How to Repair a Roof Sag

Written by chuck brown
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How to Repair a Roof Sag
Older roofs are prone to sag as the rafters age (rural house with green roof image by Astroid from

Roofs are a major part of a house, and among the most expensive parts to maintain. Roof jobs run into the hundreds and even thousands of dollars, so it is smart to keep the roof in good repair. Leaky roofs can cause serious interior and other structural damage very quickly. Saggy roofs should get immediate attention when they are noticed. Sag is a big indicator that the roof has structural damage in the trusses or the sheathing, or perhaps foundation issues. The first two problems are the most common.

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Things you need

  • Flashlight
  • 3-foot Angle iron, 2-by-2
  • 6 to 8 Lag Bolts and Washers
  • Ratchet Wrench
  • 4 8-penny Nails (Location)
  • Chalk Box
  • Scraper
  • Hammer
  • Pry Bar
  • Sheathing
  • Felt
  • Shingles
  • Roofing Nails

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  1. 1

    Enter the attic and go to the point you were able to see the sag as you viewed the roof from the outside. Use a strong flashlight and carefully inspect the entire roof truss. Shine the light into the corner of the truss, and move it slowly across the entire truss.

  2. 2

    Repair any broken, split or damaged trusses instead of trying to replace them. Drill three or four holes into one side of a 3-foot piece of 90-degree angle iron that is 2 inches on each side. Drill an equal number of holes into the other side, being sure to stagger or offset the holes.

    How to Repair a Roof Sag
    Angle iron bolted into place properly will reinforce the compromised timber (attic window image by green 308 from
  3. 3

    Place the drilled angle iron on the damaged rafter so the damage is in the middle of the angle iron. Drill through the holes of the angle iron into the rafter on the bottom and on just one side of the damage.

  4. 4

    Insert 3/8-inch diameter lag bolts and washers. Use bolts at least 2 inches longer than the space between the end of the angle iron not bolted down yet and the rafter. (For instance, if the gap is 1 inch between the loose end of the angle iron and rafter, use a 3-inch lag bolt.) Bolt the 2 by 2 angle loosely into place on one side of the damage.

    How to Repair a Roof Sag
    Lag bolts and washers will give a much stronger hold than nails or spikes (Do it yourself image by Hedgehog from
  5. 5

    Repeat the same procedure for the other end of the angle on the other side of the damage, using the same length bolts. Use a ratchet and work the bolts alternately on both sides of the damage, gradually bringing the damaged rafter back into alignment. Tighten down all six or eight bolts once the rafter is realigned.

  6. 6

    Repair broken or rotting sheathing by replacing it. Locate any edges of the damaged piece. Drive nails at four corners of the broken sheathing, between rafters, up through the sheathing, from the attic. Locate the nails driven through from inside the attic, and mark the damaged area on the roof's shingles with a chalk box.

    How to Repair a Roof Sag
    Sheathing is difficult to patch, so replacement is the best repair choice (new construction house image by Photoeyes from
  7. 7

    Remove the shingles and felt within the nailed off area using a scraper. Pull all nails from the sheathing. Use a reciprocating saw and make the vertical cuts on sheet of broken sheathing, avoiding the rafters. Pry off the sheathing with short pry bars and hammers. Replace the sheathing, felt and shingles.

Tips and warnings

  • You will need help with most roof repairs
  • Roof repair should not be put off very long.

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