Rotten Garage Roof Joists: Repair or Replace?

How to tell that joists are rotten

Garage roof joists are the timbers that run across the building and carry the deck and covering. They are usually softwood, often small in section, and many garages were built cheaply. Rot shows first at the ends, where they sit in or against a damp wall, and under any area that leaks. A ceiling or exposed underside that has dark staining, a sag in the middle, flaking timber, a musty smell or white fungus strands all point to trouble.

The simple test is a screwdriver. Sound timber resists, and rot lets the point sink in with little effort. Dry rot leaves cubical cracking and a dry, crumbling feel. Wet rot leaves the wood dark, soft and stringy. Both are caused by moisture, and neither goes away once the wood has dried. If the roof also moves when somebody walks on it, keep off it. Our page on why a garage roof leaks will help you work out where the water is coming from.

Repair or replace: how the decision is made

A repair makes sense when only a few joists are affected and the damage is in a short length. The fix is to cut back to sound timber and bolt a new length alongside, or to replace the whole joist where it can be got at. Treated timber is used, and the ends are kept clear of damp masonry with a damp-proof barrier or a joist hanger.

Replacement is the better route in other cases:

• Rot has run along most of the joists, not just one or two ends.
• The wall plate, which spreads the load along the wall, has softened.
• The joists were always undersized for the span, so they sag even when healthy.
• The deck above is rotten too, so the whole roof is coming off anyway.

When a roof is replaced from the joists up, it gives the chance to build in a proper fall for drainage and to choose a covering that suits. Our guide on whether to repair or replace a garage roof sets out how to weigh that up.

Why joists rot in the first place

Timber does not rot dry. The causes are a leaking covering, blocked or missing gutters that overflow onto the wall, poor flashing where the roof meets the house, condensation under a cold roof and ground damp rising into a wall. A roof with no ventilation at the eaves traps moist air. In an asbestos cement garage, long-term condensation drips from the underside of the sheets onto the timber below, which is hard to spot because the leak is slow.

The cure is to find the source of the water and stop it before any new timber goes in. Otherwise the new timber follows the old. If the garage has a corrugated or cement sheet roof, see condensation under a metal roof, which explains why the problem recurs.

What a repaired roof structure looks like

When joists are sistered, the new timber is the same depth as the old and runs far enough past the damaged section to sit on sound bearing at both ends. It is fixed with coach screws or bolts and not nails alone, so the two timbers act together. Where a joist end has rotted in the wall, the end is cut back and carried on a galvanised joist hanger or a bearing plate, leaving an air gap and a damp-proof layer between the timber and the brick.

If the wall plate has gone, it is replaced in lengths, one at a time, with the roof propped. It is slow work, and it is the point where a patch becomes a rebuild. Where more than a few joists need that treatment, a full replacement is faster, safer and usually less costly overall, because the deck, the fall and the covering are all done at once with new timber of the right size.

What a survey and a fix involve

We start with a free survey. We photograph the joists, the wall plate and the underside of the deck, probe the timber where it looks suspect and tell you what we find in plain words. If a patch repair is enough, we say so and quote for just that. If the structure needs replacing, the fixed written quote names the timber sizes and the covering.

Replacing a garage roof can bring Building Regulations into play, particularly if the garage is attached or the structure changes, and we handle that side where it applies. Read more on our garage roof replacement in Basingstoke page.