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THE TOP TEN

Independent glazing consultant Philip Rougier examines the 10 defects most commonly found in window installation inspections.

November 1994, Glass and Glazing Products, "Tech Talk" piece.
Copyright © Philip Rougier 1994

In the same way the Queen thinks all factories smell of new paint I only ever see bad glazing installations - probably because nobody pays me to look at the good ones. Dr Bill Horton's recent description of windows as "hi-tech products in low-tech holes" (GGP September 1994, p.74, Tech Talk) can often be appended "installed with no-tech methods". The millions spent by systems companies in perfecting solutions are so often wasted by a combination of ignorance and callous indifference later on, when their products are specified and installed. A lot of heartache could be avoided.

Here are 10 examples:

1 - BLOCKS Double glazed sealed units will break down if they have no setting blocks beneath them, because the bottom edge of the seal is in regular contact with water The water comes from penetration (no matter how good or complex the gasket) or condensation (which is inevitable). Moisture finds its way into the glazing void through capillary paths between the glass and sealant. The spacer desiccant can cope with some of it, but soon becomes overwhelmed. It is estimated that 90 per cent of sealed unit failures are caused by this inadequate installation practice, and not by sealant deficiencies.

2 - DRAINAGE Frame rebate platforms often have insufficient, or no drainage arrangements to carry water away from the housing. In a properly designed frame this requires no more than the addition of a slot or two, sometimes done at the fabricator's works, sometimes on site. If done on site the fitter should understand the profile sufficiently to avoid penetrating the reinforcement chamber. The drainage system should actually work - a cup of water is all that is necessary to test - and any necessary alterations made on the spot. Drainage pathways also provide ventilation to the rebates allowing evaporation. The moisture content of the air outside a building is usually much lower than that inside.

3 - GLASS TYPE AND THICKNESS Safety glazing recommendations are regularly ignored or circumvented. It used to be common practice for a customer to sign a waiver and have annealed glass installed to save money. If tested in Court, the installer could remain liable unless it can be demonstrated that the customer had been fully informed of all the risks rather than just the money to be saved. On legal advice in 1992 the GGF issued a warning to the industry that the Courts would be likely to apply Document N(new build) standards in every situation, even on repair work. The HSE "retrofit" requirements for risk-area glazing in workplaces to be made safe by January 1st 1996 will have far-reaching effects. As well as choosing the right type, glass must also be of the right thickness to resist wind-loadings. Even inside a building, the wind-loading requirement is deemed to be 600Pa (Newtons per square metre) and needs calculation. Computer software is available to determine the type and thickness of glass.

4 - PUTTY Despite all protestations, sealed units are still being solid-bedded in putty, sometimes even with putty fronting. In a few isolated instances this can work, provided exactly the right modern products and techniques are used. But the vast majority of solid-bedded sealed-unit installations fail, usually from one or more of a range of drainage and chemical deficiencies. Linseed oil putty should never be used near sealed units as its oil can react chemically with the sealants. Beaded solutions, used in combination with spacer (setting and location) blocks and a drained/ventilated rebate, are much easier to implement successfully and far less prone to failure.

5 - TOEING AND HEELING Opening casements are often discovered to be out of square. Tapered spacer margins give the game away, and in extreme cases the casements bind on closure. Even if glazed before arrival on site the fitter should check and adjust casement alignment and clearances. Toeing and heeling is performed by installing and adjusting location and setting blocks, adding further thicknesses when necessary and bracing the frame off the glass unit until it is truly rectangular. The procedure requires skill and patience - the geometry is determined by halving the difference between diagonals, this dimension giving (approximately) the required correction. Blocks should be inserted at appropriate points all round the frame, not just top and bottom, and the weight of the glass must be directed into the back corner.

6 - POSTS AND POLES A common mistake is to remove old (usually timber)frames whose mullions have acted as supports for the walling above, and replace them with less substantial units. This can have especially serious consequences in bay configurations, leading to complex and dangerous structural failures. Unless the new windows can be specifically calculated and designed to deal with building loads (and this would be most unusual) it is essential to first install additional steel, aluminium or timber posts or poles, and then insert the frames between them.

7 - INSETS AND PROJECTIONS Window and door frames are sometimes installed to project from walling. This can be for"architectural" reasons, but it makes them very difficult to weatherproof and leaks can develop. Frames should be inset by a minimum of l0mm into openings. Cills, on the other hand, should project by 50mm, to achieve a "drip" clear of the walling. In plastics windows many proprietary cill sections are available, often from the principal system supplier, and stories that there is "nothing suitable" are just, well, stories.

8 - LINTELS Lintel failures, or absences, account for many glazing problems. Deficiencies are often not obvious until a few weeks after installation when doors or casements bind, or head rails deflect. Ideally these problems should be identified by the pre-contract measurement surveyor, and a remedial quote given. If not seen then, they will certainly be noticed by the fitters on site (who must fix their frames to them) and should be brought to the attention of their customer at that stage. Too often they are ignored.

9 - REINFORCEMENT Windows are often fabricated without enough reinforcement to resist wind-loading, allowing the frames to deflect more than the L/175 permissible for double glazing (L/125 for single). In domestic situations outer frames seldom require reinforcement if they are connected to other structure, but mullions and transoms almost always need it. Whilst some firms offer aluminium reinforcement with the sales pitch that it does not corrode as steel does, three times as much inertia value is required since its E-value (Young's modulus) is only one third that of steel. Reinforcement requirements can be determined by subjecting specimen frames to test and interpolating the results, or by calculation for which computer software is available.

10 - FIXINGS Frames are frequently fixed to the structure with insufficient fixing points or using inappropriate components. Generally a window frame should be fixed at 150-250mm from each corner and at 600mm intermediate intervals. Through-fixings should be in expanding masonry bolts with a corrosion-inhibiting coating, penetrating at least 25mm. Lug-fixings, which should only be used when circumstances prevent through-fixing, should be in properly designed proprietary components, not just any old angle brackets or drilled steel strip. Jambs, bottom and head rails should all be fixed. If there is difficulty in fixing head rails to lintels (eg solid concrete) a high-density low-expansion fixing foam may be used, but this material is not commonly available. It should not be confused with low-density high-expansion foams obtainable from DIY shops.

Each of these problems has a remedy, but the truth is that none need occur in the first place. It is sad that a whole section of the industry is devoted to diagnosing, remedying and litigating such matters where in many cases the remedial and legal costs overtake the whole original contract value. And the many excellent minds this activity engages could be better employed. A real solution awaits the development of a system of technical education, product-specific training and quality site supervision, best led by certification initiatives from the industry's financiers and insurers.

November 1994, Glass and Glazing Products, "Tech Talk" piece.
Copyright © Philip Rougier 1994