9 Jun 2009

On Plastering Options

Plastering is the cheapest way of providing good internal wall and ceiling coverings. There are different systems of ‘plastering’ but they all come within spitting distance of £12-£15 per sq m in price. There are alternatives which can be used when you know exactly what you want – exposed brickwork, timber linings – but they are considerably more expensive than a plastered finish and are normally only built as features.

The big question facing housebuilders is whether to go for a wet or dry system of wall coverings. The wet techniques use wet-mixed cement renders and gypsum plasters: the dry systems use dry-lined plasterboards. The wet techniques are traditional British building – the dry techniques are imported from countries where timber frame is prevalent. Ceilings are almost invariably fixed with plasterboard, but here there remains a choice about whether to cover them with a wet Thistle Finish plaster, to dry-line or to comb on Artex. Pricewise, there is very little to choose between the systems – though I estimate dry-lining is a little cheaper.

Wet plastering
Plus - it is well understood by builders and favoured by most plasterers; a well-skimmed plaster finish looks fantastic – at least initially.

Minus - it’s wet. Something like one cubic metre of water (equals 12 bathfulls) is being built into the fabric of the house if it is wet plastered and this must in time dry out, which will take a summer at least. This drying out results in movement which causes cracking in the top coat plaster which looks naff and gets builders called back on site to carry out cosmetic repairs. This problem is particularly bad when plasterboard ceilings are skimmed with a plaster finish; here the movement in timber behind the boards causes hairline cracks around all the plasterboard joints. None of this cracking is in the least bit dangerous – it doesn’t mean subsidence is occurring – and many people live happily with it knowing that these bedding-in problems can be filled in at the first redecoration. However, for many unsuspecting souls it is a source of genuine grievance and complaint.

Plus - it’s dry – avoiding problems outlined above. It is relatively easy to correct out-of-plumb blockwork – you just adjust the thickness of the adhesive dabs. It also gives a comparatively soft wall with enough give for small children to bounce off unharmed, whereas a hard, plastered wall would bring forth tears.

Minus - dry-lining is not particularly difficult to learn – the plasterboard manufacturers all run cheap two- or three-day training courses – but it can be badly applied, leaving a ridged effect on walls and ceilings. Plasterboard has to be fixed more carefully than is normal trade practice so as to keep the number of cuts to a minimum. The wall finish is similar to what you would get if painting on to lining paper (which is basically what you are doing) and this may not be glossy enough for some tastes. Plasterboard walls are not as damage-resistant as traditional plasters, though repairs can be easily effected.

Another problem with dry-lining, at least when it’s applied on plaster dabs onto blockwork walls, is that it tends to perform poorly on the airtightness front, now a factor which has to be addresses because air-tightness testing forms part of the building regs. In theory the backing walls should be airtight – why do you have to put in all those expensive trickle vents in the windows? – but in practice air sneaks through gaps in the mortar and around joist ends. One solution to this problem is to seal all the joints between sheets and around openings prior to taping and jointing. However this is both expensive (Gyproc’s sealer costs around £10 per litre) and time consuming. This air leakiness problem occurs with all forms of construction that use dry-lining, but its significance is greatly reduced when you build in timber-frame, incorporating a vapour barrier in the external walls.

Blockwork v studwork
You can only apply wet render on to a masonry background and it is, therefore, not an option for timber framers. Those using studwork walls will have to fit a wallboard, usually plasterboard – though, as already noted, plasterboard will take a 3mm wet plaster finish. On the other hand, if a dry method is desired in a brick and block house, then the favoured method is to stick plasterboard on to the blockwork using the dot and dab technique which uses specialised gypsum plasters as adhesives. This is the method currently in favour with over 70 per cent of professional house builders – just goes to show how much they value not being called back because ‘there’s cracks in me walls.’

What is it? Gypsum plaster sandwiched between two layers of paper. It is characterised by being easy to cut, fairly easy to handle and it provides a good backing for paint and plaster. Note that wastage can be high when using plasterboard – up to 30 per cent on small rooms and ceilings, 10-15 per cent on walls. It is available in several different formats: square edged (for wet plaster skimming) or tapered edge (for dry-lining); 12.5mm thick for 600mm spaced studwork and 9.5mm thick for 400mm spacings; foil-backed for providing an integral vapour barrier (it’s cheaper to use a separate polythene sheet); small boards measuring 1800x900mm as well as the more normal, room-height, boards which are 1200x2400mm. Since the sound regs were beefed up in 2004, there is a big take up of heavier boards to get studwork partitions up to the required 40dB sound reduction rating. Sound deadening boards at 12.5mm are what to look out for. None of these formats is expensive: rates vary between £1.50 per sq m and £2.50 per sq m depending on what you want the board to do.

There are also a number of plasterboards which are laminated to insulation. Make sure you get the right format for the job and make sure that you’ve used metric spacings on your wall studs and ceiling joists as imperial-sized plasterboard is no longer made.

Plasterboard is a very competitive business with three companies (BPB aka British Gypsum, and now owned by St. Gobain, Knauf and Lafarge) slugging it out for the European market. Consequently, the price hasn’t really changed much in 20 years – amazing value if you think about it. There is little to choose between the rivals either on price or quality.

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