How do I repair a sagging beam under my deck in Bathurst NB?

Repairing a sagging beam under a deck in Bathurst requires identifying the cause of the sag, then either adding a support post beneath the beam or sistering a new beam alongside the existing one, depending on the severity. A sagging beam is a structural concern that affects the safety of the entire deck, and Bathurst's northern New Brunswick climate, with its heavy snow loads and deep frost penetration, puts particular stress on deck substructures.

Before you begin any repair, determine why the beam is sagging. The three most common causes in the Bathurst area are rot at the beam-to-post connection, undersized lumber that has gradually deflected under load, and frost heave that has shifted a supporting post out of position. Get under the deck with a flashlight and a screwdriver and examine the beam carefully. Press the screwdriver into the wood at every point where the beam sits on a post or where hardware connects. If the screwdriver sinks easily, you have rot, and the repair approach changes. Also check whether the supporting posts are still plumb and firmly seated on their footings.

If the beam itself is structurally sound but has deflected over the span between posts, the most straightforward fix is adding an intermediate support post. This involves pouring a new footing at the midpoint of the sag, then installing a pressure-treated 6x6 post with an adjustable post cap on top and a post base bracket at the bottom. Before setting the post, use a hydraulic bottle jack placed on a solid footing to carefully lift the beam back to level. Raise it slowly, no more than 3 millimetres at a time, checking that the deck surface above is returning to a flat plane. Once level, cut and install the new post, making sure it bears fully on the beam above and the footing below.

For beams where rot has compromised the wood but the damage is limited to one face or the area around the post connection, sistering is the preferred repair. This means bolting a new beam of equal or greater dimension alongside the existing one. Use pressure-treated lumber rated for ground contact, and through-bolt the new beam to the old one with 12-millimetre galvanized carriage bolts spaced every 400 millimetres along the length. The new sister beam takes over the structural load while the old beam provides additional rigidity. Before sistering, jack the beam back to level using the same gradual approach described above.

In Bathurst, where frost penetrates to significant depth during winter, pay special attention to the footing situation. If the sag is caused by a frost-heaved footing, the beam repair alone will not solve the problem. The footing needs to extend below the frost line, which in the Bathurst area means a minimum depth of 1.2 to 1.5 metres. If existing footings are shallower, consider installing helical piles as replacements. Helical piles are screwed into the ground below the frost line and provide a stable, permanent bearing point unaffected by freeze-thaw movement.

If the beam rot is extensive, meaning the wood is soft across more than half its cross-section over a significant length, the beam needs full replacement rather than sistering. This requires temporarily supporting the joists above with shoring posts while the old beam is removed and a new one installed. Structural beam repair in New Brunswick typically costs between $2,000 and $5,000 depending on the scope, and given the safety implications, this is one area where hiring an experienced deck contractor is strongly recommended.

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