For anyone with an experienced builder’s eye, a crooked ridge or uneven, sagging roof stands out like a sore thumb. Not everyone has that seemingly magical ability to determine if a roof is straight and even merely by looking at it.  The average do-it-yourself type may have to measure carefully to spot variations in an uneven roof, but even for the neophyte, some defects are pretty easy to notice. Maybe your carpenter-neighbour pointed them out. However defects come to your attention, a bad roof can be straightened by various means; so if you are planning to replace your old roofing, now is the right time to look at it more closely.

You mean I have to climb up there ?

You can see the ridge of most roofs from the ground, but to find problematic surface areas it’s usually necessary to head up onto the roof with a straightedge.

Let’s be careful up there, use proper ladder and roof safety protocol. Have someone else handy to help in a pinch. A word of caution is probably in order–if you get dizzy easily, or you’re not comfortable working at heights on ladders or roofs, it’s probably a good idea to hit the Yellow Pages.

Even if you’re comfortable at heights, if you have a roof that’s steeper than a 5:12 pitch, you may want to seriously consider other options. With very steep, now-trendy high-pitch roofs, it’s always a better idea to find a roofing contractor that is specially equipped to tackle roofs of that type.

“ How the problem is repaired is determined in part by the type of new roofing you plan to install.”

Inspect the roof structure

Is the ridge along the peak of the roof straight? A sagging ridge can signal structural problems such as a bad ridge board, overworked trusses, or failed collar ties between matching rafters. They can all be  replaced, repaired or straightened and reinforced, which is usually the least expensive option..

A straightedge placed horizontally across any three trusses on the roof can also reveal potential problems developing. It will reveal a weak truss, a broken truss or warped rafter, all of which will contribute to an uneven roof. Not to panic, any of these conditions can be improved by first determining the nature of the problem.

Insertion of additional supports can prevent catastrophic failure. Analyze problem structural members from the attic underneath. For a failed rafter, the fix may be the installation of a reinforcing rafter into place along side one or more structural members if necessary. Same goes for a damaged truss.

It’s really not as complicated as it sounds. A new, carefully measured and pre-cut rafter is inserted at the fascia or even through a slot in the roofing at just the right location, and the straight member is jacked into position to correct the sag. Once the plane is corrected, the new support is clamped tightly to the existing problematic rafter or truss as required . For the best result use 3″ anodized screws to laminate the reinforcing member to the top chord or rafter so they won’t separate. Think straighten and reinforce , it is as simple as lunch if you are moderately skilled and brave enough to tackle it.

Let’s get back to the roof surface

Luckily, most sags in roofing are a just a result of tired sheeting. Depending on the severity of the problem and the type of new roofing to be installed, some defects can be corrected by installing supportive blocking underneath the sheeting in select spots. If the whole roof displays sag pockets, consider re-sheeting it for asphalt shingles, or strapping it if steel sheet roofing is to be applied.

When you’re wandering around up there, mark the low areas laterally between the trusses or rafters. Problematic areas are typically caused by over-span for the specific roof sheeting used, a result of structurally weak sheeting, or stress-induced from extreme snow load. Sags in sheeting are common between trusses in wide spans on older roofs where inadequate sheeting or the incorrect grade of O.S.B. sheeting may have been used as a substitute for plywood .Again, straighten and reinforce comes to mind for minor areas.

The re-roofing plan suggests the fix required

Checking across between three trusses with a straightedge will quickly reveal sags, and if the space is minor and hardly noticeable, less than a half inch, –job done, don’t worry about it. If the space under the straightedge looks like you can get your boot under it, an inch or more and the roof droops badly, that spot is asking for some serious attention before re-roofing.

How the problematic areas are repaired is determined in part by the type of new roofing you plan to install. Minimal blocking inserted underneath and laterally between trusses can eliminate small problem areas. Use dimensioned lumber the same size as your trusses or rafter, cut the blocking accurately, and install it securely with long screws or 3″ Ardox® spikes. That’s the spiral kind . If you find you can’t force the weakened sheeting into a straight condition, consider replacing that section if  new shingles are planned. .

To install a new roof over an old surface that is sagging badly, there are several approaches that can be taken :

  • Old shingles may be removed or left in place, followed by the installation of new sheeting directly on top of the old sheeting —assuming the trusses themselves are straight. Fasten the new sheeting on the surface only at the location of the trusses, just like installing new sheeting directly on the framing itself, and the new sheeting surface will look straight.
  • Alternatively, the roof can be strapped directly on top of the old shingles vertically along the trusses with the plane corrected as the strapping is installed . The roof is then re-sheeted with plywood as in new construction, and shingled.
  • If new steel sheet roofing is your chosen material, the roof may be strapped horizontally across the trusses, correcting the plane of the strapping as required, and steel sheet roofing is then applied directly to the strapping. Ensure the spacing of the strapping is correct.
  • To be environmentally greener, use the above strapping method for steel sheeting but leave the old shingles in place. That will save you the tear-off costs, labor and landfill fees.

While you’re at it, don’t forget to check out the fascia. That’s the board along the edge of the roof. Make sure it’s straight and solid, and replace any decking on the lower roof edge that is decayed. Don’t forget the drip edging, Ice & Water Shield® and flashing.

Once you have the sags and problem areas repaired and the roof looks nice and straight, you’re good to go. Look it over. Get that buddy with the experienced builder’s eye to approve it, and while he’s there, get him to help you put that new roofing in place.