Rarely are flat roofs truly flat. Drainage is essential for all roofing to avoid water ponding. Drainage is usually about 1:80, ideally 1:40. Beyond drainage, there are other factors that play a role in the life expectancy of a roof, as well as upkeep. Here I will cover three common roofing options for home extensions, outbuildings, and garages.
EPDM Rubber
EPDM is a synthetic rubber roofing membrane supplied in large sheets. Because it is flexible, it can be moulded to fit the entire roof, creating fewer seams. It can also withstand the movement of decks as they expand and contract and has therefore become very popular with homeowners and tradespeople.
It is bonded to the deck with adhesive and sealed with special tapes at edges and seams. EPDM roofs are usually UV and frost resistant and with proper installation can last several decades. EPDM roofing is also light weight, and relatively quick to install. The membrane can, however, be damaged by puncture from sharp objects and/or careless foot traffic during installation. It is also important that the deck is clean and dry. Always check what the manufacturer’s guarantee requires.
GRP Fibreglass
GRP (glass-reinforced plastic) consists of a layer of glass matting and liquid resin, which is left to cure and toughen, before a final decorative top coat is applied. This gives a seamless, hard, tough shell which is strong enough to take foot traffic and is a viable option for balconies and roof terraces.
GRP is rigid and therefore needs a solid, stable deck to support it. Traditionally, this has been exterior grade plywood laid correctly. Movement in the deck causes stress in GRP leading to hairline cracks, especially at the joins between boards and at the edge. This happens particularly with the incorrect preparation and application of the resin due to damp or cold conditions. If correctly laid, a GRP roof can last over 25 years and has neat, crisp finishes. Repair can be done by sanding and laying new patches of GRP, however colour matching the old and new material can be problematic.
Felt, Traditional and Modern
Felt remains the more affordable option, and better versions are now available than the old bitumen sheets. High-performance felt systems now use a minimum of two or three layers and can be installed using torch-on or cold-adhesive methods. The torch-on method poses a fire risk and should be done by an experienced contractor. Cold-bonded and self-adhesive types are safer to use for a DIY project, but a competent contractor will get better results.
Because felt is so inexpensive and easy to repair by patching, it is often used as a temporary repair on flat roofs, until an improved solution can be found. Its lifespan is typically shorter than EPDM or GRP and of lesser quality for the cheaper grades. When installed properly with good laps and upstands, it can perform as well as other options. The cap sheet can be mineral finished to provide additional UV protection. Felt roofs will require more frequent inspections.
Choosing the Right One
Consider use, shape and how long you’ll keep it. An EPDM roof is a smart choice for a simple rectangular garden room. It will last a long time and is easy to work with. GRP is better if the roof has lots of corners and penetrations, or if it will be a roof that will be walked on. A garden shed or a short-term structure can have a felt roof. Felt can be a good, economical choice.
There are many details to consider. Make sure you have adequate insulation and ventilation to stop condensation, use the correct edge trims that include a drip to throw water into the gutter, and seal around roof lights and pipes properly. Keep the roof free of debris and look at it at least twice a year.
A well-built flat roof can last for decades. The key to its longevity is in the design of the fall, the deck and the covering. Consider all of these when assessing the costs and skills needed to install the roof and deciding on the roof’s purpose. The best and most cost effective time to improve a roof is before the leaks start.