How To Sister Roof Rafters

A while back, a relative of mine called me about a roof issue he was having – his roof was sagging and he thought he needed to fix some of his roof rafters. When we went up to the attic, it was clear some of the rafters were in rough shape. Our solution was to install new rafters next to the old ones, but at the time we weren’t quite sure how to sister roof rafters.

To sister roof rafters, you should use the same sized lumber as the rafter. The length of the sistered rafter should extend at least 3’ past the problem area on the existing rafter, on both sides. Glue the new rafter flush against the existing rafter, then use 16d nails plus a carriage bolt every 16” to affix the sistered rafter.

Many varying circumstances may alter how you sister your rafter. You may need to sister the entire length, at which point you’ll need to figure out how to get a 16’ piece of wood – or longer – into your attic. You may also consider the use of steel or engineered wood as a sister rafter.

In this article, we’ll go over all the reasons for sistering a rafter, the methods you can use, and the various materials available to you when it comes time for you to fix your roof framing.

How To Sister Roof Rafters

What Does Sistering Mean In Construction?

The term “sistering” means to attach a similar or identical sized piece of construction material – usually wood, steel, or engineered wood – to an existing framing member in a structure. The purpose is to reinforce that framing member in the event of deterioration or failure.

In this article, we are covering sistering a roof rafter. In new construction, roof rafters are 2×8 or greater. Sistering this type of rafter would require the use of 2×8 material placed flush against the existing rafter. The length and type of sistering material can vary for many reasons, but the idea is to reinforce the bad rafter with a new, stronger rafter fastened flush.

Is Sistering a Roof Rafter Effective?

Sistering a rafter is highly effective if done properly. Imagine a splint on a broken arm – that’s the idea of a sistered rafter. There are many instances where a rafter can – and should – be sistered:

  • Rotten – full or partial
  • Cracked
  • Sagging

In many old homes, rafters are undersized – using 2×6 or even 2×4 rafters. In time, these rafters can sag. It is critical to fix this sag because the ability of the roof to withstand its original load is compromised.

Picture this: when your old home was built, the roof was meant to withstand loads at a certain pitch. If the pitch “sags”, then it will have greater loads as snow and other loads will impact the structure more. Thus, roof failure is a distinct possibility and must be fixed.

Similarly, broken, rotten, or cracked rafters should be fixed before they fail altogether. If you don’t go into your attic often and check your rafters, you would most likely notice due to sag. If there is no snag, it won’t hurt to check the condition of your rafters. More than likely you are good to go, but it never hurts to take 20 minutes to check.

Building Code For Sistering Rafters

Building code does not include criteria for sistering rafters, as it is a repair and not part of new construction. Since there are so many variables that go into how and when you might sister a rafter, there are no clear guidelines offered by any major building compliance organizations such as the IRC.

Although there are no guidelines written in stone, a rule of thumb is to sister a rafter 3’ beyond the point of damage in either direction. If the entire rafter is sagging, then the entire rafter should be sistered.

For cracks and rot, the damage is often isolated. In these instances, a sistered rafter does not often have to be full length. For sags, where it is harder to tell the length of the deterioration of the rafter, then a full sistered rafter is necessary.

How to Sister Roof Rafters: Step by Step

Sistering roof rafters

Sistering a rafter is a fairly straightforward fix and can be accomplished by any experienced DIYer. The main problem is often getting a long piece of lumber into the attic and putting it in place, where headroom and movement are constricted. Tools and materials you’ll need for this job:

  • Hammer
  • 16d nails
  • Carriage or lag bolts with washers
  • Drill
  • Wrench, socket, and/or impact driver for bolts
  • Adjustable angle square
  • Circular, miter, or hand saw for any cuts

Once you have your tools in place, you’ll want a plan for getting your materials up to the attic. It’s best to measure your cuts in the attic first – measure three times, you don’t want to be hopping in and out of the tight space – then cut outside. Once you have your cuts, then bring your material back for the final install. Here are the step by step instructions for sistering a rafter:

Assess the Damage

Knowing the extent of the damage will determine the length of your sistered rafter. A small crack or area of rot will result in a sistered rafter of not much longer than 6’. Large sags often require sistering the entire rafter.

Measure Area to be Sistered

First, use the same sized lumber as your rafters. IF you have 2×6 rafters, use a 2×6 sister. Next, cutting a short sistered rafter is simple – no angle cuts are required. For full-length sistered rafter cuts, you’ll need an adjustable angle square.

Measure the angles of both the birdsmouth cut, where the rafter meets the top plate of the wall framing and the ridge beam cut. Then measure the distance from the top plate to the bottom of the ridge beam where the rafter meets the beam. These measurements will allow you to cut a full-sized replacement, even if the rafter is sagging.

Cut Lumber and Position

This is often the hardest part. After you cut your lumber, you have to get it into the attic. For most homes, this means angling a full-sized rafter into a tiny attic hatch, which in many cases is impossible.

One solution is to remove a section of the soffit where the damaged rafter is and slide the sister rafter up and into the attic. The soffit can be carefully removed and reinstalled without damage, and you’ll probably have to move some insulation around. Otherwise, this is your best option if your attic access is not ideal for long pieces of wood.

Finally, pre-drill your bolt holes before you lift your rafter into the attic. This will save you time and keep you from having to bring your drill up into the attic.

Jack Rafter If Necessary

If the rafter is sagging excessively, you’ll need to jack it up. This can be done by installing an under purlin, which runs flush against all the rafters in a perpendicular direction. Use 2×4 lumber and nail the purlin to the middle of the entire span of rafters.

Then place a prop beneath where the purlin intersects the damaged rafter. A prop is another piece of 2×4. You’ll need a bottle jack supported by 2x lumber that overlaps several ceiling joists. Once placed, the bottle jack can support the prop. Jack carefully so the rafter raises to the desired height.

At this point, you can either slide your sister rafter into place or get another prop – 2×4 – and nail one end to the damaged rafter and another to a ceiling rafter below. This will hold it in place to allow you to slide the new rafter in. Nail the prop to the side of the damaged rafter that will not be sistered.

Install the Rafter

Installing the rafter requires using nails and bolts. If repairing a crack or rot, using 3 16d nails every 16” is adequate. If fixing a sag, you’ll need to use that same nailing pattern, plus carriage or lag bolts every 16”, as well. Stagger the nails and bolts, so the bolts are midway between the nails – 8”.

3.5” lag bolts that are ½” in diameter will work for bolts. Pre-drilling is essential to avoid wood damage. You’ll still need torque to install them, so a socket that fits the bolt head is required. Otherwise, use 4.5” carriage bolts and use a washer with the nut.

Other Options For Sistering Roof Rafters

If you want to use a different type of material for sistering your rafter, then you have options. Use of plywood, OSB, or even steel can be used in place of dimensional lumber. Reasons for using these types of materials vary, but they can be just as strong or stronger than regular lumber.

Engineered lumber

Engineered lumber is anything that isn’t pure wood – plywood, OSB, and any other type of “fiber” product are engineered. Plywood and OSB (orientated strand board) are the most common substitutes for sistering a rafter. Since these materials only come in 8’ lengths, they are only used for shorter rafter sistering repairs.

¾” thickness for plywood or OSB is best when sistering a rafter. This will provide adequate holding power to support a damaged rafter. Use the same nailing and bolt patterns as described above. Some contractors claim the use of construction adhesive in conjunction with fasteners is a good idea, and it certainly couldn’t hurt. If you choose, use a product like PL Premium.

Finally, the rule of 3’ to either side of the damaged rafter still applies with this type of material. Always cut the width of the material to fit the width of the existing lumber.

Steel Flitch Plate

A more heavy-duty option is to sister a damaged rafter with a steel flitch plate. A flitch plate is a flat plate of steel of varying thickness, width, and length. You’ll need to find a local steel fabricator or an online vendor who does flitch plates.

Like plywood or OSB, the flitch plate should be at least ⅛” thick, although 14” is better. Make sure it is fabricated with holes for ½” bolts every 16”. Lastly, the width of the plate should be similar to the width of the existing rafter.

How to Sister Rafter Tails

How to Sister Rafter Tails

Rafter tails, which are the ends of rafters that jut out beyond the house, are the most prone to rot. If so, they should be sistered to prevent the roof ends from sagging, which could cause further water damage to the roof and foundation.

For every foot that you sister a rafter tail, the sistered rafter tail must extend at least 2’ past the top plate of the house. So if you have 24” rafter tails that need to be sistered, then the sistered rafter must extend 4’ into the attic against the existing rafter.

Use the same fastening techniques as you would with sistering a rafter from the inside – 16d nails. Bolts are not necessary unless the rafter tail is extremely rotten.

How Much Does it Cost to Sister Rafters?

The cost of sistering a rafter is the cost of the lumber – which is nominal – and the fasteners. If you are only using 16d nails for a small section of a rafter, then this project will cost you less than $75. If you need a full-sized sistered rafter plus lag or carriage bolts, then you are going to spend closer to $200.


Ignoring rafter damage is a bad idea, even if you don’t experience huge snow loads. Sagging on your roof can lead to moisture damage, which can lead to mold or roof cave-ins. A quick inspection in your attic or a visual inspection from outside for sags is all it takes to prevent future issues with your roof framing.

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