Frequently Asked Questions

Applicable to all orders


The standard adopted in the UK glass industry, is to measure in millimetres (mm).  If you measure in Imperial use inches and either one eighth (1/8") and/or one quarter (1/4") of an inch.  Multiply inches × 25.4 to convert to millimetres.  Always give the WIDTH first × HEIGHT second, this is most important for patterned glasses and insulating glass units with square/diamond leads, Georgian Grills or Duplex Bars.  It also applies to clear glass that is being BS or EN badge marked in the corner, if the width is not first, the mark may be in a different corner.

Allowances for glazing

A glazing clearance is normally the tight opening size minus 6mm giving a clearance of 3mm all round.  For larger pieces a larger clearance may be necessary.  If you are not giving a finished glazing size, you must state this at the time of ordering, unqualified sizes will be taken as the glazing size.

Template and patterns

Use a hard material, hardboard is easy for you to work with.  We probe your template on our CNC machine; the more accurate your template the greater the accuracy of the finished article.  A template should be the actual size required and need no additions or subtractions to it.  Mark the face surface.  All templates should be made as viewed from the outside.  We do not accept paper or cardboard templates.


Plus 2mm minus 2mm.  Thick Float and thick constructions have larger tolerances.

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Types of Glass

What are the basic types?

  1. Annealed - Standard glass as made by the Float or Rolled plant.
  2. Horticultural - Annealed glass for use in greenhouses and sheds where optical quality is not important and compliance to BS6206 is not required.
  3. Toughened - Annealed glass, arrissed as standard, heat treated and breaks safely into small pieces. Cannot be cut or drilled after toughening.
  4. Heat Soaked - A further process after toughening to try and eliminate spontaneous breakage in toughened glass from Nickel Sulphide inclusions.  Normally used on thicker glass glazed at high level.
  5. Laminated - Two pieces of glass with an EVA, PVB or resin interlayer, breaks safely and normally is retained within the aperture.

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Standards and Regulations

How do I know if my order must comply with a standard or regulation?

The most common references are as shown below.  Please advise us at the time of enquiring or ordering if these are a requirement.

  1. Document L & its amendments October 2010 - Building Regulations - Conservation of fuel & power.  The key targets are as follows, however, there are variations and exceptions:
    • Replacement Windows - Whole Window U value of 1.6 W/m²K or a minimum WER of C.
    • Replacement Doors - Whole Door U value of 1.8 W/m²K or a minimum WER of C.
    • New Dwellings - Standard government software, SAP, provides the calculations to comply with the predicted rate of emissions from the dwelling (the Dwellings Emission Rate) not being greater than the Target Emissions Rate (based on a notional dwelling).  The SAP calculation will be made and the whole window U value calculated on receipt of which we can then provide the specification for the insulating glass unit make up.
  2. Document N1 & N2 - Building Regulations - Glazing + materials & protection.
  3. BS476 Part 22 - EN13501-2 - 1363 - 1364 - 1365 - 1634 - Fire.
  4. EN12150 - EN14449 - EN14179 - EN12600 - BS6206 - Safety.
  5. BS5544 - EN356 - EN13541 - Anti Bandit Glazing.
  6. BS6180 - Barriers & Balustrades.
  7. BS8000 - Part 7. Code of practice for glazing.
  8. FENSA (BFRC) - The simple method - In UpVC & Timber Frames (timber depth of frame to be a minimum of 70mm from front to back) an insulating glass unit constructed with a 4mm Softcoat Low E - 16mm warm edge spacer + 90% Argon - 4mm Float, with a centre pane U value of 1.2 W/m²K will comply and no further evidence to the consumer or FENSA inspector will be necessary.
  9. Certass (TRR) - Members of this scheme will have had their products or services evaluated to show compliance with Document L2010.

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How do I know where to use a safety glass, laminated to EN14449, toughened to EN12150 or clear wired to EN12600?

Very briefly and not covering all the requirements, Document N1 & N2 of the Building regulations define areas that are deemed to be critical locations and more prone to an accident.  In these areas, e.g., glass in a door, either side of a door or within 800mm of the floor, use an appropriate safety glass.  Where abnormal behaviour can be expected, e.g., in schools or hospitals, all necessary precautions must be taken to minimise any potential risk.

In accordance with British Standards the glass will be marked as required in one corner of each piece.

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Insulating Glass Units

What is U-value?

The rate of heat loss per square metre for a temperature difference of 1 degree Kelvin, or Celsius, between the interior and exterior.

What is the "greenhouse" effect?

Clear glass will allow heat from the sun to enter the building, contributing significantly to internal temperatures.

What standard are the units made to?

EN 1279 Parts 1 to 6

Is there a date from which all units must be made to EN 1279?

April 2007

How can I improve the U-value?

  By using either a hard or softcoat low-e glass with Argon Gas in the cavity if necessary.

Can heat be kept out?

Solar heat gain can be reduced by using a solar control glass.

Does the thickness of glass improve the insulation value?

Very little.

Is there a self cleaning glass?

There are clear and tinted self cleaning glasses produced by the glass manufacturers with a hard hydrophilic coating to help the removal of dirt when it rains or hosed down.  There are other applications available that are applied to the surface of the glass prior to installation, the life of which should be checked with the manufacturers.

How should my frames be constructed?

A frame needs the U-value or WER (the Architect, Specifer or Frame Maker will give you the U-value or WER for the frame); from this information we can give you the required makeup of the insulating glass unit.  Make clear if your U-value is a centre pane U-value for the glass alone or a combined frame & glass U-value.  The size of the rebate in timber frames especially, is critical, the height of the rebate must be at least 15mm & the width, thickness of the unit, plus 6mm for glazing materials, plus the width of the timber bead (normally a minimum of 12mm).

What cavity widths (1) are available and what colours (2)?

  1. 4mm, 6mm, 8mm, 10mm, 12mm, 14mm, 16mm, 18mm & 20mm.
  2. Warm Edge Spacer GREY or BLACK. Aluminium spacer SILVER, WHITE, BRONZE, BLACK & GOLD.  Some cavities and colours are only available on request.

If I do not specify a spacer colour, what will I get?

Default spacer colours are silver for aluminium & black for warm edge.>

Are there inserts available to go into the cavity?

Georgian Bar - Straight or curved in White or Brown, 25mm/18mm × 8mm.
Gold Victorian Grill - Please submit your enquiry.
Duplex Bars - The pitch, the dimension of the flat of the bar to be applied is required.  Duplex bars are only incorporated in units that are constructed toughened glass to minimise the risk of thermal fracture.

Can units be leaded?

A self adhesive lead strip is applied to the inner and outer surfaces of the outer leaf of the insulating glass unit.  In squares, diamonds and colour film designs, details with any necessary drawings and colour references are required.  Standard designs are available.

How should units be glazed?

Glazing should be in accordance with BS8000.  See our Arbo Heel Bead Glazing System page for the basic method; contact us for other types of glazing and glazing of self cleaning glass.

Are there special requirements for glazing glass in a roof?

  1. The outer leaf is normally toughened to resist thermal fracture.
  2. The inner leaf can be toughened or laminated depending on the application.  If laminated it must be stepped back to be within the building.
  3. If the bottom edge of the insulating glass unit is exposed, the edge should be encapsulated by a U-channel or the top leaf should be longer than the bottom leaf to run water clear of the seal.
  4. Both top and bottom leaves must be "stopped" in the rebate to prevent the glass sliding down.

Can I have a cat flap in an insulating glass unit?

Yes, buy a cat flap with a round hole for fitting into glass.

After glazing what maintenance is required?

Follow the maintenance instructions below for all types of installation:

- Flush Edge Units

Inspections should be carried out after about one year and periodically,thereafter, as deterioration could take place as a result of incorrect application or vandalism.

Damaged sealant should be cut out and replaced in the affected areas.  The Sealant Manufacturers' recommendations should be followed.

Where gaskets have been displaced or damaged, or where there are gaps, they should be re-fitted or cut back and capped with silicone sealant.

With vented glazing methods, checks should be made to ensure the drainage or ventilation holes or slots, have not become blocked with dirt.  While checking these, it is also advisable to examine the effectiveness of the seal of the exposed frame joints, which may determine the weather tightness of the rebates.

- Stepped Edge Units

Routine maintenance is similar to that required for single glazing with the putty fronting. i.e. ensure that painting is carried out regularly to provide protection for the putty.  The paint line to extend onto the glass inside and out.

- Micro Porous Frames and Stained Frames

Special care must be given to the inspection of treated and stained softwood frames.  Lack of routine maintenance causes the finish to flake off and joints in the frame to open up.  This may allow water to permeate the seal and cause failure of the unit.

Visual quality standard for installed insulating glass units constructed from flat transparent glass.

It is necessary to apply a viewing standard when it is deemed an insulating glass unit may have a fault.  The glass manufactures apply a standard to their glass for imperfections in the main body of the glass and any coatings they may apply to it during their manufacturing processes.  The quality of glass used in the manufacture of insulating glass units is the same as that for single glazing.  Working with the materials supplied to us to the relevant EN standards in turn dictates the quality of the insulating glass units supplied to you and has to be taken into account for the finished product.

The viewing standard will be as follows:  Annealed glass will be viewed at 2 metres.  Toughened, laminated or low e glasses will be viewed at 3 metres.  Other coated glasses will be viewed from 3 metres.  The viewing will be at 90 degrees, in natural daylight, imperfections may not be marked to highlight their presence and will only be visible to the naked eye.  A band 50mm wide around the perimeter of the insulating glass unit is excluded as an area to be viewed.  The insulating glass unit will be looked through, not at the surfaces.

Flat transparent glass, including laminated and toughened shall be deemed acceptable if the following phenomena are neither obtrusive nor bunched: totally enclosed seeds, bubbles or blisters; hairlines or blobs: fine scratches not more than 25mm long: minute embedded particles.

Due to the manufacturing process seeds and bubbles are deemed acceptable in patterned glass.

Duplex and Back to Back Bars

The method of construction compounds tolerances and allowance should be made for this when determining the width of the bar to be applied to the surface of the glass.  The width of the bar should be the pitch of the Duplex or Back to back bar plus 3mm.  Deviation from parallel along the bar lines is acceptable.

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What are the basic edge finishes to a piece of glass?

  1. Arrissed Edge - The sharp edges are machined, not a finished edge, suitable for glazing or handling purposes or to reduce the risk of thermal fracture in annealed glass.
  2. Smoothed Edge - The edge of the glass is machined flat and the edges arrissed but not polished.
  3. Polished Edge - The edge of the glass is machined flat, the edges arrissed and the edge is polished.
  4. Bevel - The face surface of the glass is ground way and polished to various widths and thickness at the edge.  Unless the thickness at the edge is specified, it will be to our normal setting.

What diameter hole can be put into a piece of glass?

Virtually any diameter of hole can be drilled or cut.  The internal finish to a hole is generally a smooth one.  In toughened glass there are limitations as to how close to the edge the holes can be positioned.

Can you polish or bevel shapes?

Shapes can be bevelled and polished. Internal radii must exceed the minimum radius for the type of machine the glass is being worked on.  Submit details of the shape required.

Can you cut a shape from a .dxf file emailed to you?

Yes.  We have CNC machines for glass cutting and processing.

Can stencils be cut for sandblasting designs onto glass?

Stencils can be cut from your drawing, after being entered into our graphics computer, a copy will need approving before being put in hand or from your computer generated file.

Can "cut outs" or "notches" for plug sockets or other items be incorporated in the glass?

Yes, normally in toughened glass to increase the strength of the finished piece so there are rules as to how close to the edge or corner they can be, submit a dimensioned drawing for approval.

What sort of glass can be used in a barrier or a balustrade?

BS6180 defines the type of glass depending on the type of framework and building it is used in.  Your Architect, Specifier or Building Control will specify the Uniformly Distributed Load (UDL) in kN/m² and/or the Point Load in kN.  On receipt the type of building, eg domestic or commercial and the loadings, we can make a recommendation as to the type of glass to be used.

Glass floors, is there a maximum area and special frame?

We have data for the appropriate thickness up to 1m².  Submit the UDL in kN/m² and Point Load in kN and advise if a domestic floor, dance floor, corridor, or stair tread, for our recommendations.

What sealants should be used (i.e. are compatible) with the Ethylene Vinyl Acetate (EVA) laminating interlayer films you use?

We use EVA films manufactured by Bridgestone for the bespoke laminations we undertake (floor panels, barriers and the like).

Bridgestone have produced compatibility charts (link opens in a new window) to show which sealants should be used with their EVA films.

We stock clear and diffused EVA films.

What is "Cracked Ice"?

An EVA or resin multi laminate where one of the glasses used is a toughened one which after laminating, is broken and gives a "crazed" appearance.  At the time of enquiring we need to know if the piece is four edge glazed, clear, or tinted, and any processing that may be necessary.

Mirrors, how do I determine the, thickness, edgework, number of holes and whether safety backing is required?

We have tables to help determine the thickness.  If the edge is exposed or tiled up to, the edge should be polished.  A mirror will normally hang on the screws put in first, so 4 or 6 holes in the right position with the correct fixings, will be sufficient.

What thickness should I use for a shelf?

Again, we have tables and can advise you.

Is there such a thing as, "One-way Mirror"?

Yes, 6.4mm Laminated One Way.  The glass has a grey body tint with a reflective surface coating.  The glass is installed with the surface coating facing the observed area.  The glass has low light transmittance and high reflectance.  When the glass is installed with a lighting ratio of 7:1, the dark side being the side where the observer is (the obeserver should also wear dark clothing), the reflection will look like a mirror to the observed.

What is the mirror with stripes on?

"Venetian Stripe Silvered" deceives the eye by reflecting light from the bands of silvered glass, but is obviously not a secret viewing mirror.

Safety or Foil Backing

A mirror can be safety backed with a white woven film to give impact resistance on one face to BS6206 Class Bo.  Glass with a white woven safety backing cannot be fixed with a mirror mastic or low modulus silicone.  The film is coated with a release agent to enable the roll to be unwound and consequently no manufacturer of safety backing has approved their film to be fixed with an adhesive and conversely no manufacturer of mastic or silicones has approved their products to be used with a safety backing.

A mirror can be backed with an aluminium foil; as such it does not have an impact resistance classification.  Foil protects the reflective surface of a mirror and as it has no release agent it can be fixed with mirror mastic or low modulus silicone; with this method the bottom edge of the glass must be supported to take the weight of the glass.

If desired a mirror can be fixed directly to the substrate with mirror mastic or low modulus silicone provided there is an ample covering to ensure, in the event of breakage, no large and dangerous pieces are able to break off and fall or cause injury.  Support along the bottom edge is essential.

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Fire Resistant Glazing

How do I know the type of fire resistant glass I need?

  1. Your Architect or Specifier will advise you, or Building Control will stipulate, what you need to comply with in the Building Regulations.
  2. Use our Fire Rated Glass Enquiry Form (link opens in a new window) to advise us of your specification and enable us to recommend a suitable glass to meet your requirements.

Can it be clear?

Wired was the traditional fire resistant glass but the latest fire resistant glasses are clear.

Are there different types for glazing internally & externally?

Wired is for internal or external use; specify if it is to comply with EN12600.  Most types of clear, however, have an internal and external grade.

Do I need special frames?

Yes, the frame must be constructed from the correct material to give the fire rating required.

Do I need special glazing materials and glazing methods?

Yes, Intumescent, compounds, strips or sealants are needed depending on the type of glass and the rating.  When you specify the glass required we will advise on the necessary materials and glazing method.

Can you provide test evidence if required?

Yes, we have manufacturers' reports available on request.

Pilkington fire glass test reports are also available on the manufacturer's website (link opens in a new window.

What are the delivery times for cut-sizes of fire glass?

Order intake cut-off time: 12 noon
Delivery times shown are for guidance only and are subject to order
Please contact us if you have specific delivery requirements.

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Cleaning Glass

Regular cleaning of glass is important to ensure that there is no discolouration and deterioration at the glass surface.  Indeed it may be necessary according to the atmospheric conditions prevailing in the area to determine the frequency for regular cleaning, particularly where infrequent cleaning could result in obstinate dirt having to be removed by methods and labour at greater cost.  Again the usual effect of dirt on glass is to reduce the light transmittance, which was probably one of the reasons for the glass being installed in the first place.

Generally, cleaning of glass as a routine operation is by the use of warm water with soap or mild liquid detergent, followed by rinsing with clean water.  Washleather or cloth is suitable to use for transparent glass but certain types of washleather or imitation washleather have been know to cause streaking if the glass has not been initially polished with a cloth.  For glass having a textured surface into which it may not be possible to clean off with a cloth, a stiff plastic or bristle brush is effective.  Obstinate dirt in such cases can usually be removed by using either whiting in water or methylated spirits.

If dirt, contamination, staining and the like are not overcome by these normal methods, then other means may be adopted but before choosing a particular form of treatment it is advisable to determine wherever possible the actual cause of the trouble.  If the cause is an on-going one, the first step should be to prevent further trouble at source.  The remedy perhaps could be in the sealing of concrete lintels or the provision of drip channels so that the rain will not run down the glass surface.  However, where obstinate staining is present, the answer may be in the use of a specialist cleaning and polishing process by experts.

Self cleaning glass must be cleaned as soon as possible after the building work is completed by rinsing with water to remove all traces of dust, abrasives etc.  Then either spray on or apply by a saturated cloth a cleaning solution (a mild detergent and water solution is recommended) onto the coated surface.  Gently rub the wetted coated surface with a clean, lint-free towel or cloth.  Rinse with water and wipe nearly dry with a dry, clean, lint-free towel or cloth.  The use of a squeegee on the coated surface is not recommended.  If it is absolutely necessary to use a squeegee then particular care must be taken to prevent any metal parts from contacting the coating or dirt particles becoming trapped under the blade and dragged across the coating.  If the water quality is particularly hard (i.e. greater than 180ppm combined content of calcium carbonate CaCO2 and magnesium carbonate MgCO3) then rinsing water should be softened through a domestic water softener or through the addition of a couple of drops of detergent (dishwashing detergent suffices) to litre of water.

Insulating glass units should be cleaned in accordance with the types of glass used in the construction of the unit, as described for clear and self cleaning glasses.  Body tinted glasses and surface coated glasses should be cleaned carefully to avoid surface scratching.  When cleaning excess water from the surface of any unit, no water or cleaning agents should be allowed to come into permanent contact with the edge seal due to poor perimeter seals or blocked draining holes in drained and ventilated frames, they may attack the edge seal of the unit.

Fire resistant glasses constructed in a laminated form must not have water in contact with the edge of the glass. The special materials used in the lamination of the glass will be affected by contact with water.

The information provided in this document provides general guidance as to best practice with regard to glass cleaning.

It does not, however, constitute any representation or warranty with respect to the products, their suitability for any application.

Under no circumstances must a metal scraper ever be used to remove excess paint from the surface of glass, it will result in surface scratching.