Reversed draft
The relief widens as it comes towards the surface, then narrows again at the top. Anywhere the section 'flares' back in creates an undercut that the panel can't lift straight through.
Knowledge · Design guide
Getting undercut-free architectural relief.
An undercut you didn't intend costs you tool life, panel quality, or both. Here's how to spot them at design stage and design past them without losing the visual effect.
What is an undercut
Any relief the panel has to flex past to come off the mould.
An undercut is any geometry where the panel — as it lifts off the mould — has to travel through a section of the mould that's narrower than the panel above or below it. Think of a dovetail joint: the wide top can't pull straight up through the narrow neck.
In concrete tooling, undercuts turn what would be a straightforward one-piece polyurethane mould into either a two-part silicone mould (more expensive) or a mould that tears itself apart trying to demould (worse).
This guide walks through spotting undercuts on a drawing, deciding when they matter, and designing past them where they don't.
Spotting them
Four kinds of undercut we see on architectural drawings.
Overhanging cornice detail
A projecting cornice with a deeper lip that hangs over the wall face below. The lip catches on the panel as it lifts. Very common in classical detail translated into precast.
Deep letterforms
Cast letters in the panel face — especially serif letters or scripts with strokes that widen towards the surface. The bulbous serifs can lock into the mould.
Rope, beadwork or continuous ornament
Any repeating three-dimensional detail that runs continuously across a panel face. Individual beads or ropes can each be a small undercut; a run of them locks the panel in the tool.
Design responses
Four ways to keep the visual effect without the undercut.
Add draft angle
The easiest change. Draft the sides of the relief so they open towards the mould face — often by just a couple of degrees. Visually invisible at panel viewing distance. Solves most undercut problems.
Break the ornament
A continuous rope detail with slight gaps at panel joints reads as continuous but demoulds as separate segments. Same visual effect, no undercut lock.
Move to silicone
Silicone flexes past undercuts that polyurethane can't. This costs more but keeps the design as drawn. Reserve for cases where the visual effect really matters.
Split the mould
A two-part or multi-part mould that opens sideways as well as vertically. Necessary for cornice work with deep overhangs. Adds cost but is often the only clean answer for classical detail.
When to reach for silicone
The threshold where silicone earns its cost.
For shallow-to-moderate relief with straightforward geometry, polyurethane with sensible draft angles gives you the best economics. That's most façade work.
For anything with reversed draft, deep letters, overhanging cornice, or fine repeating three-dimensional ornament, silicone earns its keep. The tool costs more, but the panels come off clean and the tool lasts longer than a polyurethane trying to flex past geometry it wasn't designed for.
For anything the architect insists on keeping exactly as drawn, and where the drawing includes undercuts that can't be designed out, silicone is often the only tool that works. That's the honest answer.
Bring us in early
The best undercut fix costs nothing at design stage.
If we see a drawing at RIBA Stage 2 or 3, we can flag the undercuts and suggest small design changes that keep the visual intent and remove the moulding problem. The architect gets a design that works; the panel manufacturer gets a tool they can run.
If we see the same drawing at Stage 5 with the tool already ordered, we can build the mould, but the answer is usually 'expensive silicone with a multi-part backing frame' rather than 'straightforward polyurethane'. The tool cost is meaningfully higher and the manufacturer's programme risk is higher too.
One hour of our time at concept stage saves days of iteration later. There's no charge for that conversation.
At a glance
The idea in a picture.
How a two-part mould fits together
Swipe to see the whole diagram
FAQs
Common questions on undercut and draft.
How much draft do you need to avoid an undercut?
One or two degrees is usually enough to keep polyurethane happy. Silicone can work with zero draft on shallow relief and slight negative draft on deep relief.
Can we cast something with an undercut in polyurethane at all?
Yes, but with a multi-part mould that opens sideways as well as up. It costs more than a one-piece silicone mould in many cases.
Does silicone always handle any undercut?
No. Very deep undercuts (dovetail geometry) still need multi-part tools. Silicone flexes, but only so far.
Will adding draft affect the visual reading of the panel?
At panel viewing distance — from ground level to the top of a building — one or two degrees of draft is invisible. On a close-inspection surface (a countertop or table edge) it might just be perceptible.
Can you review our drawings before we finalise the design?
Yes. Send them across and we'll come back with any moulding concerns before you go to tender. No fee at concept stage.
Get in touch
Send us a photograph of the panel or the elevation.
One photograph tells us what surface you're chasing, and one line about the mix tells us what silicone or polyurethane will survive it.
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