How To Replace Rotten Floor Joists and Insulate A Suspended Wooden Floor in 1930s House

rotten floorOK, so in August last year I moved into a 1930s bungalow – it’s a lovely little home, with a just 2 bedrooms and a boxroom, a front room, dining room and small kitchen. I am extending it now, but after moving in I discovered some rot in the floor joists and had to replace my floor. So, here’s the story.

The bungalow had a damp, musty smell, which I put down to old carpets and wallpaper. I washed the carpets when I got there, but the mustiness in the bedroom was too much after a few months – I started waking in the night wheezing and coughing, so decided it was time for the carpets to go.

I emptied the room, pulled up the old carpet and underlay, and noticed that the floorboards along one wall were very bouncy – in fact, they just sunk underfoot. I removed a couple of floorboard sections, and discovered rotten joists, rotten wall plates, as well as rot on floorboards, skirting, and wall battens that had been used to put some plasterboard on the external wall. That, and the wallpaper was rotten too. At this point, I had never done any real DIY.

I called a couple of companies for a quote to fix, and total refurb of the room was quoted at about £4000 to strip out old plasterboard and replace a few boards, then add insulation to wall and underfloor, plaster skim and paint. To repeat through the house would end up costing around £25000, so that was totally out of the question. I’d have to do it myself!

Remove All Floorboards

So I started by ripping out the plasterboard and battens, and then taking up the floorboards and removing two full rotten joists and the ends of 3 wall plates. I also cleaned the subfloor – it was covered in about an inch of dust and dirt. I also cleared out the 2 air bricks (partially painted over on the outside and filled with decades of dirt on the inside), and cut out 2 new air bricks. Then I lagged the radiator pipes, installed new wall plates, 2 joists and bought some chipboard flooring to replace rotten boards.After replacing the joists I used timber preservation to help stop mould and rot developing – it will also protect against woodworm.

floorboards removed

I insulated the sub-floor with 25mm solid board insulation overlaid with 50mm of Knauf Earthwool, and used 50mm Kingspan solid board insulation on the main external wall, with 25mm insulation board on the front wall, and also insulated behind the skirting board at the front with 25mm board. I used 9.5mm plasterboard on the external walls, and made good with some filler. I was careful to seal all joints and gaps with insulation tape and expanding foam.

insulated under wooden suspended floor

Finally, I decorated the whole room and had a new carpet fitted. Total cost? I think about £500 – yes, £500 for wall insulation, new plasterboard, replacement joists, floorboards, floor insulation, decorating the ceiling, walls, skirting, window cill and door, and putting in a new carpet.

I did take me 4 months, and some parts were a lot harder than others – but I did have a broken hand when I started, and took a break over Christmas.

So far, no more problems. The room is now warm and cosy, just like a modern room. If money had permitted, I would have put more insulation in the floor, and had the wall skimmed, but I decided to just paint the plasterboard to save money. It will be covered with wardrobes and pictures soon anyway.

Pictures and full details on how to do it all to follow.

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  2 comments for “How To Replace Rotten Floor Joists and Insulate A Suspended Wooden Floor in 1930s House

  1. John Ryan
    November 11, 2019 at 2:16 pm

    Would love to see some more information and detail on the process if possible. I have a 1930s bungalow and an identical room where i need to do the same job.

    Will you be posting anything more on this job,as you mentioned at the end of the article?


  2. Jon
    November 12, 2019 at 8:09 pm

    Have done another 3 rooms since, and insulated walls too. If you have any questions, fire away! I do hope to write more, but seems to be swamped with DIY, work and life!

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