with coarse gravel or macadam which breaks off the capillary action so the moisture from the ground cannot rise. Old buildings have none of that.........

Careful! That's not the case here, where not only do we have bricks below ground, we pretty much have to have bricks below ground, as concrete has to finish a certain distance below ground level to avoid frost damage. Nor do we drain our foundations. Nonetheless, the point you make about isolating the masonry from the concrete and thus the moisture it "pushes" sideways is 100% valid.

Modern houses properly built with a DPM and DPC, ground bearing concrete slab and so on, work beautifully. Old houses, with breathable and flexible materials and a good amount of controlled ventilation also work beautifully. Where problems arise is in hybrid situations, where a really thoughtful analysis of the situation needs to take place before modern materials can be used in conjunction with older materials and details.