Rebuilding joists on Victorian suspended floor

[heimlaga]

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.
(.....)

Strange........ bricks in contact with ground near enough to the outside of the wall to allow them to freeze crumble veery rapidly from frost cracks in our climate. The inside of very thick usually hollow brick exterrior walls and brick partition walls often have bricks all the way down into the ground and there the bricks last because they don't freeze even when it is -30 degrees celsius outside.

We have different climates and therefore build in different ways!

 

[MikeG.]

......Strange........ bricks in contact with ground near enough to the outside of the wall to allow them to freeze crumble veery rapidly from frost cracks in our climate.......

We're better at making bricks than you.

Seriously, our bricks are tested for frost resistance, and in some circumstances, a very high grade of brick known here as an engineering brick, is used below DPC. The word for a brick destroyed by frost is "spalled".

 

[heimlaga]

I have never seen nor heard of any such bricks.
Either you are better at making them or you have better clay.
After all this Brexit squabbly you really deserve a chance to cheer yourselves up...... so let's say it is all about the brickmaker

 

[scholar]

Bricksit innit

 

[Marineboy]

Love the description “Brexit squabbly “

 

[J_Ashley]

Many thanks for all the advice here – it’s much appreciated, and will definitely be factored in to the decision making.

Before we put in the new joists, I just wanted to check something regarding potential damp. Near the bay window the earth of the sub-floor void is slightly darker and does feel as though it contains moisture. Photo attached.

This slightly damp area sits about 2 bricks below the airbricks, which are themselves at ground level outside. Is this therefore normal, or does this represent a potential damp problem that needs addressing?

 

[MikeG.]

Yes to all your questions.

Most of your answer lies outside. Firstly, and before you do anything else, reduce the level of the ground outside until it is 6" (150mm) below floor level inside, and make sure there is nothing growing in front of your airbricks (or that they're not obstructed in any other way). Also, check your gutters, downpipes, and other drains in the area. Downpipes need examining at their junction with the gutter, because a poor connection there can (and very often does) have the water from the roof running down the outside of the downpipe, and therefore not into the drain at all. Put a hose on your roof, and check that your downpipe is working properly. If it isn't, fix it. If your drains aren't working properly it is often just because they are blocked with rotten leaves and humus. If they are......sort them.

How many airbricks have you got, and how big are they? A photo or two will help.

Next, internally, dig all the soil away that you can which is built up against the inside of the outside wall. Ants, believe it or not, are a major cause of this problem (such soil is normally gorgeous, BTW......put it on your veggie patch!). Check your mortar with a pointing trowel? Is it soft and damp? If so, scrape as much out as you can, and repoint with whatever was used originally......be it lime mortar or cement. DON'T mix the two! Don't do this until we've decided on the airbrick situation, because you might be needing to knock another couple of bricks out to get some more air in under the floor.

Above all, don't worry. Your fixed-up floor will be separated from any damp, and most damp problems are easily sorted with the sort of access you have. And once you done all the necessary work, it might still take 6 months or a year for everything to get perfectly dry under there. No point looking in a few weeks and thinking that you've failed.

 

[J_Ashley]

Thanks for the speedy advice – thankfully none of that sounds too daunting to undertake.

We've already cleaned out the airbricks. Airbrick location wise, in the bay window itself we have three; the original central (below radiator), and two subsequent additions about 2 feet either side. There’s also one more airbrick on the front wall just outside the bay window area, although ventilation to that is probably compromised by it’s close proximity to the mains electrical connection. I'll pour a few buckets of water into all the guttering tomorrow to check that everything in draining is proper order.

One question:

reduce the level of the ground outside until it is 6" (150mm) below floor level inside

By “below floor level inside” do you mean below the height of the suspended wooden floor, or that the exterior should be lower than the earth base of the sub floor void? If it’s the former then we’re probably already good, but the latter means we’ll need to potentially excavate a good foot down.

It’s probably worth mentioning, we are unfortunate enough to have a cement plinth/render (circa 8” high) running at the base of the exterior wall on the front of the house, which I can’t imagine is doing any favours. I assume we want to gently chip this off, repoint in lime mortar and do our best to minimise any potential splashback to the wall?

 

[MikeG.]

I meant the timber suspended floor.

 

[MikeG.]

......It’s probably worth mentioning, we are unfortunate enough to have a cement plinth/render (circa 8” high) running at the base of the exterior wall on the front of the house, which I can’t imagine is doing any favours. I assume we want to gently chip this off, repoint in lime mortar and do our best to minimise any potential splashback to the wall?

That's a pity, but quite probably, something you can't fix. The problem with sand/ cement render is that it normally takes the face off the bricks that it's applied to, so if you take the render off you're likely to ruin your plinth.

 

[sammy.se]

By the way, is it worth adding noggins between the joists? does that add structural strength?

Especially if the joists are approx 2" x 4", and not, 2" x 6"?

 

[MikeG.]

Yes, it helps a lot. It makes the floor act as one. They're not noggins in a floor, BTW. It's called blocking.

 

[sammy.se]

Thanks. I'm currently flooring my Victorian loft, and was thinking of adding blocking to them while I'm up there... They are 2"x 4".

Sent from my Nexus 5X using Tapatalk

 

[MikeG.]

Ping a line up the middle of the span, and then stagger your blocking alternately left and then right of the line (so every second one is to the right of the line, and the others to the left). This way you can fix straight through the joists into the end of the blocking without having to skew nail.

 

[J_Ashley]

That's a pity, but quite probably, something you can't fix. The problem with sand/ cement render is that it normally takes the face off the bricks that it's applied to, so if you take the render off you're likely to ruin your plinth.

So you’d leave the cement render as it is? (photo attached).

Should it be removed, I had in mind that we might be left with a rather unsightly mess so was considering; essentially recreating the plinth but with lime mortar, or applying a breathable exterior paint.

 

[MikeG.]

Oooh, that's a nasty plastic airbrick!

If you've got a hidden bit of plinth around the side or back, you could try taking some off, and see what you see. It certainly isn't helping you. You might have a better chance than some, because those bricks look harder and less porous than a soft red.

Ideally, you'd not have hard paving up to the face of the plinth like that. A soft surface, usually pea shingle, stops the rain bouncing so far up the wall, and you can clearly see the effect of that in your photo. Even 9 inches width of shingle would be a big improvement.

 

[J_Ashley]

We have plans to redo the front area outside the house, and we’ll be inserting a run of pea shingle immediately before the wall as well as making sure everything drains away properly. We’ve got quite a lot going on presently, so we're prioritising trying to get the inside more comfortable first though (e.g floors!).

The plinth is only of the front, but on the other side of the bay window it is quite concealed from view behind a small storage unit so that’ll be an ideal area to test.

 

[J_Ashley]

We’ve bought some c16/c24 kiln-dried regularised treated joists and they have been delivered with a higher level of moisture content than I was expecting, some of them above 30%. Do you think these will be suitable to use for joists as they are, or should I source some more/seek to return them?

Looks like they have been stored outside at the lumber yard, which I would have thought slightly defeats the points of kiln drying.

 

[MikeG.]

It's not furniture! Frankly, they're better slightly damp, as they twist all over the place otherwise, generally.

 

[screwpainting]

Oooh, that's a nasty plastic airbrick!

If you've got a hidden bit of plinth around the side or back, you could try taking some off, and see what you see. It certainly isn't helping you. You might have a better chance than some, because those bricks look harder and less porous than a soft red.

Ideally, you'd not have hard paving up to the face of the plinth like that. A soft surface, usually pea shingle, stops the rain bouncing so far up the wall, and you can clearly see the effect of that in your photo. Even 9 inches width of shingle would be a big improvement.

As usual, spot on.
I would also hack that crazy paving up and dig down up to a foot if you can. I bet if you ran a water level or lazer datum from outside to the interior of that room and checked the levels they would be virtually the same. Do you know at what level any cavity starts? is it full of crud? I bet whoever put that wonderful air vent abortion in was sorting out the same problem and probably made it worse.
Its always depressing stress when you find the 'can' everyone else has been kicking down the road and fatal to assume everything has been done properly. You have at least got the interior clear and proper exposure work done and that in its self is no mean task.
You will be an expert by the time your done 8) best of luck with it.