Suspended floor - best way to support joists on piers

[shanghaiman]

I've had to take up a 4m x 5m suspended floor in a Victorian building because of wet rot damage and would appreciate any advice on how to reinstate it.

A lack of air vents (I'm rectifying this btw) meant that capillary bound moisture in the two intermediate wall piers (2' wide, 3' high, built of stone using lime mortar) had slowly caused the supporting beams laid along them to develop wet rot where they were in contact with the pier, and so lose their structural integrity. (The actual joists then sat perpendicular on top of these beams, clear of the pier itself.)

My guess is that I should be using a row of engineering bricks set into mortar along the top of the wall piers, instead of a wooden beam. Would that work? Should I be using a layer of waterproof building paper between these bricks and the joists? Any insights very much appreciated. Terrible internet access, so it might be a few days before I can see replies. Thanks.

 

[vinn]

Its not quite clear to me what you've got so a couple of posted pics would help?
FWIW: if you go back & follow my posts (& others) on this kind of question you will see lots of detailed advice - research on here or the building forum.

 

[shanghaiman]

Fair point! Here's a pic of the floor after I'd started taking it up. The main joists are running left to right (obviously!) but the bits I'm interested in are the beams that these are sitting on, to the left of the shot (there are others out of shot). These are the beams that had been slowly rotting away, because they're simply sat on top of 3ft supporting walls, directly in contact with the stone/mortar. I assume there's a correct modern way to use these supporting walls, with some kind of waterproof barrier between them and the main joists.

I'll also have a look back at previous posts—cheers for the pointer—I think I failed to find anything in searches because I don't know exactly which terms to use for these beams and walls.

 

[vinn]

Thanks for the pic.
What you have is a centre (or are there more?) unventilated knee wall, with two sill beams sitting on the wall in direct contact with their bedding of sand & cement. Its an unusual arrangement to have two sill beams.

The knee wall joist ends are notched and /or showing signs of decay.
The joist tails going into pockets at the right hand also appear to show some decay and notching.

It might be best if you carefully lift the remaining floor boards - remove all nails as you work - dont leave proud nails in any material.
When all the joisting is exposed then all and any decay will be revealed. Then post further pics.

Typically, a length of DPC material should be laid under any sill plate. No engineering bricks required.
And any joist tail going into a masonry wall pocket should be capped in DPC material.

Best practice is for sleeper walls to be honeycombed for through ventilation from front elevation to rear elevation.
Suspended floors need air bricks every 2m at a min. any wall gable, front or rear wall should have air bricks.
When replacing the floor you will have an opportunity to add insulation.

Are there any issues of damp in the walls at damp course level?

 

[shanghaiman]

Thanks very much for this, Vinn—it answers a lot of q's I've had. I'll use new sill beams, then, and keep them out of contact with the knee walls with dpc material. Needless to say I'll be using suitably treated timber, and I'll be adding a natural blanket insulation to let the old structure breathe.

There are actually two knee walls (one out of shot to the left), and the only "ventilation" between the three voids is the gaps between the joists. There's only one air vent to the entire floor, which is why the sill beams had all developed wet rot to their undersides and the woodworm had attacked the joists and boards to the point where some were eaten through. The Victorians were normally good at that kind of basic stuff, and I'm surprised the other sections of the subfloor didn't have any vents. I'm having generous new holes drilled through to air the subfloor.

The twin beams in the pic were simply because the room was too long (I imagine) for the joists they had in the 1890s—the other knee wall has a single beam. All the joists rested on sill beams at their ends rather than being set into the walls, which saves using dpc to cap them.

The walls have no dpc, and are basically big hunks of stone with lime mortar. The earth floor and walls are dry—the only problem is that lack of ventilation over a century has gradually caused problems.

Thanks again!