Building components · Substructure & Ground
Filter & Separation Layer
Explains the filter and separation function as one positioned layer, and why losing it causes a granular layer to fail slowly.
Educational reference entry. What this element has to do, and whether any particular arrangement of it is adequate, depends on the whole assembly, the loading, the exposure, local requirements, the manufacturer's instructions and qualified professional design.
Overview
What a filter layer is
A filter and separation layer sits between two materials of different grading — commonly a fine soil and a coarse granular layer — and does two things at once. It allows water to pass, and it prevents the fine material working its way up into the voids of the coarse one.
The failure it prevents is gradual. A drainage layer that has silted up looks unchanged from the surface but no longer drains, and a granular layer contaminated with fines no longer spreads load the way it did.
Boundaries
How it differs from nearby elements
The distinctions that keep this entry from being a restatement of its neighbours.
- It is not a barrier. A barrier stops water; a filter layer is intended to let water through and stop solids.
- It is not a blinding. A blinding creates a working surface; a filter layer manages what moves between two materials over the long term.
- It is not a root barrier, which addresses a different intrusion and does not need to be permeable in the same way.
Terminology
Common names and aliases
These names describe the same element. The library keeps one entry per component so that a trade term or a regional name never becomes a second, thinner page.
- separation geotextile
- filter layer
- filter fabric
Dependencies
What it depends on
What has to be right elsewhere for this element to do its job.
- The grading of the materials on either side of it, since filtration is a relationship between the two rather than a property of the layer alone.
- Continuity, because migration will occur wherever the layer is missing, torn or badly lapped.
- Whether it is protected during construction, as placing and compacting coarse material over it is when damage occurs.
- The water regime, including how much flow passes through and in which direction.
Considerations
Topics that genuinely apply here
Subjects worth discussing about this element. These are discussion topics, never measured values, ratings or performance claims.
The layer's whole purpose is to remain permeable while retaining solids, and a layer that blinds over stops doing the first of those.
Contamination of a granular layer is progressive and largely invisible, which is why the separation matters more than it appears to.
Damage during placement of the layers above is the most common way separation is lost.
Assemblies
Building systems this element is met in
The role this component fills inside each assembly. A role describes what the element does there — it is not a statement that this component suits that system in any particular building.
Where it is met
Application contexts
Building contexts where this element is commonly encountered. Association is not recommendation: appearing here does not mean the component suits every instance of that context, or any particular one.
- Basement
- Boundary Structure
- External Works
- Hardstanding
- Site Drainage
- Substructure
Related elements
Neighbouring components
Typed relationships to other elements. There is deliberately no generic “related” category: an edge that cannot be classified does not exist.
Clarifications
Commonly misunderstood
- That any fabric will do. Filtration depends on the relationship between the layer and the soils either side, which is a design consideration.
- That separation is a refinement. Without it a drainage layer silts and a sub-base contaminates, both of which end the layer's usefulness.
Conversations
Questions for qualified professionals
Questions worth asking someone with the project in front of them. This page cannot answer them.
- What materials sit either side of this layer, and what has the separation been selected against?
- How will the layer be protected while the material above is placed and compacted?
- How are laps and edges being handled so the layer stays continuous?
- What flow is expected through the layer, and does that affect what is required?
Go deeper
Related Build Design Hub guides
Planning guidance behind the decisions this element involves.
Preparation
Related planning checklists
Owner-side preparation before the conversation where this element comes up.
What this entry does not do
- No grading, permeability, lap or product characteristic is stated here.
- Filter and separation selection is a geotechnical design decision based on the soils and flows involved.
Substructure & Ground Elements
The prepared surfaces, layers and buried members between undisturbed ground and the first structure above it.
Browse all substructure & ground entries →