Repairs: floors and flooring

Problem 1: Collapsed suspended timber ground floor

At first the problem is hardly noticeable - a little springing in the floor perhaps; then a little more vibration; then the ornaments on the TV start to wobble as you walk past. Eventually the TV may topple over!

The most advanced case the author has seen involved a thick rug, which was the only thing that prevented one from wading ankle deep through the floorboards.

The usual cause is lack of sub-floor ventilation. Older properties, especially in England (the Scots were well ahead on this), generally had no over-site finish applied to avoid moisture evaporation under the floor (and condensation).

If you are lucky, the joists or floorboards will only have gone soft; possibly there may be some wet rot.

If the defect is as simple as that, repairs are usually straightforward, the minimum being to improve the sub-floor ventilation and replace the defective timbers.

However, it would be risky to assume that there are no other contributory factors. So an investigation is essential:

Check the gaps between the brick sleeper walls are clear. (The floor void may be used as a free skip by builders, especially on refurbishment projects. Is this a building that has been builder modernised?)
Check the joist ends. Has the damp-proof course (DPC) been compromised on the main walls externally (or internally)? Is there a render finish, cement plinth, a new patio or footpath? Has it bridged the DPC? If air vents have been blocked the DPC has probably been bridged too. Even if the DPC is not fully covered up, a reduced clearance can trigger surprising amounts of damp ingress as rain splashes off ground surface finishes.
Does the patio or path drain back towards the house? Even if the air vents are not bridged they may fill up if the patio floods and water is being dumped into the floor void.
Check for dry rot – even if in the early stages. This can be hard to detect if there is no full-blown outbreak. Fugenex™ detectors may prove particularly useful allies in monitoring any repairs.
Check the condition of water service pipes. Old lead water pipes can develop tiny pinhole leaks as they age. A small fountain of water may have been erupting under the floorboards for some time before a problem is noticed. If the joist ends are affected it might also be the neighbour’s water pipe which is leaking onto a party wall, and should be considered if the pipework proves sound in the affected property.
Check that any heating pipes are not leaking. This may involve running the system up if it is dormant during the summer months in order to make suitable checks.
Check if there have been any historic floods or water leaks (burst pipes, tanks in roofs etc.) which have been cured but without proper drying out elsewhere.

You may find (particularly in cottages) that the walls step in thickness – the ledge forming a bearing for the upper floor joists. For example, in Surrey you may find a 215mm (9 inch) ground floor wall and a tile-hung half brick thick first floor wall. Occasionally the first floor main walls are of timber construction with tile hanging. It would pay to check that the joist end bearings are suitably stiffened where they rest on the ledge with nogging pieces to reduce the risk of the timbers twisting under load, which could weaken the bearing position.

Once you have established that there are no hidden extra contributory factors to the problem, it should be safe to proceed with a repair.

Solution 1

If the timbers are affected and have softened or decayed but there is no rot, it should be safe to replace with like for like. A sensible extra precaution is to sleeve a layer of modern plastic DPC material on top of the sleeper wall and beneath any timber plates or joists.

Builders' merchants and DIY stores now sell small plastic wedges to help with levelling up new joists to old for a perfect repair. But it should be a simple matter for a builder to make up timber wedges on site, and opposed or 'French' wedges are a quick and traditional way of making even fairly large changes to level by tapping the wedges in or out.

If joist or plate ends have been affected by decay it is also sensible to wrap the new joists in some DPC to isolate them from any future damp risk.

Folding or 'French' wedges: it is easy to make quick changes to levels by tapping the wedges 'in' or 'out' against each others.

Cost sequence - Repairs to timber ground floor

Carefully remove existing skirting boards and set aside for re-use.
Assumed floor size/room size 5m by 5m.
Break out existing floorboard covering. Condition is assumed to be poor and reuse not feasible.
Carefully break out existing joists/wall plates assumed to be sat on sleeper wall construction.
Cut down all timber lengths and carefully bag up and cart away to minimise risk of dust or spore contamination, even if dry rot or other fungal defects not identified.
Clear out spaces in between existing honey-comb sleeper walls. Allow for 1m3 of cart-away debris.
Ensure all existing underfloor ventilator bricks are unobstructed and operating satisfactorily.
Lay PVC damp-proof course material across the top of all sleeper walls. Fix down new timber plates nominal dimensions 100mm by 50mm treated softwood assumed. Allow for levelling of plate using proprietary plastic folding wedges/slips.
Reinstate floor using 22mm floor grade chipboard. Allow for all appropriate cutting in.
Re-fix skirting boards, caulk all joints at mitres. Bring forward, 2 undercoat and 1 top white gloss assumed.

Repair to timber ground floor - cost £1,700 without floor insulation: £2,300 with floor insulation.

Small works pricing

Since the original price estimating in 2008 the economic turmoil of the last few years has caused some disparities in pricing - if you are reviewing these figures in the London area they may look laughable - if you are located in Hull they might still look a bit expensive. Please treat them with caution and of course always consider obtaining more than one quote against which to test prices. Reference to the BCIS or a small works pricing book such as Spons or Laxton's will help assess costs and the hourly labour rates upon which they are based.

Solution 2

One of the most straightforward repair strategies is to bolt a steel angle section to the wall, but give added consideration as to:

how to protect the joist ends from damp walls; and
how to conceal the steel angle, if appropriate.

Note: Ensure rubble from under the floor is cleared out, bagged carefully and disposed of off site. This will eliminate the slightest risk of spreading any dry rot spores (or any other rot, for that matter).

Solution 3

Although they cannot prevent a sagging floor from continuing to sag, noggings are extremely functional, and the cross-bracing and stiffening effect may be very beneficial in helping to arrest any further movement (perhaps in conjunction with a scheme to double up the joists to spread existing loads).

Floor joist repair (decayed old joist cut back):

'plated up' floor joist;
consider counter-batten to drop new ceiling line OR notch plated joist to set it to original joist line (notched over ‘shelf ’) and re-make ceiling to original level;
plastic DPC roll to isolate steel and timber from damp solid wall;
steel angle shelf fixed by resin-socketed stud bolts or chemical anchor.

Cost sequence - Inserting noggings

Assumed floor 4m by 4m suspended timber joist.
Carefully lift all floorboard coverings and set aside for reuse.
Cut timber block nogging pieces slightly short of the joist depth so that these do not interfere with the existing ceiling fabric.
Insert 2 no. nogging pieces between each joist centre at approximately 1m from the end bearing of each joist. Allow for screw fixing only, with skewed in screws to prevent vibration damage to plaster and ceiling surfaces below. Ensure service runs are not interrupted.
Reinstate floorboard covering with all boards screwed back into place.
Note: No allowance has been made for reinstatement of damaged chipboard coverings where this finish is in place.
Allow for localised making good to flush off proud edges to floorboards leaving in clean sound and splinter free condition. Use of butt edge floorboards (not tongue and grove boards) is assumed.

Insert noggings - Cost £340