How to Build a Flat Roof (UK)

Flat_roofing_Farnborough fibreglass

How to Build a Flat Roof

The construction of a flat roof differs greatly from that of a pitched roof, and it’s important to know the basic requirements of a flat roof structure.

Ideally, the roofing joists should be laid across the walls of the structure, which in most cases are the upright items. It is recommended to use steel joists in the walls because they provide greater stability.

A typical distance between the joists is around 400mm, although this may vary depending on the roof size and material being used. That way, less pressure is placed on the top. As always, we recommend to use a professional roofing service to deal with this as roofing in general can be a dangerous job.

How do you build a flat roof step by step?

Flat roof systems are usually constructed using trusses or purlins. The trusses are made up of two parallel beams with cross members connecting them together at right angles. They can also be called girders.

Purlins are similar to trusses but have no diagonal braces. Instead, they’re attached directly to the wall.

To construct a flat roof, first layout the trusses or purlin layout. Then cut the necessary pieces for each one. Here is a step by step method:

1) The first thing you need to do is lay down the base plates for the roof. These must be at least as thick as the wall thicknesses. They can be made of plywood or metal sheets.

2) Once the base plates have been laid out, you’ll need to cut them into strips. This will depend on the area of ​​the roof you want to cover. For example, if you’re covering an entire room, then you’ll only need one strip. If you’re covering a smaller area, such as a porch or balcony, then you might need two or more strips.

3) After cutting the base plates into strips, you’ll need to attach them to the wall using screws or nails. You’ll also need to install the rafters, which are pieces of wood that run along the length of the building.

4) Now comes the tricky part: attaching the ridge beam. A ridge beam runs along the centerline of the roof, and it holds everything together. To make sure your ridge beam fits properly, you’ll need to measure its width and height. Then, once you’ve found the right dimensions, you’ll need to mark where the edges of the roof meet.

5) Next, you’ll need to nail the ridge beam to the base plate, making sure that the ends of the beam fit snugly against the edge of the roof.

6) Finally, you’ll need to add the sheathing to the underside of the roof. The sheathing is basically just plywood, but it has a special adhesive applied to it. When installing the sheathing, make sure that it goes all the way up to the ridge beam.

How Do I Measure Flat Roofing?

A flat roof is easier to measure than a pitched roof since it is easier to find the square meterage. Measure the length and width of each roof plane, including the dormers, before multiplying the numbers. You will then be able to figure out each plane’s area.

Add the final calculations from the two or more planes together to determine the size of the flat roof. It will enable you to determine how much of each material you need and how much you may have to pay for it.

Types of Flat Roofing

You can choose from two basic types of flat roofing systems. Which one to choose will depend on the requirements of the building, taking into account the level of insulation required, heat loss, and building codes.

Warm Flat Roofing:

The insulation used in warm roof  has no air gaps above or below the roof deck. This approach requires a vapour control layer beneath the insulation. A ventilated roof is not required with this approach.

Cold Flat Roofing:

The insulation used in cold roof is usually stowed between the rafters of the roof. Above the insulation, a 50mm air gap must be left, and the roof perimeter must be ventilated.

Common materials used for flat roof construction

Hot tar and gravel

This type of roof is commonly used in industrial applications. Hot tar and gravel roofs are very durable and require little maintenance. However, they are expensive because they use large quantities of tar and gravel.

Modified bitumen

This type of roof uses modified bitumen. Modified bitumen is cheaper than hot tar and gravel roofs. However, it does require regular maintenance.

Asphalt shingles

This type of roof consists of asphalt shingles. They are inexpensive and easy to maintain. However, they do not last as long as other types of roofing.

Metal roofing

This type of roof system is made of metal panels. Metal roofs are popular because they look like traditional slate or tile roofs. They offer excellent durability and fire resistance.


This type of roof has become popular because it is easy to apply and maintain. Fiberglass roofs do not require any ventilation holes. They are also lightweight and cost-effective.

Rubber membrane

This type of roof consists of rubber membranes also known as EPDM rubber. Rubber membranes are inexpensive and easy to install. However, they are vulnerable to damage by extreme weather conditions.


Can I build my own flat roof?

No. Unless you are a professional roofer, it’s quite challenging to build your own flat roof. You also only need some general roof repairs, so always go down this avenue before moving forward with a roof replacement.

 What is the thickness of a flat roof?

The minimum recommended thickness is 18mm. This differs when working on commercial projects.

What size joists do I need for a flat roof?

It is best to span the shortest distance possible between joists. 200mm x 50mm, 175mm x 50mm, and 150mm x 50mm are the most common joist sizes. Joists are normally spaced 400mm apart but should not exceed 600mm.

What Pitch is a Flat Roof?

To shed rainwater, flat roofs must have a slope, known as a fall. Ideally, it should be between 1:40 and 1:80.

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