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Property Surveying Blog
December 11, 2017

Why do Roofs Sag?

Written by Tughan Musa

In my first blog post, I am going to be looking at something that all properties have in common, the roof!

Roofs are essential to the fabric of every building, they weatherproof it while also channeling water away from the upper parts of the property.

Roofs traditionally are constructed to form a shape of a triangle with the sides, known as the pitches constructed of timber rafters joined together by purlins, binders and joists, which then rest on the wall plate beneath.

Roof slopes are designed to be level and straight, therefore when you see “dishing” to a roof, that can often have a dipped or curved appearance, it should be a red flag that something is not quite right! More important than the roofs aesthetics is its structural integrity. A sagging roof is not just an eyesore, but can be an indication of a failing roof, where in extreme cases may lead to collapse.

So, what causes a Roof to sag?

Generally speaking, there are several potential causes that homeowners should be aware of:


Excessive weight load on a roof is one of the most common causes of sagging. Every roof will be designed to handle a certain weight and carry a certain load, if exceeded it can cause the roof to deflect that has a dipping or sagging effect to the appearance of the roof surface.

Victorian properties constructed around the turn of the century traditionally had slate roof coverings. However, in the 1980s, many had their roof coverings replaced with concrete tiles as the previous coverings reaches their end of life. While concrete tiles were quicker and easier to install, they were considerably heavier than the original slates, therefore causing the rafters to bow under the additional load.

An unknown and slowly developing issue we expect to see more of over the forthcoming years is overloading caused by solar panels, as these add weight in the same way that concrete tiles did.

Weak Roof Timbers

As confirmed above, roofs are built up using timbers. While these timbers were fit for purpose when constructed, over the tenure of their life they can be weakened for a number of reasons, such as botched loft conversions or the installation of roof lights without property strengthening the surrounding rafters.


These are the timbers that follow the pitch of the roof and form the inner skeleton of the roof. If these are undersized or incorrectly positioned, it can cause the roof to sag and dip.


These are the timbers than run from flank wall to flank wall of the property and aid in the support of the rafters above them. If Purlins are undersized, missing, or poorly connected to the party wall to support the rafters it can result in deflection which often has a dipping or bulging appearance to the roof surface.

Another issue can arise when homeowners unknowingly remove struts and collars in their loft to create extra space for storage, again this will cause roof deflection.

As confirmed, purlins run from wall to wall. If the wall surfaces they meet are damp or wet, this can lead to the purlin end rotting, which in turn leads to roof deflection.

Roof Spread

Roof spread occurs when there is a failure to adequately support the main rafters. This occurs when the ceiling joists is no longer holding the walls together, which then means the rafters push the top of the walls out causing the roof to dip in the middle.  That in turn can cause the purlins to drop, subsequently resulting in cracking to the edge of the gable wall.

Beatle Infested or Rotten Timbers

Decay caused by moisture is the timbers’ greatest enemy. Dampness leads to mold growth, wood rot and beetle infestation.

It is unusual to find significant rot to roof structures, except where timber has been exposed to damp for long periods due to leaks. One example is when the lead flashing is defective causing damp to penetrate the room/area beneath.

When the moisture content of wood gets above 15% it can become attractive to beetles and other pests. However, in my experience, it is rare for infestation to be an issue in well-ventilated lofts.

Solutions and Fixes

With any property defect, there is usually a simple and tried and tested solution for repair, in the case of roof sagging/deflection this is as follows;

Overloading Roof Solution

If the roof timbers are overloaded with heavier coverings, weakened with botched extensions, or undersized when originally built, then it may be necessary to reinforce them with additional purlins and struts.

Defective Purlin Solution

Rotten purlin ends need to be cut out, replaced and then supported on new metal hangers.

When removing a load-bearing wall, it is important to engage a Structural Engineer to advise on how to transfer loadings to the main walls before undertaking works.

Roof Spread Solution

If the ceiling joists are not properly secured, they will need to be tied in to prevent further movement.

To prevent rafters from spreading, bracing struts can be used to tie the rafters to the ceiling joists, or a metal tie bar spanning through the building at ceiling joist level with metal plates on both sides of the wall.

Beatle Infested or Rotten Timbers Solution

Once the cause of the problem is identified, the rotten timbers must be treated or cut out and replaced.

While many damp specialists will recommend chemical treatment, generally it is unnecessary if the source of the dampness has been identified, has been removed and the timber given the opportunity to dry out. Reducing moisture and dampness is key to an effective solution for fungal beetle attack.

The roof is an integral part of your home, and it is important to maintain it properly and ensure you are doing all you can to preserve it. Roof repairs can be expensive and disruptive. If you think you might be on the receiving end of a potential roof issue, give one of our Surveyors a call now and we will be happy to advise.