How Much Does It Cost To Fit/Repair/Replace Solid and Suspended Sub Floors?



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Cost to Fit/Replace/Repair Sub Floors


job
Description
labour
1
Your living room floor is springy in the bay window. You have cleared the room of small stuff, the builder will move necessary furniture, roll back the carpet then put it all back again. This will take 2 men 1 day including materials.
£335
2
The whole living room floor structure is rotten and needs replacing but most of the floor boards are ok. There’s a bay window and a chimney breast. This will take 2 men 3 days.
£850

Plus materials etc. for the above
 £135
3
Your bedroom floor creaks horribly. Below are all those quotes you made him provide, all of which will require the following procedure. So this price must be added to the 4 below. Removal and eventual replacement of bedroom furniture, carpets and all floorboards.This will take 2 men 2.5 day s
 £700
3a
 Fit 6 joists between existing ones and completely “nog out”. This will take 2 men 1.5 days
 £450

Plus materials etc. for the above
 £175
3b
 “Nog out” between existing correct joists (inc. materials) 2 men half a day
 £190
3c
 Clear the room downstairs also and put it all back afterwards. Completely remove the bedroom floor joists (and ceiling below). Fit a new floor and plastered ceiling below. This will take 2 men 4.5 days.
 £1275

Plus materials etc. for the above
 £350
 3d
Clear the room below and put it all back again on completion. Remove, replace and plaster the ceiling.   This will take 2 men 3 days.
 £850

Plus materials etc. for the above
 £100
 4
Let’s have a change. Remove a solid floor (and all fittings) in the 12’ x 10’ kitchen and replace it with a new, properly laid, insulated and screeded floor ready for tiling. Jack hammer up the floor, dig down, fill the skip, lay the old floor as hardcore, then construct a complete concrete floor as described below. This will take 2 men 5 days.
 £1350

Plus materials, skip, building inspector fees etc. for the above
 £800

Information Sheet on Floor Construction

Solid Floors

If the house is Victorian or older, any concrete ground floor slab will most certainly have no means of preventing rising damp. This is because it would have been laid straight onto the ground, usually in the kitchen/scullery. This was because of the ubiquitous fire which would always be burning and anyway, only the servants ever went into the kitchen and clogs were waterproof!

Even in the fifties, a kitchen’s concrete floor only had a damp proof membrane by default so to speak, because bitumen or pitch was used on top of the floor slab to stick down the thermoplastic floor tiles. I bet if you live in a 50s house they’re still there festering under the carpet or whatever. They’re a *** to get off!

It will surprise most people to discover that DPMs (damp proof membranes) did not become a legal requirement under solid floors until 1967. 
1967… cor blimey, I was playing at being a hippy then, ahh…’Penny Lane’, flower power and free love. It was free then too but I’ve been paying for it ever since!

Still….I digress.

Solid ground floors today are excellent in comparison. A typical floor can go something like this…..First there’s 150mm of hardcore, then 50mm of sand, then a polythene sheet which is the DPM, then 100mm of concrete, then 75mm of insulation, then a 60mm screed which is the smooth surface the carpet or whatever sits on. That’s a composite floor over 400mm (16inches) thick. That, my boy, is progress! If the floor has underfloor heating, this is incorporated into the screed.

Suspended Ground Floors

These are almost always timber but composite concrete floors are also being laid nowadays.

Let’s look at the way a suspended timber floor is made, starting from the lowest ground level (beneath the floor).

This will have been dug out from whatever earth the house stood upon and may remain just as it looked at that time. It is sometimes call the oversite or sub-floor and will be a void around 600mm deep

Next there will be a number of short walls made of brick or blockwork. They will sit on foundations beneath the ground level of the oversite and around 300mm will be visible. These are known as sleeper walls and they are designed not only to take the weight of whatever stands on top of them, but also to keep it away from the damp and also to aid air circulation. These walls are also built with occasional holes in them, which further allow air to circulate. Sleeper walls are built at approximately 1800mm intervals, with the ones at either side being close to the external walls of the building.

On top of the sleeper walls will be a damp proof course (DPC) which may be made of a variety of materials including slate and plastic and will depend on the age of the house.

Next, laid directly on top of the sleeper wall and damp proof course (DPC) is a 100x50mm (4x2inches) timber board known as a wall plate whose job it is to support the joists.
Joists have the same dimensions as wall plates, but they are laid on edge and run at right angles to the wall plates and about 400mm apart.

Finally come the floorboards or floor sheets.

Ventilation

The whole of the ground floor should be constructed to allow air to travel freely around under the timber floor. This ventilation is vital to prevent mould growth and to dry out the sub floor brickwork which is all below the general damp proof course level.

The air enters via the air bricks which should be sited every 1500mm (5 feet) or so all round the building. These should never be blocked, or worse still, built over. Have a good look at your air bricks every couple of years or so. Most are brick sized but replacing these with bigger ones when other work is going on, will be a few bob well spent. It’s not worth getting builders in specially though.

If you are having a (solid floor) extension, or particularly a conservatory built against a wall with air bricks in it, make sure the builders fit adequate tubing under the new floor to extend the openings to the new outside wall, or 50% of your ventilation may be lost. The building inspector should see this is done in the case of an extension but most conservatories don’t need to be overseen by inspectors.

If you have a proper fire in your living room and the house has suspended floors, it’s worth getting a grille put in the floor close to the fireplace. This is particularly relevant if you have double glazing or weatherproofed windows. The fire needs oxygen and it will pull air from under the floor rather than creating a draught under the living room door. Ever wondered why all the edges of your auntie’s carpets were blackened where they met the skirting boards? It was caused by dirty air being sucked up from the sub floor by that coal fire she always had.

Problems with Ground Level Floors
i) It is solid concrete and was built in the 1950s and is rising slowly in the middle of the room. You have sulphate attack. This is a chemical problem brought on because the hardcore under the slab might have been old power station waste and it is swelling. You need a new floor I’m afraid BUT look on the bright side, get underfloor heating put in and bring those fuel bills down.

ii) The house is built on clay and there have been a couple of dry summers and there may be a large tree nearby. This combination will have removed thousands of gallons of water from the sub soil. When it rains for the next six months, it all flows back again, the ground heaves and the floor cracks. Same result as the previous paragraph I’m afraid!

iii) You have a suspended wooden floor that’s springy or downright loose. Unlike bedroom floors, ground floors don’t tend to fail due to too few or incorrectly sized timbers. They usually fail in corners or on either side of a chimney breast or right by the front door. Normally it’s caused by a failure of the DPC and damp has set in. Do not ignore this. Wet rot is one thing but if the conditions change or already have done, then dry rot can grow.

You don’t really know what dry rot is but you DO know you don’t want it,…. don’t you?

You, or your builder can take up the floor, remove the rotten wood, have a good look for dry rot, sort out the reason why the timber is wet, fit new timbers and rest easy.

One reason for wet wood other than a failed DPC is the transference of water from the exterior walls to the adjacent wall plates. Ordinarily these don’t touch each other but in old houses, the plaster behind the skirtings can deteriorate. It comes off the wall, drops under the floor, rests against the wall plate on the handy ledge created by the slate DPC and allows the water that’s in the wall to pass across to the timber plate, which rots happily away. It takes 20 years but if you don’t sort it, your tele will eventually slide into a hole just like in Poltergeist. Get it fixed. Now!

Insulation
Your suspended timber floor will not be insulated. Ten years ago it wasn’t necessary as most of the heat loss from buildings is upwards (through the roof) but with today’s spiralling fuel costs it might just have become economical, particularly if you combine it with allied flooring work. If you are going to do it, all the boards must come up and I suggest you use 75mm thick polyurethane insulation sheets cut to fit snugly between the joists. (Kingspan is one company which produces these but there are many others). A few nails belted in at the bottom of the joists will keep the stuff in place and as most pipework and cables will have been laid in notches cut out of the joist tops, it all goes underneath quite happily. If any pipework is to remain underneath the insulation, make sure this is insulated as well though.

As insulation is usually placed at the perimeter of buildings to stop heat escaping outside, there’s little point in insulating the bedroom floors as they are all part of the interior. However if you live in a flat, let the people below heat your living space but insulate your ceilings to prevent that noisy lot upstairs benefiting!

If you have solid floors then you can only insulate them if you have enough ceiling height and maybe put laminate flooring on top of the sheets afterwards but if you only put say 25mm of insulation down is it worth doing? If you do put 75/100mm down, be prepared to trip up as you come through the front door.

Suspended Bedroom Floors
These will almost always be timber but concrete is possible. If yours are concrete then it’s very unlikely that you will ever experience any problems. However timber floors can fail and it’s usually expensive to fix.

If your bedroom floor just creaks a bit get the boards screwed properly down and that should stop it.

If it’s a bit springy then stop jumping up and down on it!

If it’s very springy and moves all the time, back gingerly out of the room, close the door gently and call the builder in.

Problems with Suspended Timber Bedroom Floors
There is only one way to fix a moving floor, and that’s to firstly find out why it’s moving. It may be that it was never designed to be a floor. Are you in what was once the attic? Has the conversion not created a proper floor and just left you walking on what is actually a ceiling?

Bedroom ceilings are made from 100x50mm (4x 2inch) joists which can span about 3.6m (12’) and are only designed to keep a sheet of plaster board in the air. Bedroom floors are made from 180×50 mm (7 x 2inch) joists which can span the same distance but keep beds, wardrobes and the odd piano up in the air.

If a 180x50mm floor is springy it’s because there is, or has been, too much weight put on it, or it’s been badly designed. For example, the joists are spaced too far apart, the end bearings are inadequate (they need to be at least 100mm (4”), the span is too great for the size of joist used. It may also be the case that a succession of plumbers, electricians and odd job cowboys have cut far too many notches, far too deeply into the timber joists, all in the centre of the room, not around the edges where they do far less damage.

There may also be galloping woodworm, dry rot (unlikely on the first floor) or rotting of the joist ends due to a leaking bathroom (unlikely again, as you would have seen the water on the wall or ceiling below).

The solution is to add more joists. These really have to be the same depth as the originals otherwise either the floor level or the ceiling level in the room below will have to change. To do the job, either the floor boards or the ceiling below have to go and new timbers are bolted alongside the existing ones. You might as well do them all, you’ve gone to all this trouble so make sure you solve the problem first time. You may even decide to get wider ones put in i.e. 180x75mm (7x 3inch).

Questions to ask the Builder during his Quotation Visit

Why is my wooden bedroom floor springy and do I need to strengthen it?

You might just be making a mountain out of the proverbial. Get his opinion.

He might tell you that it’s nothing untoward. If this is the case, give him a drink as he’s just saved you a serious amount of aggravation.

If he says it does seem serious (lots of cracks on the ceiling below for instance) he won’t know whythere’s a problem until he does a fair bit of exploration. At this point you are going to need to either clear the room below and loose the ceiling or clear the bedroom and have the floor up.

If the joists are too shallow, putting more in won’t cure the problem, you need a new deeper floor (that’s the ceiling below definitely gone then).

If the joists are too few, then one can be added in between each existing pair. Do each space between, don’t skimp!

There may be no noggins.
No this isn’t a word I just made up, a noggin is a timber which is the same cross section as the joist, it is fitted between 2 joists at right angles to them and it’s job is to brace the 2 joists to stop them moving sideways and /or “flexing”. Noggins aren’t fitted in isolation. That would be pointless. With 12 foot long joists there should be 2 lines of noggins about 4 feet apart crossing the room.

The most vital ones fit very snugly between the two outermost joists and the walls. These finish the job off. The joists can now neither move sideways, nor flex (wobble).

Any competent builder can do the remedial work described above and they tend to be cheaper than specialist companies but if you suspect dry rot, get a specialist company in to eradicate it. There’s loads of free information on dry and wet rot on the internet, that’s why we haven’t attempted to describe identification and eradication processes here….. just Google away!

What do we do now then?

He should go away and prepare several quotes.

One to clear the bedroom, take up the floor and see what’s wrong then replace everything when he’s done the job.

One to simply nog out because there aren’t any existing ones.

One to fit new joists between each pair of existing ones and then nogg out.

One to remove the downstairs ceiling and the too shallow existing joists and fit a completely new floor and ceiling.

One to fit a new ceiling in the room below when the old one just decides to fall in the middle of the job.

On top of all this ask:

How much do you charge per man, per hour/day for any extra work?
This sort of job can often extend due to unforeseen circumstances. If he has to do extra work you can then monitor his extended bill relevant to man hours spent on site.

Why is my wooden ground floor springy and loose?
He will know the answer to this based on where the problem is. If it’s against a wall its due to rot. If it’s in the middle, it’s either rot again or there’s a sleeper wall missing.

He can’t quote for either because he doesn’t know the extent of the rot or how a high a wall he may have to build. You will have to trust him but make sure you know what his hourly/daily charges are per man, before he starts.

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