Floorboards sagging, suspended floor repair/insulation Qs FAQ

A further point to add more information (which doesn't contradict your comments); I can tell that it is the *end* of the floorboards which have 'gone' - ie. the 'sag' starts abruptly, at a join.

I do slightly wonder if it is a T&G floor and they have left one end hanging in mid-air (I presume you should join over a joist) with only the tongues and groove to hold it. And then over the years these have deteriorated. Only examination will tell...

J^n

Very common here, normally in the wall.

Also seems common in the USA but I've never seen such access in the UK, possibly because the headroom beneath the joists in most UK houses with a suspended wood floor is so low.

How about buying yourself a boroscope - less than a tenner as a plug-in for a phone, on Ebay. You'd only need to drill a few 10mm holes in the floorboards to take a look underneath.

Under a ground floor, suspended floor, the pipes usually run below the joists - there is no need to go to the effort of running through them when the bottom of the joists are well clear of the ground. It's only upstairs, where there is a ceiling below the floor joists where you'd need to run through the joists.

Ours are too, but most of what you lot call bungalows on sloping ground do have one of those hatches. We don't have basements like the yanks do tho.

If you have a mirror handy, you should be able to get a glimpse! :-)

Just a basic get a board up, and a look under the floor really needs to be the first step - that will tell you loads very easily. The construction, whether the boards have a problem like rot, or it is the joists not supporting them, or the piers not supporting the joists etc.

Well that tells you the board is probably *not* the problem. Boards will normally join over a joist so that the cut ends can't flap about. So either there really was a dodgy builder (or a half arsed attempt to lift a floorboard in the past), or the joist is not doing what it should - probably because it is failing itself (e.g. rot) or it is not being supported. (settlement of a pier for example).

Yup, that would be possible. You even occasionally meet T&G boards where they glued them, and did not bother putting the cuts over joists. (the nail patterns and positions ought to tell you).

To ventilate the joist bases if you had no vapour barrier there. Heck, I have a suspended concrete block and beam floor and above that is waterproof styrene foam and a screed and they STILL insisted on underfloor ventilation. I suppose it stops water building up inside .

Since it was there, I punched holes in the blocks to fit air vents for the open fires...

That's good. You have absorbed all the info and made a reasoned choice. If you intend to carpet , you can use lots of construction techniques And use the opportunity to reroute pipes and cables or add underfloor heating etc.

Good luck with finding a competent anyone these days.

take the floor up.

You need to try and get some UNDER the joists to fully vapour proof them And reduce cold bridging

What's the chance that, as this was an extension done after the fact, that the joists they used were undersized for the span they had to cover? Was that floor always a bit springy? If so perhaps supporting the joists in the middle would be enough. We've got a section of floor here over a concrete slab with just that problem. Always been springy, and on the to-do list here is to get some supports under all the joists.

Definitely over a joist.

In my case in the US, the crawl space was accessible from outside the house. I ran some underfloor TV cables there. I also could see that the house was not bolted to its foundations, which omission I had remedied. Save my bacon in the '89 earthquake.

I actually have something like that already ... tried it when we had some (unrelated) drain problems a few years ago. I think I would be better off with a powerful torch and the mobile phone camera itself (having lifted a board), but it's worth a thought, thanks.

Isn't it required in some parts of the UK to prevent the build up of Radon gas?

You don't say if the house is on a slight slope, so the extension is several steps up where the front is level or vice versa. Most on the level type extensions on terraced properties round here are just concrete floored it seems. The fact that yours is boarded makes me wonder exactly how well the floors are constructed and in what way. I'd guess from your description that it might involve rotted Joists. A neighbour had this issue being the end terrace and water got underneath. Cost quite a lot even 20 years ago, but was done quite fast with it seems some concrete, some bricks and new good quality wood and mitigation of the water destination!

I think its going to need to all come up and a proper survey done of why its so different to the original, after all a lot of houses floors have stayed good for many more years than yours and they seem to have been built on brick piers just the same. Also there are usually air bricks below the floor and these should not be blocked. Brian

My home office/workshop/garage is now 3 years old, we're on clay, and it's concrete beam and block. I like the idea that there's an airgap between the floor and the soaking Somme-like clay we have around here. The foundations are chest deep!

Andy

My grandfather hid his whisky stash down there because Grandma liked a tipple (or two,three,...) during the day. He moved the grand piano over it hoping to make it extra secure but she had other ideas.