How To Measure A Flat Roof

How Are Flat Roofs Constructed

Rolled Steel Joists or RSJs are used to support joists in flat roofs

On a basic level, flat roof construction comes in the form of roofing timbers, or joists as they are more commonly known or steels that are laid across two uprights .

The steels, or RSJs are often built into one or both walls to give added stability to the roof structure.

The joists are then either laid on top of the steels or cut into them. These joists are spaced at a specified distance apart, usually 400mm depending on the width or span they are covering and the surface that is to be applied.

In the majority of cases the covering will be plywood or OSB timber which is more commonly called Stirling board.

When timbers are used in flat roofing they obviously need to be waterproofed so boards are normally laid across the joists and then a felt or rubber is used to cover the boards to make them waterproof. Where two boards meet and the joint is not on a joist, tongue and grooved decking boards make the joint strong enough to support the flat roof load.

Stirling board with tongue and groove for fitting to flat roof joists

Why Is Roofing So Expensive

Tile and Asphalt are the most used materials for roofing, and theyre manufactured with the usage of concrete, clay or oil. The rise of oil prices can directly impacted asphalt shingles to become more costly. Furthermore, the cost of disposing old and damaged materials has also increased in the recent years.

Understanding The Minimum Roof Pitch

Internationalbuilding code guidelines recommend a minimum roof pitch of 1/4:12. This meansthat for every 12 inches of the roof’s runwhich is the horizontal distance ittravels as it covers a homethe elevation of the roof should change by at least1/4 of an inch.

Keep in mindthat the building code requirements for minimum roof pitch may vary from onelocation to the next. Similarly, maximum roof slope rules can vary bymunicipality and may be determined by the type of building being constructed.In general, it’s rare for a roof pitch to xceed a 45-degree angle, or a pitch ratio of greater than12:12.

Recommended Reading: How To Shingle A Shed Roof Pictures

Draw A Diagram Of Your Roof

The first step in measuring your roof is to get a general idea of the area you will be measuring. Take a piece of paper and outline the appearance of your roof so that it is as close to its actual dimensions as possible. Not only will this give you a good idea of what youll be measuring, but also a handy place to record your measurements as you go. Increase these numbers by 15% to account for waste. Once you have a rough sketch of the roof, its time to go upstairs and start measuring.

Learn More: How To Vent A Dryer Through The Roof

How To Measure For Your New Flat Roof Skylight

It’s totally understandable question. After all, how often does the average homeowner need to order flat roof skylights in their life? Not often.

It’s worth taking the time to measure for the flat skylight properly, in order to avoid disputes and costly extra work. It’s essential that you inform your builder / roofer clearly and correctly as to what size to build the timber kerb. Perhaps most importantly, and this is where it can go wrong, it’s not just the width and length sizes that the builder / roofer needs, they also need to know whether those sizes they are being given are internal kerb or external kerb sizes.

Here’s 3 simple steps to ensuring you have measured for your flat roof skylight correctly and that everyone is on the same page.Step one

Find out from your architect or builder what size they had in mind for the flat roof skylight. On the other hand they might not know and ask for your input. In this case, decide on the rough size you think will work in your build.

Step two

Tell your builder the following: Our flat roof skylights are measured to the internal kerb size, and we recommend an 60mm deep timber kerb. You should also print off and give your builder / roofer this document. For example, a flat skylight 1m x 1m would require timber upstand that is 1m x 1m internal kerb size.

Step three

Order the skylight through us either through our website, over the phone or by email.

You May Like: How To Calculate Aluminium Roofing Sheets

How Pitch Affects The Cost Of Roofing

The pitch of a roof will have a definite impact on the cost to install or replace it. Steeper roofs cost more to install, so bear that in mind when choosing the right pitch for your project.

A low-slope roof might cost 10% more to install than a flat-roof, while a steep-slope roof might cost 20-30% more. Roofs with a very steep pitch might cost even more than that to install.

If youre considering a new roof, we suggest getting several estimates to learn more about the right pitch for your roof and understand the cost of installation. We also cover more about roof replacement cost in detail in our cost guide.

The Importance Of A Pitched Roof

If a home orbuilding’s roof didn’t have any pitch at all, it would be at a high risk ofcollecting standing water that ends up deteriorating the roof itself,presenting a serious and inevitable risk of leaking through the roof and intothe building. Even on roofs that may appear flat to the naked eye, a slightpitch has probably been built into the roof to facilitate water run-off fromrain and snow.

Pitched roofsare even more important in regions where snow can accumulate on a roof. Thelower the pitch, the greater the amount of snow that can accumulate on theroof’s surfaceand the weight of this accumulation could potentially break theinterior structure of the roof itself. With a higher roof pitch, snow and heavyrainfall can be sloughed off before structural integrity becomes a concern.

Roof pitch canalso have the following implications on a home:

Also Check: Is The Tesla Solar Roof Worth It

Calculating The Area Of Complex Roofs

If your roof is not the classic Gable shape, or if it has multiple sections such as hips and valleys, wed recommend splitting the roof into each section and working out the areas individually. If youre unsure of how to do this, get in touch with a roofing professional. Learn how to find a good roofer.

Or, alternatively get in touch with us! Our customer service team will be able to help you calculate your roof area, and anything else you might need for your project. Call them on 01295 565 565, email , or use the handy live chat below.

Can I Replace Roof Myself

The truth is, anyone can replace their own roof, with the help of the many DIY websites. For residential houses, roof replacements typically take two days the first day to remove the old roof, and the second day is to install the new one. A day or two more can be necessary depending on the size of the roof.

Read Also: What Does Tpo Roofing Stand For

How To Measure Roof Pitch

There are several reasons why you may need to work out the pitch of your roof. For example if youre planning an extension, purchasing a sky light, or replacing tiles.

When calculating your roof pitch, you are trying to find out how much the roof rises over one foot. To do this, you will need to access the roof, so read this article on how to do this safely before you begin.

There are several ways you can measure your roof pitch, here are the most common

How Do You Calculate The Slope Of A Roof

When referring to roofs, the term pitch means two things. Most of the time, people use this term to refer to the roofs steepness. But for carpenters and handymen, they believe that the terms more accurate meaning is the overall dimensions of the roof. Either way, lets see how you can calculate the slope of a roof without using a roof slope calculator:

1. Measuring the slope from the rafters

• The most precise way to find the roofs slope is by measuring it from the underside. Access one of the rafters then take the measurement this way.
• Use a level against the underside of the rafter youre measuring to get the most accurate value.
• Locate the levels 12-inch mark. If it doesnt have this mark, use a tape measure to locate the mark and draw a mark on it.
• Take the measurement of the vertical distance from the rafter to the level.
• Write the slope down using the measurements youve taken.

2. Measuring the slope from the surface of the roof

Also Check: What Voids A Roof Warranty

How To Measure Your Roof With Google Earth

Using your computer to get a ballpark estimate of your roof area is easy and fast. However, using satellite imagery may not be an ideal solution if your structure is in a very remote location that hasnt been 3D scanned. Thats because well need to grab an accurate representation of your roofs pitch for measuring purposes. From above with a 2D image, everything looks flat. If you were to just measure your roof area from directly above, you wouldnt be accounting for its pitch, or slope.

Consider two homes with the same base area square footage, but one of them has a much steeper roof pitch than the other. That structure would need to be taller, and there would be a lot more roofing material to account for. If youre just measuring from above in 2D, both of these structures might look identical. If you dont account for roof pitch, youre going to underestimate your materials requirements, which could be a costly mistake.

Satellite technology has progressed a lot in the past few years. With , you can easily find your address and a 3D replica of your structure, including your roofs pitch. Lets get to it.

Once you load the site, youll be presented with our beautiful planet:

While you can switch to 2D mode by clicking the button in the lower right, youll want to stay in this 3D view so you can get a good view of your roofs pitch.

Finding The Total Roof Area

Once the area of the roofs footprint is known, the overall roof area can be found by accounting for the roofs pitch. The pitch of the roof is the rise over a 12-inch run. Use our roof pitch calculator to find the pitch of your roof.

Next, multiply the footprint of the roof by the multiplier below for your roof pitch to find the overall roof area.For example, a 4/12 pitch roof that is 100 square feet:

100 × 1.054 = 105.4ft2

Roof Pitch Area Multipliers

Read Also: How Do I Estimate The Cost Of A New Roof

Expressing Slope In Standard Pitch

Note: All of these calculations work the same whether you are using inches or centimeters. You just have to plug the numbers in. A roof that rises 10 cm for every 30 cm of horizontal distance has the same slope as a roof that rises 4 inches for every 12 inches of run.

In the U.S., the slope of the roof is typically given in standard pitch, which is a ratio expressed as rise-in-run, X-in-12, or X:12.

Traditionally, 12 is always used as the run, because there are twelve inches in a foot, and you are stating how many inches the roof rises vertically for every foot the roof spans horizontally.

Pitched roofs commonly have slopes between 4-in-12 and 12-in-12 . A roof that goes up at a 45-degree angle has a slope of 12:12.

How Moss Can Affect A Flat Roof

Problem: Without an adequate fall on your flat roof, water can accumulate in puddles laying on the surface of the roof.

Seeds and spores can drop into this fertile environment and in no time at all, the flat roof is covered in moss and even weeds. The moss in particular holds on to the water every time it rains and when wet doubles the weight on your roof.

This weight leads to sagging of joists which in turn leads to splitting of the roof covering.

A moss covered flat roof can lead to sagging joists and leaking roofs

Solution: Add firring pieces to the roof to increase the fall and keep the roof free of moss.

Fibreglass and rubber flat roofs are far less likely to attract the same amount of moss and weeds that felt will especially if some effort is made to keep them free from grime.

A modern EPDM rubber or fibreglass roof with firring pieces can prevent water buildup and moss growth

Read Also: Can You Put Elastomeric Roof Coating Over Shingles

What Is A Cold Flat Roof

Having laid out the rules in simplistic terms above, the most common type of flat roof these days is one which combines a warm insulating layer, with an air flow to protect the joists from rot, more commonly known as a cold flat roof .

This is a very simplistic diagram designed to show the user the principles of the cold flat roof.

If you are designing or building a flat roof at home you need to do a little more research to show you how and where to incorporate a vapour barrier into your roof and whether or not you need to add a further layer of roof decking timber to the top of the insulation.

As mentioned above, you will also need to ensure that you have an adequate amount of air change or ventilation to ensure that no condensation is allowed to build.

It is impossible in a generic flat roof project such as this to “cover all bases” so please ensure you investigate your own project thoroughly.

One point to note with cold flat roofs is the they are not a recommended solution in areas of high humidity and due to this are not generally recommended by Scottish Building regulations.

It can be seen that warm air can rise up through the plasterboard into the void.

Any insulation present in the roof void will further warm this air which will rise until it hits the cold underside of the flat roof decking.

It will then condense and water droplets can fallback onto the insulation and even soak through to the plasterboard ceiling. Do not underestimate the dangers of condensation.

Sketch A Basic Diagram

While not necessary, it can be helpful to create a visual diagram of your roof. Start by sketching the shape of your roof, including important details like dormers, valleys, hips, and ridge lines. You can also add in protrusions like chimneys and skylights to make the sections of roof easily identifiable.

Using the calculated measurements for each section of roof, mark down the length and width of each plane, along with the square footage of each section. Ideally, you want your sketch to show your roof broken down into simple geometrical shapes which can make it easier to complete your calculations.

You May Like: What Are Metal Roofs Made Of

How To Use The Roof Pitch Calculator

This roof pitch calculator or roof slope calculator is a simple, easy to understand tool to use when you need to perform construction work in your home. To use this, you must take the required measurements so you can input them in the required fields. Here are the steps to follow for the roof angle calculator:

• First, enter the value of the Rise and choose the unit of measurement from the drop-down menu.
• Then enter the value of the Run and choose the unit of measurement from the drop-down menu.
• Finally, enter the value of the Rafter Length and choose the unit of measurement from the drop-down menu.
• After entering all of the values, the roof pitch calculator automatically gives you the values of the Roof Pitch, Roof Pitch , and Roof Pitch .

Calculate The Surface Area Of The Roof

Take the areaof the roofs footprint, say its 5,000 square feet, and multiply 5,000 by 1.118.

That gives you 5,590 square feet, which is the actual area of the surface of the roof. And thats what you were looking for.

This applies to rafter lengths as well as areas. A rafter for a roof with a 6:12 slope will be 1.118 times as long as the horizontal length that it covers.

You May Like: What’s The Average Cost Of A New Roof

Schedule A Free Roof Evaluation

Measuring the area of your roof is not always a simple process. In fact, it can be challenging to determine the square footage, especially if you have an irregular-shaped roof or if your roof has multiple dormers, valleys, hips, and ridge lines. However, getting a precise measurement is crucial if you want to purchase the correct amount of materials. If you are not confident in your ability to get a precise measurement, or if you do not feel comfortable going on your roof, consider hiring a professional to make the measurements and calculations for you. Contact the roof replacement contractors at Beyond Exteriors to schedule a free roof evaluation.

Walk To Your Roof And Measure Physically

Once youre on the roof, its time to check out the map and tape measure so you can get started. Carefully measure the length and width of all sections of the roof, paying special attention to the dimensions in the corresponding sections of the diagram. If your roof has triangular sections, such as a cornice, it is important to measure these as well, as they must also be shingles. Once youre done measuring the entire steep roof, its time to lower the ladder and use the measurements effectively. Finally, increase the total by 15% to account for any waste.

You May Like: What Is A Roof Boot