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How to Build a Flat Roof

how to build a flat roof

How to Build a Flat Roof

If you’re looking to construct a flat roof on a property, it’s important to know the scale of the project, materials required and the process involved. Building a flat roof isn’t as daunting as some may expect and if you decide to take on the challenge yourself, we’re here to help ensure you have everything you need.

Please note: It’s always best to get the advice of a professional roofer before undertaking any project.

Right, let’s get started!

How to measure flat roofing

To find the square meterage of a flat roof, the width and the length of each plane on the roof should be measured and the figures multiplied together. This figure is the total area of each plane. If more than one plane is required, simply add the final calculations together and this will provide the overall size of the flat roof. Remember, the more accurate the measurements – the less you will waste in materials and costs!

Flat Roof Tool List:

  • Roofing timber
  • Plywood
  • Steel joists
  • Exterior covering
  • Water-based adhesive
  • Foam with a fibreglass backing
  • Insulation

How to build a flat roof

The construction process begins by laying roofing joists across two upright items, typically the walls of the property or structure. These joists will provide the required stability for the roof and they are usually found in at least one wall. The distance between the two joists is around 400mm, although this does depend on the type of material that is going to be applied and the size of the roof itself. It also helps to reduce the amount of pressure that is placed on top.

The next stage is covering the joists with plywood, with the width in-between this determined by the weight that is placed on them as too much weight can lead to an unstable roof. Flat roofs should also incorporate a small incline in the design, this is added through the use of a frame and will ensure rainwater is sufficiently drained.

Decking and joists are then slotted together using tongue and groove fitting, this further enhances the stability of the roof and increases the integrity of the materials used.

Boards used for flat roofing must then be waterproofed with an additional layer of material used with a sealant to protect the inside of the building.

How To Build A Flat Garage Roof?

When building a flat garage roof, you will need a flat base for the roof covering made from WBP plywood with a thickness of at least 18mm. You may also want to consider installing insulation if you are constructing a warm roof. The flat base is then made by joining the plywood and the joists, with the plywood panels laid staggered to provide more stability. Timber noggins can also add extra strength.

How to repair a flat roof

The first sign of a flat roof in need of repair can be found in the room below it! Whether this is water dripping down or dampness in the ceiling. If you do need to repair your roof, we recommend taking extra care when stepping onto it. If it has been leaking for some time, the surface may not be suitable for standing, always consult an expert if you are unsure about starting a project.

To repair a leaking flat roof, you first need to find the source in the surface of the felt. Unfortunately, the leak in the top layer is usually not directly above where the leak is coming through the roof; the water will be spreading under the top layer before finding a way through the boarding.

Check the overall condition of the flat roof, if there is obvious damage such as splits or tears – the whole flat roof may need replacing. If you have noticed a leak adjacent to a wall, check the flashing for the leak spot first.

Starting from the spot where you’ve noticed the damage from below, work outwards to find cracks, blisters or splits in the material.  Roof repair kits are readily available and are ideal for making localised repairs, these typically consist of adhesive and tape. Clean the area thoroughly before starting your repair job, if there is any mould, combat this with a fungicide immediately and leave to dry out. Roof repair is always advised after a long period of dry weather!

Repair a blister in a flat roof

Clean the surface around the blister and use a sharp knife to make a cross cut in the top layer, being careful not to use too much pressure. Peel back the segments and dry the area using a heat gun emitting warm air across the whole area. Apply bitumen adhesive to the exposure surface and fold back the four segments, press down firmly to secure the area. Create a patch for the area using a piece of new felt that’s big enough to give a 2-inch overlap. Apply the adhesive over the repair and press firmly into place, focusing on the edges to make sure they are very well sealed.

Repair a split in a flat roof

Carefully lift and clean the space below the split or tear in the flat roof. Fill the space underneath the lifted split with bitumen adhesive and press down firmly. Fit any other tears with bitumen compound. Follow the steps above to cut a new patch and complete the repair.

How to felt a flat roof

The felt on a flat roof is what keeps it waterproof. If you need to add felt to your flat roof, we have the simple steps you need to complete the job, and get it right the first time!

Step 1 – Prepare the Roof

Safety first! You need to make sure that the roof is in a good enough condition to walk on before undertaking the project. Look for any boards that show signs of rot, these will need to be replaced before felting. Remove any existing felt from the roof along with any nails used. Roll the new felt out on the roof and leave it there for at least 30 minutes, this time will make the felt much easier to work with!

Step Two – Attaching the Felt

With galvanised clout nails, secure the first layer of felt and ensure that it remains straight and even as you work. Start nailing the felt in the centre and work outward from there. Once the first layer is down, apply a thin coat of adhesive over the layer before placing the second later over this. Make sure that there is an overlap in the felt to increase the water tight quality. Remove any air bubbles by walking over the roof once the second layer is secure.

Step Three – Final Layer

The third layer of felt is the final step in the process. Cover the edges of the flat roof with strips of felt before following the same steps as above to place the felt layer. A final coat of adhesive should be applied to the top for optimum waterproofing, gravel can then be added to the top for extra strength.

Warm Flat Roofing Vs Cold Flat Roofing

There are two types of flat roofing, warm and cold. The difference between the two is the application of the layers. A cold roof involves applying insulation between the rafters while warm roofing this is placed on top of the roof.

The type you select will depend on the building you are working on. You will also need to consider insulation levels needed and building regulations.

  • Warm Flat Roofing involves placing the insulation on top of the roof and has no air gaps above or below the deck. A vapour control layer is placed below the insulation and does not require ventilation. As you can see from the diagram below, the insulation is fully packed between the plasterboard, decking and joists to prevent any water vapour from occurring.

warm flat roof diagram

  • Cold Flat Roofing involves applying insulation between the rafters with a 50mm air gap above the insulation. You will also need ventilation at the roof perimeters. As you can see in diagram below, there is room between the insulation and the decking where warm air rises and condensation is present.

cold flat roof diagram

Which flat roof system is best?

There are many types of flat roof construction and the best system for you depends on your property. Reinforced bitumen membranes (RBMS), Ethylene Propylene Diene Monomer (EDPM), asphalt, Glass Reinforced Plastic (GRP), tiles, single ply, lead and even green roof systems all have their own unique benefits, so it’s important to find the one that’s right for you. 

  • Reinforced Bitumen Membranes (RBMS) comprises of layers of waterproof yet flexible materials which are then bonded together with hot bitumen that forms a waterproof membrane.
  • Asphalt is an older style of flat roof in which felt is bonded then rolled out and secured with asphalt cement. This involves a number of layers, one of which is gravel coated. This is often inexpensive but can require regular replacement.
  • Tiled flat roofs are another type which are often required if the roof is to be walked upon, for example a balcony or roof terrace. It is essential that the roof can support this weight and regular usage.
  • Single Ply involves using a strong polymer material which is held in place with mechanical fasteners to the structural deck.
  • Ethylene Propylene Diene Monomer (EDPM) is a style of flat roof that uses rubber based materials that are longer lasting and more durable than traditional materials.
  • Lead flat roofing is often found on older buildings such as churches. It has a long life expectancy and can be formed to fit most positions due to its flexibility. However, it has a tendency to buckle, crack and corrode.
  • Glass Reinforced Plastic (GRP) is a type of flat roof often used on commercial or public buildings as it can be walked upon and is highly durable. GRP usually has a life expectancy of around 40 years if well maintained.
  • Green flat roofs are an eco-friendly option which use turf, sedum matting or soil and are used for those wanting to construct a roof-top garden in a suburban area.

Flat Roof FAQs

We answer some of your most common questions on flat roofing…

What Size Joists Are Required For Flat Roof?

When calculating the size of the joists required for a flat roof, you will need to take into consideration a number of factors. You need to ensure that the joists are going to support the weight of the roofing materials. Weather conditions also need to be taken into account as the joists will need to be durable enough to withstand any harsh conditions or any added weight from snow fall. Check the BS EN 1995 code for guidance on choosing the right joist sizes.

How many years does a flat roof last

Some flat roofs (especially built up flat membrane roofing systems) have a lifespan of around 40-50 years, however, the manufacturer warranties may only cover around 10-15 years of this.

How often does a flat roof need to be replaced?

It is advisable to replace a flat roof after ten years, this is enough time for it to have normal wear and tear and for water to start coming in.

When Is A Flat Roof Needed?

  • A flat roof is usually required when a pitched roof is not suitable. Planning permission may not be given if the roof obstructs view or light for neighbours.
  • A flat roof is also needed if you are building an extension on your property.
  • If you currently have a flat roof that has a number of problems and fixes are not going to resolve the issues, then an entire new flat roof would be required for safety and structural reasons so as to adhere to building regulations.

Safety Note – If you replace a flat roof, care should always be given as asbestos may be present. Roofs with asbestos need to be replaced appropriately and in accordance with local authority regulations.

How Does Moss Affect a Flat Roof?

Moss can often accumulate on flat roofs due to surface water stagnating on the roof. This can then lead to spores, seeds and ultimately moss and weed growth. This can lead to added weight on the roof and effect its structural integrity and joist sagging.

To prevent moss growth on flat roofing, add firring which will increase the fall. You should also consider choosing fibreglass or rubber for your flat roof as these are less likely to become susceptible to moss and weeds than felt types.

How much does a flat roof cost?

The cost to construct a flat roof will depend on the type you choose and the scale of the job you are carrying out. Be sure to get quotes or estimates from a number of different companies and compare. It’s not always best to go for the cheapest option available. Also look for reviews to ensure they can provide a quality service.

For more helpful guides related to roofing, we also recommend reading our related articles:

For advice and guidance on the tools needed for your flat roof construction, contact a member of the BC Profiles team today. Remember, we sell a wide range of roofline products including roof ventilation, roofing membraneroof shingles and more, all in stock and at low prices. 

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