Floor joist replacement on a Victorian House

[DeanSpoors]

Your opinions please.

We planned on removing some dated pine floorboards (30mm) in our Victorian property. We decided to go for 22mm solid oak unfinished boards and planned on originally taking up the boards, gluing a 8mm strip on top of the original joists to offset the plank thickness and going from there.

The floor construction is as follows, joists (5x3) are raised off the ground by dwarf walls at opposite ends of the room with a sleeper wall in the middle, on these walls are more joists laid down flat (3x5).

So I have a few couple of small problems;
* The dwarf wall joists are rotten, due to no damp proofing method being used.
* The joists are twisted, bowed and bent.

I've now removed all the joists in the room. I plan on, installing an 8mm bed of mortar along the dwarf walls (using some pre-cut wood shims along the length to ensure mortar does not get squeezed out before setting)((this is to make up the 8mm difference between pine and oak floorboards). Place a damp proof membrane on top of the mortar, place 3 of the good and square joists on top of the membrane effectively replacing the rotten joists, and then buy new joists to replace the originals.

My questions are;
The original joists are 5x3, but I'm finding it very difficult to find these, would there be an issue with 5x2 instead? (400mm centres) or any suggestions on a supplier who will deliver to Durham.

Will 50mm cut clasp nails be adequate in pairs at each joist? (trying to keep the original look) I'm concerned with 5x2 joists that I will have to hammer the nails in very close to the edge of the new floorboards and they may split, even with pre-drilling.

Thanks,
Dean

 

[Bonni]

When fitting timbers to span rooms, you follow the regs on which sized timbers span the maximum length for the centres to which you are going by.

https://www.rightsurvey.co.uk/floor-joist-span-tables-for-surveyors-floor-construction/

But those are minimum timber dimensions, you are more than welcome to supersede this scale by fitting larger timbers. To make a floor less bouncy, opt for a timber size than whats specified.

 

[Mr Chibs]

I had a similar situation in my lounge...

I ended up using 6 x 2 @ 400 c
Had dwarf walls on one side, and retrofitted joist hangers on the other walls.

I then used noggins to really firm the whole floor up.

The deeper the timber Section, the stronger it is.

Due to trying to get the floor heights all the same I have one room in 18mm solid oak and the rest is 23mm.

I used rock wool insulation in between joists, with a building paper layer on top, before nailing The oak down.

Been down a few years... All seems good.

 

[JobAndKnock]

Surprised that you can't get 5 x 3in timbers. A few months back we got to the end of a project where we used 3in thick C24 joists throughout, more than 5,000 of them in total in various depths from 125mm up to 200mm. I'd suggest that you look for a better timber merchant.

As to replacing 5 x 3 with 5 x 2, I don't think that is a good idea because the thinner stuff will be less able to carry the load - you'll just end up with a bouncy floor because strutting is no replacememt for joist strength.

To get to the same stiffness as 5 x 3 you would probably need to go to something like a 6 x 2 or 7 x 2in joist. Look for a C16 (or better still a C24) structural grade, and because there is evidence of rot, if you sre going into wall pockets I'd end wrap the joists (with roofing felt). I'd also opt for treated timbers as well. To accommodate the deeper timbers in the walls the pockets would need to be opened out a bit (with a breaker), and bricked back in once joisting is complete (this is to prevent the joists from twisting and moving)

Alternatively a treated timber ledger with joist hangers (pre fixed at 16in or 400mm centres) could be resin anchored to each of the outer walls, ideally with a layer of DPM between wall and ledger. In this instance the joists probably don't need to be treated timber - only the ledgers. Using ledgers is actually faster to do than re-pocketing, I find, despite the many twist nails you need to drive.

I wouldn't try to bed the middle joists onto DPM on a mortar bed. Mortar is likely to break up unless you have something on top of it such as a couple of courses of brick. Better to put in a DPM further down, then a couple of courses of brick before getting to the bearer layer which could be cut down concrete lintel or even cut dowwn paving slab (in low loading situations). you use kare as your bearer you won't need a DPM as slate is impervious.

Unless your joist span is more than about 15ft you shouldn't need mid span solid strutting between the joists, but it is advisable to carry through the joist tails which meet over the pygmy wall a little (so they overlap) and connect the joist ends together (bolt or nail). Solid strutting or blocking immediately over the pygmy wall is a good idea - just leave out the ones at each end between the last joists and the walls.

Your outside joists should be 50 to 70mm off the walls at each side and you should have a 5 to 10mm gap between the flooring and the masonry when new boards go in - this is partly to reduce floor creaks and possible water/rot transmission paths into the floor structure as well as providing expansion space. The skirting should cover this gap, but you could always add additional draught proofing by installing a compressed gap filler tape such as Compraband in the gap before fixing the skirting

 

[Bouy]

Strength wise a 125 x 50 would be 2/3 the strength of a 125 x 75 so to compensate for this your spacing's of the joists would need to be reduced by 1/3, if 600 down to 400. If you maintain the same spacing 155 x 50 would be the equivalent strength wise of what you have got at the moment but actual deflection would be slightly reduced. Your floor does not appear to have any air flow so treated joists are to be recommended. Anything between 2.5 and 4.5m should be strutted in the centre

 

[DeanSpoors]

Thanks for the knowledge guys! Could I use 6x3 joists and notch them out at each end and centre section to reduce them to the correct floor height?
As the timber ledgers are already on a mortar base around the fireplace, should I remove this and place a thin cement layer instead to give the ledgers a good flat base?

 

[foxhole]

Easier to adjust height top down once you have the joists in place .