Saturday, July 13, 2024
HomeMust ReadHow To Cut Roof Rafters

How To Cut Roof Rafters

Stair Gauges Lessen The Chances For Error

Cutting Common AND Hip Rafters: Simple Solutions for Roof Framing

The most useful accessories for laying out rafters are a pair of tiny clamplike devices called stair gauges . Each gauge is a small hexagonal nut with a slot through the middle and a thumbscrew for securing it to the square. Clamped to the legs of a framing square at rise and run points, stair gauges act as stops to ensure that the rise and run settings remain consistent throughout the layout process. As the name implies, these little gadgets also are handy for laying out stair stringers.

Making A Birdsmouth Joint

Now that we have marked our rafter length and also marked our plumb line that defines the position of our birdsmouth, the next stage is to mark and cut our birdsmouth.

Before we start marking or cutting anything you have to be aware of that fact that your birdsmouth should be no more than a 1/3 the total depth of the timber you are using. For example, if you are using 6 x 2 timbers, the birdsmouth should be no greater than 2 inches deep.

With the rules covered, its time to mark our cut. There are several different ways of doing this but we are going to use a two stage method.

Firstly, lay the timber flat so that its depth is facing upwards and its as horizontally level as possible. Measure its total depth and then divide by 3.

This will now tell you the total depth that the cut can be at.

Using this number, measure and mark two points either side of your vertical line that depicts where the birdsmouth is to go, measuring up form the base of the timber and then draw a line between them.

At some point along this line it will intersect with the plumb line that states the Birdsmouths location. This point marks the location where the seat and heels cuts meet at & 90deg to form the birdsmouth joint.

The final job now is to join this mark to the underside edge of the rafter at the correct angle so that it sits fully flat and level down on the wall plate timber.

Using a roofing square to mark out the birdsmouth seat line

What Is The Difference Between A Truss Roof And A Cut Roof

A Trussed rafter is a fully engineered product and as such does not have the architectural advantage of being able to be left exposed. A cut roof however can be engineered to show exposed rafters, purlins, joists which may be required to give a more open feeling in parts of a new building or extension.

Also Check: How Much Is An Aluminum Roof

Additional Steps For Roofs Without A Ridge Beam

If you arent using a ridge beam, then youll want to use plywood gussets to reinforce the connection at the apex of each set of rafters. For an 8×10 shed, one sheet of plywood should be fine.

You are going to cut out triangles that will cover the apex of each set of rafters. The rule of thumb is that the gusset should be twice the width of the wood you use for your rafters. So roughly 7 8 long for a 2×4.

Each side of the gusset will be 8 long, but only 8 tall. Layout your sheet of plywood on sawhorses. Facing the long side of your material, measure down from the top edge 8. Then draw a line across the width of the plywood from that mark. Use a circular saw or jigsaw to make the cut.

Now you have a piece of 8 x 48 plywood. Use this piece to mark out the rest of the plywood. Now you need to cut your smaller piece into three triangles. Starting at one long edge, mark off points every 8. Now do the same on the other long side. Connect the dots so you have three triangles, each 8 high and 16 long.

Use a circular saw to cut them out. Use the finished gussets as templates to mark out the rest of your wood, and cut accordingly.

Attaching the gussets requires 1 wood screws and construction adhesive I use Gorilla Heavy Duty Construction Adhesive and it works great. Make sure you use gloves when handling. Remember that your sets of rafters on either end will only have a gusset facing the inside of the shed. All interior rafters will have gussets on both sides.

How To Cut A Birdsmouth Joint

Hip Roof Drawings

Now that we have looked at exactly what a birdsmouth is and when and where it is used, its time to get to the nitty-gritty and actually look at how we cut one.

When it comes to cutting a birdsmouth joint, we are going to look at doing this in the context of cutting and installing a roof rafter or joist as this is where its most commonly used, so in this scenario, the joint itself is only a part of the process.

In terms of cutting a birdsmouth joint, we are going to need to make three separate cuts the ridge or plumb cut where the rafter meets the ridge board, the birdsmouth where it meets the wall plate and the tail cut or overhang.

The three cuts needed to fit a rafter including a birdsmouth cut

To ensure that the birdsmouth is cut in exactly the right place, we first need to make the ridge cut as this will become our point of reference for working out the position of the birdsmouth, but before we can make the ridge cut, we need to work out the pitch of the roof.

So, to summarise, the full process is as follows:

  • Calculate pitch of roof
  • Use roof pitch to cut ridge cut
  • Use ridge cut as point of reference to form birdsmouth

For the purposes of this project we are going to be working with a simple gable roof e.g. pitched roof, with a central ridge.

Recommended Reading: How Do You Measure Pitch On A Roof

Can You Replace Trusses With Rafters

Although its technically possible to replace trusses with rafters, its best to check with a structural engineer before you make any decisions. Redesigning a roof space potentially interferes with the structural integrity, so not only should you consult an expert, but you should also properly plan your remodel before beginning the process. If you fail to do so, it could lead to expensive and potentially dangerous problems with your roof.

If you decide to make adjustments to your roof trusses, please follow these steps to ensure the proper outcome:

  • Put temporary load bearers in place as support. This includes:
  • Underneath the ridge of your roof, run a support header after finding the correct type and size with the help of an engineer.
  • Add support to both ends of the header and one in the middle. These new beams should go right to the foundations to make your roof as structurally sound as possible.
  • Then, youll have to boost the load-bearing capabilities by adding extra rafters between the sidewalls and new support header.
  • After that, youll be ready to cut out the trusses. Make sure you have an engineer check the floor of your space.

What Is The Difference Between Ceiling Joists And Rafters

Rafters and joists are both used in the construction of buildings, but they have a few differences. Its essential for those who work in the construction industry to understand the difference between the two, and its also useful for homeowners to have a basic understanding in case repair and maintenance are needed.


Lets start with joists. They are parallel horizontal beams that run across an open space. They join the opposite walls in a building to support both the ceiling below and the floor above. Theyre usually made from timber, and they come in a variety of widths .


On the other hand, rafters run diagonally from the wall plate to the apex of a roof structure. They form the foundation for attachment of the roof boards onto which the final roof covering is fixed. They are typically about 8-10 inches wide.

The main difference between joists and rafters is that joists are usually more horizontal to the ground while rafters are used for steeply sloped roofs.

Don’t Miss: What Are Roof Shingles Made Of

For The Rafter Seat Cut:

He uses a Bigfoot 10-1/4 inch beam saw with a swing table. The saw pulls a LOT of power, so use a pfaat extension cord.

The bevel is 63.5 degrees in this case. Sometimes you may need to cut at half the saw depth and then the full depth if the saw has difficulty. Use a sharp blade.

The rafters Tim is cutting require rips to the tails, so each one must be cut individually to that extent. For simple rafter tails, the roof would be cut now.

Cutting The Birdsmouth Joint

Beginner Rafter Layout | Speed Square | Common Rafter

Using your circular saw, cut along the plumb and horizontal lines. Be careful not to cut too deeply. If your circular saw cant reach all the way to the triangle of the birdsmouth joint, use the handsaw or jigsaw to finish the cuts. Remove the little triangle of wood and admire your new birdsmouth cut.

Next, test the birdsmouth cut by setting the rafter in place. Does it have the correct angle for the roof pitch you want? Does the rafters top-end reach the peak of the roof precisely? After double-checking everything, you may need to adjust your calculations and cuts. However, if the birdsmouth seems like a perfect fit, you can use it as a template to cut birdsmouth joints in the rest of the rafters youll be using.

As with any project involving saws, make sure you follow all safety protocols. Keep power tools like the saws away from children, pets, and adults who dont know how to use them properly. Depending on the size of the roof youre constructing, you may also need a friend or two to help you lift the rafters into place.

Recommended Reading: How Long Slate Roof Last

What Is A Binder In A Roof

Ceiling Binders These can be used to restrain and hold the ceiling joists above, especially if the ceiling joists are long. They can be supported either by the structure or built into the fabric of the building. They provide support to the rafters along its length and push loads back into the structure.

Chainsaw Kickback Is Caused By Binding The Blade Usually At The Nose

According to Tim, for kickback to occur, one of two things must happen: the nose of the bar needs to hit something or the bar must bind in the wood.”

Factcheck: True.

There are some other things that can contribute to chainsaw kickback, like really loose chains that are really dull.

McCulloch says kickback is usually caused “when the chain gets snagged,” usually in the kickback zone – the top corner of the nose.

Neither of these is the case in Tim’s video, and the saw slides through the 2x12s like butter.

Also Check: How Much To Fix Flashing On Roof

What Is The Standard Shed Roof Rafter Spacing

For standard shed roof rafter spacing, see rafter span chart.

There is no standard spacing it depends on what size of rafters you want to use, and the span of your shed. For instance, #2 grade 2x4s at 16 on center will span just over 9. If you get premium, kiln-dried lumber, then you could span 10.5 with the same size of lumber and rafter spacing.

When it comes time to install your rafters, consider rafter size, cost, and especially safety. Be sure your lumber and rafter spacing adheres to the load chart.

Lastly, if you live in a place with heavy, wet and regular snow, then consider your load. Consult the heavy load table, which has different values for lumber sizes, spans, and rafter spacing.

How Do You Calculate Rafter Length And Birdsmouth

Cutting and Installing I

On the rafter, you want to cut, measure its overall depth, and divide it by 3. Using this figure, measure and mark two points on each side of the vertical line, which indicates where the birdsmouth will be cut. With your carpenters square, draw a straight line between the two points you just marked.

Recommended Reading: What Material Is Metal Roof Made Of

Setting Out Rafter Lengths With A Ready Reckoner

To show how easy it is to use, let’s imagine the roof has a 3.265m total span from outside to outside of the wall plates, the ridge is 45mm thick and the pitch is 30°.

First thing to do is to subtract the thickness of the ridge from the span of the roof so that the ridge will fit perfectly between the two common rafters. Next, divide the remaining measurement by two. This will give us the span for each individual roof rafter:

3265 – 45 = 3220mm

3220 / 2 = 1610mm

Now, when we turn to the 30° page in the rafter ready reckoner and when looking up the common rafter lengths, one of the things it tells us is

The length of rafter per meter of run is 1.1543. This means that for every 1m the rafter travels horizontally, the angled length of the roof will be 1.1543m.

All we do now is use a calculator to multiply 1.610 by the measurement in the book:

1.1543 x 1.610 = 1.858m

So, the rafter lengths 1.86m long – Easy!

Mark And Cut The First Rafter Ends

The first step in building the roof rafters is to cut one rafter board to length with the ends angled properly and then to mark and cut the birds mouth and seat on the rafter.

The ridge end of the rafter, and often the eve end, will need to be cut to the angle of the roof slope. The roof on this shed is a 4 in 12 slope which makes it a 18 and one half degree angle on the board ends. A rafter or speed square will have marks for both of these angles.

Measure from the eve end of the rafter board to the long point that will be the ridge of the rafter and place a mark on the board, this is the overall length of the rafter.Use the square to make a square line on the board that goes from edge to edge of the board. Now take the square and put the pivot point, on the point where the line you made meets the edge of the board. Pivot the square until the 4 on the Common markings lines up with the edge of the board. Mark the board with this angle. You will also notice that the 18 and one half mark on the Protractor markings will be lined up with the edge of the board. Use a saw to cut along this line. Make sure to cut on the waste side of the line so the rafter size will be as accurate as possible.If you have a chop saw you can set the angle of your ridge cut on the saw and make the cuts without using the angle from the framing or speed square.

Recommended Reading: A Good Roofer Corbin Ky

How To Calculate The Length Of A Common Rafter

To lay out the common rafters, we will need to know

  • The building width
  • The width of the ridge

If there were no ridge, the run would be half the width from the center to the outside of the wall. In reality, youll need to subtract half the ridge from the measurement.

So, after marking the center line, also mark each edge of the ridge. And don’t forget to include the exterior plywood wall sheathing in the dimensions. It doesn’t always make it up to the top of the wall plates when someone is pulling numbers for rafter measurements.

Hook the outside of the building with your tape and measure to the inside of the ridge. Thats the run, 14 feet, 11-3/16 inches in this case.

Pitch is shown on the plans and is indicated as a fraction, usually with 12 being the denominator. The first number is the rise the second is the run. A 6/12 roof rises 6 inches for every 12 inches of run.

So how do you get from expensive new lumber on the saw horses to a perfect fit on top of the walls?

With a calculator and then a saw. Punch this into your calculator.

Roof Framing: Key Terms And Definitions

How To Make Common Rafters [Measure, Mark & Cut]


The rafters are the diagonal members that directly support the roof deck.

Ridge Board

The ridge board serves as a nailing surface for the opposing rafters and provides no structural support.

Rafter Tie

The rafter tie resists the outward forces imposed on the load-bearing walls. As the rafters sit on top of the wall plates at an angle, they exert horizontal forces on the exterior walls. Rafter ties, which often double as ceiling joists, prevent these horizontal forces from causing the walls to “pancake.”

Ridge Beam

Some framing systems use a ridge beam combined with gable-end posts instead of a ridge board, which provides structural support to the rafters. For roofs with less than 3:12 pitch, codes require ridge beams because they reduce the extreme outward forces on the walls. A ridge beam can eliminate the need for rafter ties, which is often paramount for the cathedral ceiling aesthetic.

Don’t Miss: How To Soft Wash A Shingle Roof

The Rafter Length Calculator Formula: Determine Rafter Size In Minutes

Before building a gable roof or hip roof, you need to know your common rafter sizes.

Seasoned framers know how to calculate this crucial measurement no matter the project requirements thanks to millennia-old trigonometry.

To put you on par with the experts, we provide the formula carpenters use to figure out common rafter sizing and where to mark cuts.

All you need is paper, a pencil, and a scientific calculator.

It might trigger traumatic memories of high school math class, but the sheer bliss in knowing that you sized your rafters correctly is well worth it.


Most Popular