Installation Is the Product: What to Write Into the Contract Before the First Fixing

Article

October 26, 2026

The U-value is the number on the datasheet, and it is the number buyers compare. It is also the number that changes least in service.

Installation is the only part of the purchase performed by someone who is not the manufacturer, in a place the manufacturer may never see. That is precisely why it has to be written down before the first fixing — not because installers are dishonest, but because the moment the first screw goes in, every subsequent decision becomes a negotiation about who should have said something earlier.

This article breaks installation into eight actions that can actually be inspected, gives a sentence for each that can go straight into a contract, sets out where the responsibility boundary lies between the factory, the importer and the installer, and names the three checks and the evidence chain that decide a dispute two winters later.

Why the Installation Clause Matters More Than the U-Value

A window specified at 1.8 W/(m²·K) and installed with a four-millimetre gap along one edge, no backer rod, and sealant applied over dust will perform like a considerably worse window. Nothing in the paperwork will record this. The datasheet will still say 1.8, the invoice will still say 1.8, and the draught will arrive every January regardless.

This is not an argument against good windows. It is an argument that the specification and the installation are two halves of the same purchase, and that only one of them is normally written down in any detail. The buyer who writes the second half into the contract is not being difficult; they are closing the only gap in the transaction that the manufacturer cannot close for them.

Eight Actions You Can Actually Inspect

“Installation quality” is not inspectable. Eight specific actions are. Each of them can be checked by a person with a torch, a probe and a phone camera, and each can be written as a clause.

Two notes on the numbers. The fixing centres and corner offsets belong to your local code, not to this article — use the values your code requires, or those on the approved shop drawing, whichever is tighter. And the sealant, the primer and the backer rod diameter are named by the sealant manufacturer for the joint width in question; a clause that says “sealant to manufacturer’s instructions” is weaker than one that names the product.

Who Is Responsible for Which Part

Three parties touch a window before it is finished, and most disputes are not about any one of them failing. They are about the handover between them being undocumented.

The factory is responsible for the dimensions on the approved sheet, the fabrication tolerance, the glazing, the drainage slots as designed, the packing, and the documents: shop drawings, hardware schedule, dimensional inspection report and installation drawings. What the factory cannot do is verify that the opening on site matches the sheet it was given, because it has never seen the opening.

The importer or distributor is responsible for the survey being right, for storage, and for the sequence. This is where the most expensive and least discussed failures happen: a window that arrives correct and is stored flat on the ground for three weeks can be twisted before anyone opens the crate, and a window installed before the wall is weathertight will be blamed for water that came from somewhere else. Storage upright, dry, on packers, and installation after the wall is closed — these are cheap rules that are almost never written down.

The installer is responsible for the damp-proof course, the fixing pattern, the joint, the drainage check, the commissioning and the protection. Every one of those is on the list above, and every one of them is checkable at the time.

The boundary that matters most in cross-border work is the first one: the factory warrants the product, the importer warrants the survey and the storage, the installer warrants the workmanship. A contract that assigns all three to one party is a contract that will be argued about, because two of the three are outside that party’s control.

Three Checks at Handover

None of these requires equipment, and all three can be done in an hour on a normal house.

Operating force. Open and close every unit by hand. A sash that needs two hands, or that catches at one corner, is either out of square or has its hinges set wrong. This is the single fastest way to find a badly installed unit, because it fails immediately rather than in the first storm.

The drainage test. Pour about half a litre of water into the sill channel and watch where it leaves. If it comes out on the inside, the slot is blocked or the frame is back-falled. Do this before the internal finishes go on, not after.

Seal continuity. Run a hand, or a smoke pencil, along the whole internal perimeter joint. Any movement of the flame is a gap, and the gap is where the water will arrive.

Record all three. A handover sheet that says “installed and working” is worth nothing in a dispute; one that records handle force per unit, the result of the drainage test and the location of any re-sealed joint is worth a great deal.

The Evidence Chain to Keep

A dispute about a leaking window is almost never a dispute about whether it leaks. It is a dispute about which of four sources the water came from: the frame or corner joint, the glass and its gasket, a blocked drainage path, or the junction between frame and wall. Those four have different owners, and the party that can produce dated photographs from the right stage resolves it in ten minutes.

Four things are worth keeping, and they cost almost nothing to keep:

• Photographs at four stages: the opening before installation, the fixing positions before sealing, the finished joint, and the completed unit in the wall.
• A handover sheet per unit, not per project — because a defect is always in one unit, and a project-level sign-off proves nothing about which.
• The sealed sample and the dimensional inspection report, held until the warranty period ends. These are what you measure a later batch against.
• The approved dimension sheet, signed. It is the document that decides who pays when an opening and a window disagree.

None of this is adversarial. It is simply the difference between a conversation that starts with “it leaks” and one that starts with a photograph of the joint on the day it was sealed.

What a Factory 8,000 km Away Can and Cannot Do

It is worth being precise about this, because “we support installation” is a phrase that means very different things depending on who says it.

What we can do: issue shop drawings for approval before production; supply installation drawings and a hardware schedule with every order; provide video guidance for the standard sequences; produce a first-article sample with a full dimensional inspection report and a sealed sample that the batch is measured against; and dispatch a technician to site on request. Lead time is 25 to 35 days from shop-drawing approval, which is production time and not door-to-door.

What we cannot do: supervise the installer, verify the damp-proof course, or be present at handover. There is no local installation network behind these units. We also do not publish a warranty period on the product page, because it is agreed per order with the factory — ask for it in writing, and treat the answer as one of the lines in the comparison.

That division is not a limitation of willingness. It is a limitation of geography, and it belongs in the contract rather than in the sales conversation — because a buyer who knows where the boundary is can plan around it, and a buyer who discovers it after the first leak cannot.