You rarely see a flat roof, which is why it only gets attention when a damp patch appears on the ceiling. Yet the roof covering is under heavy strain: sun, frost, standing rainwater and temperature changes work on it day in, day out. At some point renewing is wiser than continuing to repair. This article explains how to recognise that moment, which types of roof covering there are, what applies to insulation, fire safety, asbestos and the permit when renewing, and what a roofer does. If you have water coming in right now, first read Leak: which tradesperson should you call? for what to do immediately.
Signs that your flat roof needs attention
Look preferably after a rain shower, from an upstairs window or a dormer, and watch out for these signs:
- Blisters. Bubbles in the roof covering form because moisture or air is trapped under the covering and expands in the heat. A small blister is not immediately a leak, but blisters can split open.
- Cracks and open seams. Small cracks in the top layer and open overlaps between strips let water run under the covering.
- Puddles. Water still standing on the roof days after a shower points to insufficient fall, a sagging substrate or a blocked outlet.
- Weathered top layer. With bitumen, the protective layer of granules or slate chippings washes away over time, after which the black bitumen is exposed and ages faster under sunlight.
- Edges and upstands coming loose. The upstands, where the covering runs up against a wall, dormer or roof edge, are vulnerable. Detached upstands and loose roof trims are a common cause of leaks.
- Leaks or damp patches inside. The clearest signal, but also the last. Water can run a long way under the covering, so the location of the stain is not always the location of the leak.
Preferably have a flat roof checked every year; how that fits into the maintenance year is explained in the Maintenance calendar for your home.
Repair or renew the whole roof?
Not every leak means a new roof. A roofer can usually repair a cracked upstand, a loose seam or a leaking roof penetration locally. With bitumen, a new top layer can sometimes also be applied over the existing covering, if the underlying layers are still well bonded and dry.
Renewal is called for when the problems are no longer local: blisters and cracks across the whole roof surface, a top layer that is weathered everywhere, new leaks keep appearing in different places, or moisture has already soaked into the insulation or the roof deck. Wet insulation insulates poorly and hardly dries out under an impermeable covering, and a rotten timber roof deck must be replaced before a new covering can go on. If in doubt, a good roofer opens up the covering in a few places to assess the build-up. No fixed lifespan can be given for any roof covering; it depends on material, workmanship, fall, orientation and maintenance. So be guided by the condition of the roof.
Which roof covering: bitumen, EPDM, PVC or TPO
On Dutch homes you mainly see three types of roof covering, each with its own strengths and points of attention.
- Bitumen. The most widely used covering on flat roofs. A bitumen roof usually consists of several layers: an underlayer and a top layer with a protective finish. There are two main types of modified bitumen: APP (plastomeric) and SBS (elastomeric). SBS stays more flexible at low temperatures, APP is often cited for its resistance to heat and sunlight. Bitumen is traditionally torch-applied, with a burner that softens the underside of the strip, but it can also be cold-bonded, mechanically fixed or laid as a self-adhesive strip. Advantages are its robustness, the extensive experience in the industry and the ease of repair. A point of attention is working with an open flame in the torch-applied version.
- EPDM. A synthetic rubber covering that is usually applied in one layer, on home roofs often as a single made-to-measure sheet. EPDM is bonded, mechanically fixed or laid loose under ballast and needs no torch, which makes it suitable for roofs where open flames are undesirable. Points of attention are the seams and details, which must be skilfully bonded or taped, and repairability: repairs require materials compatible with EPDM.
- PVC and TPO. Plastic roofing membranes that are usually mechanically fixed or bonded and whose seams are hot-air welded. PVC contains plasticisers and generally may not come into direct contact with bitumen; when renovating over an old bitumen roof, a separation layer is then required. TPO, also called FPO, is a plastic without plasticisers. Welded seams are strong if made properly, but welding requires specific expertise and equipment.
A green roof is not a separate type of covering, but a build-up of substrate and planting on a root-resistant covering, with extra requirements for load-bearing capacity and drainage. More important than the material is often whether the roofer has a good command of the system. Ask why they recommend a particular system for your roof and whether they apply it regularly themselves.
Fall, rainwater drainage and emergency overflow
A flat roof is never completely flat. The roof must have a fall: a slight slope towards the outlets, so rainwater does not stand. The Vakrichtlijn gesloten dakbedekkingssystemen (the roofing industry's professional guideline for sealed roof covering systems) assumes a design fall of at least 1.6 per cent, so that after deflection of the structure an effective fall of at least 1 per cent remains. On older roofs the fall is often too small or the structure has sagged over the years. When renewing, the fall can be improved with tapered insulation, insulation boards that are themselves tapered.
The Bbl (Besluit bouwwerken leefomgeving, the Dutch Building Decree) requires a roof to have a provision for collecting and draining rainwater with a capacity determined according to the NEN 3215 standard. When renewing, the roofer should check whether the number and position of the roof outlets are right and whether they connect properly to the new covering. A flat roof with a raised edge should also have an emergency overflow: an opening in the roof edge that drains the water if the normal outlet is blocked, so that no large quantity of water remains on the roof. How large and how high it must be depends on the load-bearing capacity of the roof; if in doubt, have a structural engineer determine this. A blocked outlet without an emergency overflow can put a heavy load on a roof in a short time.
If water drains from the flat roof via a gutter or hanging gutter, also look at its condition. How to tackle a blocked or leaking gutter is explained in Cleaning or replacing a gutter.
Insulating when renewing: warm roof or inverted roof
If the roof covering has to be replaced anyway, that is the moment to look at the insulation too. The covering is coming off already, so insulating costs no extra work on the roof covering itself. There are two ways to insulate a flat roof on the outside:
- Warm roof. The old covering is removed, a vapour control layer goes on the roof structure, the insulation boards on top of that, and the new roof covering on top of those. This keeps the roof structure on the warm side of the insulation. The vapour control layer prevents moist indoor air from condensing in the insulation; a missing or poorly sealed vapour control layer is a well-known cause of damp problems in flat roofs.
- Inverted roof. The insulation, usually of extruded polystyrene or cellular glass, lies on top of the roof covering and is held in place with ballast such as gravel or tiles. This is an option if the existing covering is still good, but the structure must be able to bear the weight of insulation and ballast. If the covering has to be replaced anyway, insulating under the covering is recommended.
Insulating on the inside, against the ceiling or between the joists, is trickier with a flat roof, because the roof structure then becomes cold and there is a greater risk of condensation. Also bear in mind that insulation makes the roof build-up thicker: roof edge, upstands, thresholds of doors onto the roof and the connection with a dormer must be adapted accordingly. A dormer has its own rules; you can find them in Installing a dormer: permit-free or permit required?
What does the Bbl require for insulation, and is there a subsidy?
For insulation during alterations, the Besluit bouwwerken leefomgeving (Bbl) applies. The starting point for alterations is the rechtens verkregen niveau (legally acquired level): the quality after the alteration may not be lower than before. For insulation this has been tightened. When renewing or replacing insulation layers, a roof must have a thermal resistance (Rc) of at least 2.1 m²·K/W, or the legally acquired level if that is higher.
The higher new-build value does not apply to an ordinary roof renewal. It does apply to a major renovation: if more than 25 per cent of the total building envelope (roof, façades and floor together) is renewed integrally, including all layers. The new-build value then applies to the renewed parts, which for the roof of a home is Rc 6.3 m²·K/W. With an ordinary roof renewal, in which the roof deck stays in place, this is not the case, and with only the flat roof of an extension that threshold is usually not reached. If the roof deck is also renewed on a home with a large flat roof, have it recalculated whether the threshold is exceeded. The new-build values always apply to the complete construction or complete renewal of a dormer or an extension; if only the roof of an existing extension is renewed, the renovation value of Rc 2.1 applies.
For roof insulation, as an owner-occupier you may qualify for the ISDE, the national subsidy administered by RVO (Netherlands Enterprise Agency). The main conditions for roof insulation: it concerns insulating the existing roof, the newly applied insulation material has an Rd value of at least 3.5 m²·K/W, the insulated surface area of roof and any attic floor together is at least 20 m², a company carries out the work in full, and the home was built before 2019. You apply for the subsidy after the work has been carried out, within the period stated by RVO. Check the current conditions on rvo.nl before you award the contract. More about the scheme can be found in Sustainable renovation: subsidies and options.
Fire safety: torching or flame-free working
Torch-applying bitumen is skilled work with a real risk. Fires mainly start when the flame comes into contact, via gaps or seams, with combustible material under or next to the covering, such as a timber roof deck, insulation, timber roof edges or window frames. A smouldering fire under the covering can break out later.
For fire-safe working on roofs there is the NEN 6050 standard, which is elaborated in the Vakrichtlijn gesloten dakbedekkingssystemen of the roofing industry, to which VEBIDAK and Dakmerk, among others, contributed. Its core is that at fire-hazardous details, such as upstands against façades, overhanging parts and the connection to timber, no open flame is used within a certain zone and these details are executed flame-free, for example with self-adhesive or cold-bonded strips. Insurers also often set requirements for fire-hazardous work, such as extinguishers on the roof and a follow-up check after the work is finished.
Ask the roofer which details will be made flame-free and whether a check will be carried out afterwards. With a lot of timber around the roof or combustible insulation, a fully flame-free system may be preferable.
Asbestos and tar in old roof covering
In homes from before 1994 there may be asbestos in the roof build-up, for example in old bituminous roof covering or sealant, in base boards and roof decking, or in the roof trims and fascias. Whether this is the case can only be established by a survey by a certified company. How to recognise asbestos and which rules apply is explained in Recognising asbestos when renovating.
Old roof covering can also contain tar. Tar-containing roof covering is hazardous waste and must be kept separate on the building site and disposed of separately; ordinary bituminous roof covering must also be disposed of as a separate waste stream if it amounts to 1 m³ or more. Ask in the quote how the old covering will be disposed of.
Permit, listed building, solar panels and roof terrace
Replacing the roof covering with a comparable covering is ordinary maintenance. For an ordinary home you normally do not need an omgevingsvergunning (environmental permit) for this, as long as the detailing, profiling and design of the structure do not change. That is different if the appearance changes. A thicker insulation package can mean that the roof edge becomes higher; at the rear a new fascia is often permit-free, but at the front or a side facing the street a clearly higher roof edge may require a permit. A roof extension generally requires a permit; if in doubt, do the permit check in the Omgevingsloket (the government's online permit portal).
For a rijksmonument (national listed building), necessary regular maintenance aimed at preserving the heritage values is permit-free, as long as the detailing, profiling, design, type of material and colour do not change (article 13.11 of the Besluit activiteiten leefomgeving, the Dutch Environmental Activities Decree). A different type of roof covering, a higher roof edge or an insulation package that changes the appearance may therefore require a permit. For municipal listed buildings the rules are set out in the omgevingsplan (local environmental plan) or the heritage regulation, and in a protected townscape or village view extra rules may apply.
If you want solar panels, renew a roof that will need replacing in the foreseeable future before the panels go on. On a flat roof, panels usually stand in a frame with ballast, and the structure and the covering must be able to bear that weight. More about this in Having solar panels installed. If you want to use the roof as a roof terrace, the structure and the roof covering must be designed for foot traffic and loading, a fixed barrier is required for a height difference of more than 1 metre and a permit is often required; a roof terrace on an extension, for example, falls outside the permit-free rules, as explained in Permit-free extensions.
VvE, who does it and quality marks
If you live in an apartment, the roof generally belongs to the communal parts and renewal is a matter for the vereniging van eigenaren (VvE, owners' association). What exactly is communal is set out in the splitsingsakte (deed of division) and the regulations. The roof is then included in the long-term maintenance plan and the VvE chooses the roofer. More about this can be found in Renovating a VvE apartment.
Renewing a flat roof is work for a roofer who specialises in flat roofs. Carpentry, such as a new roof deck or rot repair on fascias, is done by the roofer themselves or they call in a carpenter.
In the industry, two names are useful to know. VEBIDAK is the trade association for companies that apply bitumen and plastic roof coverings; members must meet criteria for business management and continuity. Dakmerk is a quality mark for flat roofs; a certified roofer can give a watertightness guarantee that is backed by an independent guarantee fund. Check on the quality mark's website whether a company is genuinely affiliated. More about quality marks in construction can be found in Quality marks in construction explained.
What should be in the quote?
Preferably request two or three quotes based on a visit to the roof, and check that they state:
- the surface area of the roof and the length of the roof edges and upstands;
- what happens to the old covering: removal, disposal as a separate waste stream, or overlaying;
- the system: material, number of layers, brand and fixing method (torch-applied, bonded, mechanical, self-adhesive or welded);
- the insulation: warm roof or inverted roof, material, thickness and the Rc or Rd value, and the vapour control layer;
- the fall, the number and position of the outlets and the emergency overflow;
- the adaptation of roof edges, roof trims, upstands and thresholds to the new roof build-up;
- how additional work is charged if the roof deck turns out to be rotten or if asbestos is found;
- how the company works fire-safely and how safety on the roof is arranged;
- the guarantee on materials and workmanship, any quality-mark guarantee and the business liability insurance.
How to compare quotes fairly side by side is explained in Comparing quotes: how to read a building quote.
In short: have a local problem repaired quickly. If the whole roof surface is affected, renewing is wiser, and that is the moment to insulate and to improve the fall and the outlets. Choose a roofer who has a good command of the system and ask how they work fire-safely. On VerbouwPro you post your job free of charge and roofers from your region respond.