Help with existing suspended floor to new

1 Dec 2019
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United Kingdom
Hi i have an existing suspended timber floor to 2 rooms that are being opened up into a new extension (as below). The existing timbers are 4x2 sat on sleeper walls at each end and are decent but slightly twisted after 100 years. They have a 300mm ventilation void between the ground and the underside of the joist.


The extension is being constructed with a suspended timber floor which the SE has spec'd in 8x2's @ 400c. At the same levels this gives a 200mm ventilation void which I understand is in line with BC requirements.

I need to remove 40% of the joists to install a pad foundation for teh steel columns so is it best to bite the bullet and just remove the existing joists and sleeper walls and rebuild it all in 8x2's breaking the spans into 2 4m areas as below? If so are sleeper walls the best method or wall plates and hangers with a sleeper in the middle?


Thanks in advance for the help.
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I honestly think that if your original joists are twisted you'd be better off lifting them out and replacing the lot with new joists onto a properly constructed central sleeper wall.

If your walls are good enough and you have no dry rot I think the easiest approach might be a ledger (wall plate) bolted or resin anchored onto the masonry at each side with standard twist nailed joist hangers. These can be set out and nailed in place on the bench before passing them down into position. They need to be treated timber as they are in contact with walls near to ground level and/or external walls. You might also consider inserting a layer of DPM between the back of the ledger and the wall and you will need a layer of DPM between the top of the sleeperveall and the timbers. Where the joists meet in the middle you should nail the joist ends together (side to side) as well as blocking (solid strutting) between the joists. This will stiffen the floor.

Attempting to install zip hangers (i.e. the sort that fix directly into the mortar joints) into an existing old wall can be an exercise in frustration and futility as they will rarely line up.

Unless I am mistaken by "half and halfing" your joists over a sleeper wall you should be able to reduce the size (on a 4.2 metre span) to something like 7 x 2in C16s. Ask the SE

Longer joists tend to be more expensive and will potentislly require solid strutting to ensure a bounce free floor. They can also create problems getting them into the house
Thanks. So if i am reading your response correctly you are suggesting what i was thinking of doing;

# Remove all existing
# Build sleeper wall along the existing rear external wall of the house
# Install a wall plate to the inner wall of the house
# Install a wall plate to the new rear wall of the extension
# Install joists between wall plate in hangers and sleeper wall nailed to sleeper wall head plate
# DPM to all areas joining ground via masonry
# Brace between joists

The max span if constructed this way will be just over 4m so it should give a solid bounce free floor structure?

The rear wall of the extension will be constructed out of 7n concrete blocks so that will be fine to fix a wall plate to. How do i establish if the existing brickwork is sufficiently sound to fix the internal wall plate to?
This is the wall where the ledger board would be attached to carry the joists. From teh looks of it is there anything that would suggest it is not suitable to bolt the board to?

When fixing a ledger board do you dry pack between the board and the brickwork to take out any minor bumps etc?

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If i am going to install 8x2 joists does the ledger board also need to be 8x2 also?

Are these 2 products suitable for fixing to the brickwork and at what centers do you recommend?


Chem Fix
Its easier if ledger plate is the same height as the joists.
Fixings- easiest is use a few long screws (100 x 6 or so) and plugs to get the thing on the wall. Once you've got it set and level, fix with frame anchors or sleeve anchors (the type where you can drill the masonry through the timber), make sure you hit brick centre and avoid where your hangars will be. Shim (double glazing packers) at fixing points if needed.
If i am going to install 8x2 joists does the ledger board also need to be 8x2 also?
The ledger needs to be the same depth (8in) as the joists

Are these 2 products suitable for fixing to the brickwork and at what centers do you recommend?


Chem Fix
Yes, they are suitable. Resin anchors are ideal if the walls are of variable quality masonry. If the masonry is good you could use Rawlbolts (shield anchors) instead, which are cheaper. Centres should be 600 to 800mm. I normally wouldn't use screws (even 6.0mm) other than as a temp fix whilst you get the resin in and it goes off

The ledger should be treated timber, C16 or higher structural grade and you can save yourself time and effort by fixing your joist hangers to the face of it at 400mm centres with twist nails (never screws, every hole filled with a nail)

You could put the ledger below the level of the joists. This gives the advantage that it doesn't necessarily need to be level as the joists can be notched to level them, however that is extra work and in order to make your floor as stiff as one installed on hangers you'd need to fix solid strutting between the individual joists. It also positions your ledger lower down and nearer to the point where moisture could be taken up by the timber from the masonry
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So do you fix the hangers to the ledger before installing it?
Yes. Use a small offcut from the end of a joist to check the positioning of the hangers
If i am using M12 threaded rod with Chem resin fixings do i drill a 14mm hole or 16mm?
14mm. It's just very wasteful of resin to go larger. Make sure you clean out.the holes of all waste after drilling (vacuum cleaner?). Makes for a better bond.
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Thanks. 14mm it is. Should back the ledger board with dpm or is that not necessary.
If it's below ground level we always do it..A bit belt and braces, but for the sake if an extra 5 to 10 minutes and a bit of DPM...

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