Roof Sag

When looking at a prospective property, keep a little tip in mind.

If the house has a flat front with no bays, stand under the front wall. Then, look up at the gutter. If it is badly bowed and not straight, walk away. The house therefore has roof sag.

Unfortunately, roof sag is very expensive to fix. The worst case scenario is removing the roof.

How does roof sag occur? Firstly, roof timbers are subjected to extra weight. However, this is weight they were not designed for. Case in point: Slates being substituted with concrete tiles, a very common occurrence nowadays.

Overall, concrete tiles are cheap and easy to install. They are heavier than the slate originals.

It is therefore very important to reinforce roof rafters before working.

Normally, a roof involves a series of rafters. These are fixed to a wooden wall plate at their base. Then, they go up to a ridge at the top.

Roof Sag detail

Roof Sag detail

The rafters are fixed to the wall plate using a bird’s mouth joint. This method of securing the bottom of the rafter means they are unable to slip out of position.

Furthermore, if a builder then overloads the rafters by installing heavy concrete tiles, one of two things might happen:

a) The wall plate will be screwed securely into the top of the wall. Therefore, the downward pressure on the rafters will push the top of the exterior wall outwards. As well as this, it will bow the brickwork. Bowed brickwork front or back is serious and will almost certainly involve the roof.

b) The wall plate will not be fixed to the wall. Thus, the downward pressure of the concrete on the rafters will slide it over the wall and in danger of eventually ending up in the street.


Now if, as is often the case, the fascia board holding the gutter is attached to the end of the stressed rafters. As a result, the guttering will move accordingly, hence the bow-shaped alarm signal.

Another reliable thing to look for in regards to roof sag is the line of the ridge. It should be horizontal. If it is badly dipping in the middle, it is because the rafters have sagged or bowed. This pulls down the ridge down with them.

From my experience, particularly of terraced housing, the rafters never move much near the party walls. However, they are affected mostly towards the centre which is why warning signs are always bow-shaped.

One final tip. Unscrupulous builders sometimes mask the dip in the ridge by packing the tiles in the middle with extra sand and cement. Hence the ridges near the walls are flat on the tiles. The ones in the middle have up to 40mm of mortar underneath.

Remember, unless it’s a Tudor timber-framed building that will twist and sag safely to great aesthetic effect, avoid anything that is bowed. You will be doing yourself a favour.

Roof spread was detected recently in a property where the concrete tiles had pushed the front and back flanks down together with the top flat roof. Gaping cracks and gaps meant the structure needed a complete rebuild.

At the back, not the fascia and guttering had the tell-tale signs of bowing and being pushed away from the brickwork.

When the client advised me they would be moving in the next 5 years, I agreed to stabilise the situation by adding purlins and “H” frame timbers in the loft. We then attended to the waterproofing of the roof itself.

As I explained to the owner, any future purchaser would be made aware by a surveyor of the measures we have had to take and almost certainly would recommend a rebuild of the roof to incorporate a loft.

Photo Illustrating Condition

Roof Sag Condition

Call The Roofing Crew on 07973 429 945. Alternatively,