Everything you need to know about flat roofs, from construction to repairs

There is much to consider when it comes to flat roof construction details. If you have been asking yourself: "How to build a flat garage roof?" or "how to frame a flat roof?" you have come to the right place. Flat roofing is a popular choice for garages, homes, garages, extensions and many other types of buildings. The roofs have a timber frame as the supporting structure. Flat roof joists spanning the shorter length of the roof form the load-bearing members of the structure. A timber decking is then constructed, and its weight supported by the joists. The joists are attached to load bearing walls by two common methods:

  1. Using timber wall plates.
  2. By way of metal flat roof joist hangers.

This article covers all manner of flat roofing venting methods and details on how to build a flat garage roof and how to frame a flat roof, and also encompasses flat roofs, roof decking, roof coverings (including flat roof felt), how to prevent leaks at abutments, timber flat roof construction details, and parapets, and common flat roof problems.

Take a look at our range of flat roofing products.

Flat Roofing ConstructionFlat Roofing

A typical flat roof is made out of about sixteen components which all serve different purposes. Ensuring the flat roof has a fall is a critical step in the construction and the fall should be a minimum 1:80 for smooth surface such as corrugated metal or plastic roofs. A fall of 1:60 is required for roofing felt’s and other coarse roof coverings. The fall serves the following purpose:

  • Allow the roof to shed rainwater. If the fall is shallower than the recommendation there is a high risk of puddles (Standing Water) forming on the roof. This is a common cause of water leaks, especially with flat roof felt, when the covering breaks and the pooled water seeps through the roof.

Roofers achieve adequate fall by way of strips of tapered timbers known as furrings nailed onto the joists or by installing a specially designed insulation scheme called a ‘cut to falls’ system.

 

flat roof construction details

How to build a flat garage roof?

A flat base is required to lay the roof covering on. This can be either Smartply OSB3 or good quality, structurally sound WBP plywood. A minimum thickness of 18mm must always be used. Depending on the building use insulation could be required to construct a ‘warm roof’ build-up. The sheets of plywood are fixed to the flat roof joists to make a flat base, hence the name flat roof. The panels are laid with the longer edges across the flat roof joists and staggered to offer a more stable surface. To improve strength timber noggins are sometimes used between the joists.

Roof decking construction

A flat base is required to lay the roof covering on. This can be either Smartply OSB3 or good quality, structurally sound WBP plywood. A minimum thickness of 18mm must always be used. Depending on the building use insulation could be required to construct a ‘warm roof’ build-up. The sheets of plywood are fixed to the flat roof joists to make a flat base, hence the name flat roof. The panels are laid with the longer edges across the flat roof joists and staggered to offer a more stable surface. To improve strength timber noggins are sometimes used between the joists.

Some flat roofing venting methods and systems require an expansion gap between the decking boards and therefore many build ups will require the joints to be taped to prevent product dropping through those 2-3mm gaps. It's flat roof construction details like this that must be taken on board.

Types of flat roof coverings

Industrial flat roof finished with EPDM

Other flat roof construction details often refer to the types of flat roof coverings. A waterproof exterior layer is essential and there are a number of options including bitumen based felt or Asphalt, EPDM rubber and PVC based systems, as well as cold, applied liquid systems Bitumen coverings.

When considering flat roof construction details, it's worth noting that there are two types of bitumen based coverings, namely asphalt and bitumen flat roof felt. Bitumen felt is the most common covering type in use for domestic buildings. The types are:

  1. Mastic asphalt is waterproof and is made from bitumen and withstands the weather extremely well. Commonly used for commercial roofing jobs, the asphalt is melted, poured and spread while hot. This is a hazardous job best left to professionals.
  2. Flat roof felts are bitumen impregnated sheets, which are applied in layers. They are available as either self-adhesive sheets or bonded together with hot or cold bitumen.

View our article on Flat Roof Product Comparison here.

Cold-Applied roof coverings

Bitumen is a traditional roof covering material for flat roofs and has a long lifespan, when installed correctly. Modern roof coverings promise even longer lifespans as well as more efficient performance. EPDM, PVC & liquid systems are flexible membranes which are easy to lay and virtually maintenance free.

Preventing leaks at abutments and parapets

The most common source of leaks when it comes to flat roofs is where the flat roof meets the house or parapet walls. Turning the roof covering up the wall to form a skirting is the commonly applied method of ensuring a water-tight seal. This should be factored in when considering flat roof construction details.

Fault-finding flat roof problems and making repairs

How you approach repairs to a flat roof depends on the age and condition of your roof, as well as the extent of the damage you are facing. It's always worth considering all flat roof construction details before proceeding. Minor repairs are well within the abilities of an average homeowner to tackle, however, when the deterioration covers a wider area, you need to call in the experts.

Locating the source of the problem

Fault-finding on flat roofs can be incredibly difficult and locating the source of a leak on any roof is a challenge. Water tends to follow the path of least resistance and flows away from the source of the leak. When you notice a wet patch on the ceiling, take a tape measure and measure the distance to the nearest wall. When you go up on the roof, measure the same distance and you will be closer to locating the source of your problem.

Splitting and blistering

Blisters in a flat roof

An extreme example of blistering occurring on a flat roof.

You don’t need any expertise to spot splits and blisters on an asphalt or bitumen flat roof felt. Flat roof construction details, such as these, are usually hard to miss. However, if there are chippings laid on top then they may cover the splits and make the detective work a little more involved. You will need a blowtorch to soften the bitumen so you can scrap the chippings away and make repairs, but again this is probably best left to the professionals. Ensure the surface is flat and clean before patching it up.

Movement of the substrate will also cause splits in the felt. Trapped moisture will cause blistering and if you press it you should be able to spot the flaws in the covering. Moisture will escape through pinholes and once you locate these seal them with patches to avoid further deterioration.

Patch repairs

Patch repairs are effective for small splits in the felt using reinforced liquid repair systems. Remove chippings to get access to the flat roof felt covering, clean the surface, apply the required primer to give a strong bond, followed by the system which usually comprises of a base liquid followed by a strong reinforcement fleece finished off with a final waterproofing coat. This should be a minimum of 75mm in all directions. A small tear at a joint will require patching up and to prevent further splits treat the whole joint. When patching up small areas ensure all the edges are perfectly sealed by pressing the tap firmly.

Dealing with blisters on felt roofs

Unless blisters contain water and are causing leaks, it is best to leave them alone. Blisters in asphalt or bitumen coverings can be flattened by heating the blister up with a hot-air gun or blowtorch and then flattening with a heavy object, or block of wood and hammer. This method will work if the blister does not contain water. If your flat roof construction details highlight the presence of blisters, the appropriate action should be taken.

In this instance calling a professional should really be your first action.

Whole surface treatment for flat roofs

If your flat roof is beginning to resemble a patchwork quilt, then it’s time to consider a more extensive treatment with your timber flat roof construction details. Applying a liquid waterproofing agent will often add a few more years to the roof. A popular method involves a cold application bitumen emulsion which is applied onto the surface. Using a fibreglass membrane or polyester fleece will reinforce the waterproof capability of this solution if applied correctly. The steps for this treatment are as follows:

  1. Sweep the roof and remove any dirt and debris.
  2. Apply a solution that kills off lichen and moss and ensure the surface is dry before proceeding.
  3. Next spread a generous coating of primer, according to the manufacturer’s instructions, and wait until it cures.
  4. Use a brush or medium fleece roller to apply the first coat of the waterproofing material, embed a layer of reinforcement and apply the second coat of the liquid in accordance with the manufacturer’s installation instructions. They will specify coverage rates per square meter. Any reinforcement fleeces used will need to overlap usually at a minimum of 75mm.
  5. Some systems require drying times between coats whilst some can be installed whilst they are still wet, saving installation time.

Most systems will require a period of what is known as ‘full curing time’ following the initial ‘touch dry’ time. Until the system is fully cured (dried), sharp objects and foot traffic could possibly damage the new membrane and are strongly advised against. It's always worth considering all flat roof construction details as this way you can make an informed decision before proceeding.

 

This article has covered flat roofs in detail, from how to frame a flat roof and the construction of a flat roof joist and other timber flat roof construction details, to troubleshooting and carrying out small repairs. If you are having constant leaks and your roof is old, you may be better off contacting a roofer so you can receive expert guidance.

Related articles:

What is a Skylight upstand?

Warm Roof vs Cold Roof. What's the difference?

Installing roofing felt on a flat roof

Acrypol + Flat Roofing Repair

 

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