Tapered Roof Insulation & Drainage Falls

1 September 2026

PNP XPS SLOPE boards are factory-tapered extruded polystyrene insulation boards that create drainage falls on flat roofs, terraces, and inverted roofs in a single layer, eliminating the need for a separate fall-forming screed. XPS 300 SLOPE (300 kPa compressive strength) and XPS 150 SLOPE (150 kPa) are the primary products for this application, both manufactured by PNP Orange Kft. in Várpalota, Hungary, and conforming to EN 13164:2012+A1:2015. The built-in taper reduces dead weight, shortens installation time, and maintains constant thermal performance for the lifetime of the roof.

What XPS SLOPE Boards Do

On a flat or near-flat roof, standing water accelerates membrane degradation and adds structural load. The conventional remedy — a fall-forming screed — adds significant dead weight and requires curing time. PNP XPS SLOPE boards solve both problems: each board is factory-tapered so that, when laid to a design layout, the insulation layer itself forms the required drainage fall. No separate screed is needed.

Two SLOPE products are available from PNP Orange Kft.:

XPS 300 SLOPE — compressive strength 300 kPa; suitable for standard and accessible flat roofs, terraces, and green roofs where moderate to significant foot traffic or maintenance loads occur.
XPS 150 SLOPE — compressive strength 150 kPa; suitable for non-accessible or lightly loaded flat roofs and inverted roof assemblies.

Both products share the key material properties of PNP XPS: a homogeneous closed-cell structure, negligible long-term water absorption, and excellent dimensional stability — properties that keep thermal performance constant over the roof's service life. Detailed declared values (λD, WL(T), DS, tolerance class) are given in the current TDS and DoP for each product; consult those documents before specifying.

Why Use a Factory-Tapered XPS Board?

Single-layer solution: fall formation and thermal insulation are combined, reducing the number of trades and wet processes on site.
Lower dead load: eliminating a fall-forming screed (typically 50–150 kg/m² depending on fall height) reduces the load on the structural slab — relevant on refurbishment projects and lightweight structures.
Faster programme: no curing time; waterproofing can follow immediately after board installation.
Consistent thermal resistance: XPS boards maintain their declared λD values under long-term moisture exposure (WL(T) rated by EN 13164); a screed does not.
Suitable for refurbishment: SLOPE boards can be laid over an existing roof system that has no fall, restoring drainage without structural intervention.

Recommended PNP Products for Tapered Roofs

Product	Compressive strength	Typical use case	Product page
XPS 300 SLOPE	300 kPa	Accessible terraces, green roofs, standard flat roofs with maintenance loads	View product
XPS 150 SLOPE	150 kPa	Non-accessible flat roofs, inverted roofs with light traffic only	View product

Where a flat base layer of insulation is required beneath the SLOPE layer (to achieve the minimum insulation thickness before the taper begins), standard PNP XPS boards are used in combination:

XPS 300 or XPS 300 W — for assemblies requiring 300 kPa compressive strength throughout the build-up.
XPS 150 W X-Grip — where adhesion to a waterproofing membrane or applied layer is required on a lower-load assembly.

The selection of compressive strength class is a structural engineering decision. A qualified designer must verify that the chosen product's compressive strength — and, where relevant, its compressive creep class CC(2/1,5/50) — is adequate for the imposed and permanent loads. See the current TDS and DoP for declared values.

Typical Layer Build-Up (Indicative Only)

The following build-ups are indicative and derived from the PNP solutions catalogue. Actual layer specifications, thicknesses, and materials must be confirmed by a qualified designer in accordance with local building regulations and the relevant national application standards.

Non-Accessible Reinforced Concrete Flat Roof (Warm Roof)

Reinforced concrete structural base
Vapour barrier
PNP XPS thermal insulation (base layer — flat boards, e.g. XPS 300)
PNP XPS SLOPE layer (fall-forming tapered boards)
Geotextile or fibreglass separation layer
Mechanical fasteners
Waterproofing PVC membrane

Non-Accessible Ballasted Inverted Roof

Reinforced concrete structural base
Vapour barrier
PNP XPS SLOPE layer (fall-forming tapered boards)
PNP XPS thermal insulation (flat boards above the SLOPE layer)
Geotextile or fibreglass separation layer
Waterproofing PVC membrane
Geotextile protection layer
Gravel ballast

Green Roof / Inverted Roof with Landscaping

Reinforced concrete structural base
Geotextile or fibreglass separation layer
PNP XPS SLOPE layer
Waterproofing PVC membrane
Root barrier
Geotextile or fibreglass separation layer
PNP XPS thermal insulation
Drainage and storage profiled membrane with filter
Vegetation layer

Vehicle-Accessed Inverted Roof / Podium Deck

Reinforced concrete structural base
Geotextile or fibreglass separation layer
PNP XPS SLOPE layer
Waterproofing PVC membrane
Geotextile or fibreglass separation layer
PNP XPS thermal insulation (flat boards, high compressive strength)
Drainage profiled membrane with filter
Base course (crushed rock)
Road pavement

Sloping Surfaces — Dual SLOPE Layer

For complex geometry or larger falls, the solutions catalogue shows a two-layer SLOPE configuration:

Reinforced concrete base
Vapour barrier
Extension layer of PNP XPS flat board
PNP XPS SLOPE A1/A2 (primary slope layer)
PNP XPS SLOPE B1/B2 (secondary slope layer)

Specific taper angles, minimum and maximum board thicknesses at toe and ridge, and layout geometry must be designed by a qualified engineer using the product TDS data and the DoP values.

Installation Notes

Layout design first: tapered board schemes require a fall layout drawing prepared before ordering. Boards are supplied to a specific taper and must be placed in the correct orientation. Errors in orientation reverse the fall direction.
Minimum fall: the required drainage fall (typically expressed as a percentage or ratio, e.g. 1.5 % or 1:67) is set by the roof design standard and the membrane manufacturer's requirements, not by PNP. Confirm with your waterproofing supplier and applicable national standard.
Staggered joints: offset board joints between layers by at least half a board length to avoid continuous linear gaps in the insulation.
Base layer flatness: the structural substrate or vapour barrier surface must be adequately flat and free of protrusions before SLOPE boards are laid. Irregularities can create voids that compromise both thermal performance and board stability.
Mechanical fastening: in mechanically fastened single-ply membrane systems, fastener layouts must account for varying insulation thickness across the SLOPE scheme. The membrane manufacturer's fixing pattern specification and pull-out tests govern fastener selection.
Adhesive bonding: where boards are bonded rather than mechanically fixed, confirm adhesive compatibility with the XPS surface. The smooth skin of SLOPE boards differs from the waffled (W) surface products; check the adhesive manufacturer's requirements.
Abutments and upstands: ensure the SLOPE scheme directs water away from upstands and towards outlets. Additional flat boards may be required at outlets to maintain a consistent bearing plane.
Temperature at installation: XPS boards have an operating temperature range of −70 to +75 °C; however, adhesives and membranes have their own temperature constraints — follow those manufacturers' instructions.

Thickness Guidance

Because tapered boards vary in thickness from one end to the other, the relevant dimensions are the minimum thickness (at the low point / drainage outlet) and the maximum thickness (at the ridge or high point). The taper angle determines how these relate to the bay width and the target fall.

Minimum insulation thickness is governed by the thermal resistance (RD) required by the applicable national energy regulation and the designer's condensation / dew-point analysis. Maximum thickness is governed by structural and architectural constraints.

Specific taper angles, minimum board thicknesses, and available increments for PNP XPS SLOPE boards are given in the current TDS for each product. See the current TDS for exact values; do not assume dimensions from catalogue illustrations. A qualified designer must select thicknesses to satisfy both thermal and structural requirements.

Where the minimum thermal thickness exceeds the minimum SLOPE board thickness, a flat base layer of standard XPS boards is laid first to bring the insulation up to the required minimum before the tapered layer begins — this is the approach shown in several PNP roof build-ups above.

Relevant Standards and Compliance

EN 13164:2012+A1:2015 — Factory-made products of extruded polystyrene foam (XPS) — Specification. PNP XPS SLOPE products are manufactured in conformity with this standard. The DoP (Declaration of Performance, CPR 305/2011) for each SLOPE product provides the declared performance values.
National energy performance regulations in the target country set the minimum RD or U-value for the roof assembly. These vary across the PNP supply markets (HU, RO, DE, CZ, PL, SK, HR, BG, and others). The designer must apply the correct national standard.
Roof drainage design (outlet sizing, fall requirements) is governed by national plumbing and roof drainage standards (e.g. EN 12056-3 for gravity drainage) and the membrane system specification.
Fire performance of the complete roof assembly is subject to national building regulations and must be assessed by a qualified fire engineer. XPS 300 SLOPE has Euroclass E reaction to fire (see DoP); XPS 150 SLOPE — see the current DoP. Assembly fire classification may differ from the board classification alone.

Documents to Download

The following documents are available from the PNP documentation library:

XPS 300 SLOPE DoP A1-B1: DoP PNP 300 SLOPE A1-B1_en.pdf (updated 2025-03-17)
XPS 300 SLOPE DoP A2-B2: DoP PNP 300 SLOPE A2-B2_en.pdf (updated 2025-03-17)
XPS 150 SLOPE DoP: see the current TDS (contact PNP sales for the latest issue)
EPD — PNP XPS (Várpalota plant): environmental product declaration per EN 15804 (updated 2025-03-19) — available from documentation library
PNP BIM Libraries — building information model objects (updated 2025-02-24) — available from documentation library

For XPS 300 and XPS 300 W (base layer boards used in combination with SLOPE):

TDS pnp_xps300-300TB_600x1250_techcard_en (updated 2026-07-22)
DoP PNP XPS 300 EMI_en (U) (updated 2025-10-14)
TDS pnp_xps300W-300WTB_600x1250_techcard_en (updated 2026-07-22)
DoP PNP XPS 300 W EMI_en (U) (updated 2025-10-13)

Contact PNP at sales@pnpinsulation.com or visit the contact page for project-specific technical support.

Sources

XPS 300 SLOPE product description and DoP references (PNP product data, retrieved 2026-09-18)
XPS 150 SLOPE product description (PNP product data, retrieved 2026-09-18)
DoP PNP 300 SLOPE A1-B1_en.pdf (updated 2025-03-17)
DoP PNP 300 SLOPE A2-B2_en.pdf (updated 2025-03-17)
XPS 300 technical data sheet: pnp_xps300-300TB_600x1250_techcard_en (updated 2026-07-22)
DoP PNP XPS 300 EMI_en (U) (updated 2025-10-14)
XPS 300 W technical data sheet: pnp_xps300W-300WTB_600x1250_techcard_en (updated 2026-07-22)
DoP PNP XPS 300 W EMI_en (U) (updated 2025-10-13)
XPS 150 W X-Grip technical data sheet: pnp_xpsX-Grip_X-GripTB_600x1250_techcard_en (updated 2026-07-22)
PNP solutions catalogue — Roof build-ups (PNP internal data, retrieved 2026-09-18)
EPD — PNP XPS (Várpalota plant), EN 15804, updated 2025-03-19
PNP BIM Libraries, updated 2025-02-24
EN 13164:2012+A1:2015 — Factory-made products of extruded polystyrene foam (XPS) — Specification

Frequently asked questions

What is the difference between XPS 300 SLOPE and XPS 150 SLOPE?

The key difference is compressive strength: XPS 300 SLOPE is rated at 300 kPa and is suitable for accessible terraces, green roofs, and roofs subject to maintenance loads, while XPS 150 SLOPE is rated at 150 kPa and is intended for non-accessible or lightly loaded flat roofs and inverted assemblies. Both create drainage falls in a single insulation layer. A structural engineer must confirm which class is appropriate for the specific load case.

Can XPS SLOPE boards be used as the sole insulation layer, or do I need a flat base layer too?

SLOPE boards alone are sufficient where the minimum board thickness at the low point meets the required thermal resistance (RD). Where the minimum tapered thickness is less than the thermally required minimum, a flat base layer of standard PNP XPS boards is laid first to make up the deficit, and the SLOPE boards are then laid on top. The designer must verify this using the declared λD from the current TDS and the applicable national energy standard.

What fall percentage do PNP XPS SLOPE boards create?

The taper angle — and therefore the fall percentage achieved for a given board width — depends on the specific SLOPE board type (A1, A2, B1, B2 variants indicated in the DoP references). The exact taper geometry is given in the current TDS; see the current TDS for precise values. The minimum fall required on a roof is set by the membrane specification and applicable drainage standard, not by the insulation manufacturer.

Can XPS SLOPE boards be used on an inverted (upside-down) roof?

Yes. The PNP solutions catalogue includes inverted roof configurations where the SLOPE layer is positioned below the waterproofing membrane (between the membrane and the structural slab), directing drainage at the membrane level. In such configurations, the flat XPS insulation boards are placed above the membrane, over the SLOPE layer. The specific build-up must be designed by a qualified engineer.

Can XPS SLOPE boards be used on a green roof?

Yes. XPS 300 SLOPE is identified by PNP as suitable for green roofs. The closed-cell XPS structure provides negligible long-term water absorption, which is important in permanently moist green-roof conditions. Root barrier requirements and the full layer build-up must be specified by a qualified designer in accordance with the applicable guidelines for green roof assemblies.

Are XPS SLOPE boards suitable for refurbishment over an existing flat roof?

XPS 300 SLOPE is described by PNP as suitable for refurbishments where the existing roof has no fall. Before specifying, a structural engineer must assess the existing roof's load capacity (the new insulation layer adds dead load) and confirm compatibility with the proposed waterproofing system. The existing roof surface condition and any existing insulation also need to be evaluated.

What fire classification do PNP XPS SLOPE boards carry?

XPS 300 SLOPE has a Euroclass E reaction to fire classification as stated in its DoP. For XPS 150 SLOPE, see the current DoP. Note that the fire performance of the complete roof assembly — including membrane, substrate, and any surfacing — is determined by system-level testing and must be assessed by a qualified fire engineer in accordance with local building regulations.

Where can I get a tapered roof layout design using PNP XPS SLOPE boards?

Contact PNP at sales@pnpinsulation.com or via the contact page for project-specific technical support. A tapered layout drawing must be prepared before ordering, as boards are supplied to specific taper configurations and must be placed in the correct orientation on site.