Roof cement is primarily used to hold roof tiles, hip tiles and ridge tiles securely in place, it also keeps out rainwater.
The correct mix ratio is 3 parts sand to 1 part cement, this is much stronger than bricklayers use (5-1) as the cement on the roof must withstand more driving rain than a typical brick wall. Strong cement isn’t likely to wash out.
If an incorrect ratio is used, this is the outcome (click to enlarge):
As you can see from the photo, the cement has now turned to dust and is no longer securing the tiles to the roof.
Here are my 6 steps to remove and correctly replace the cement under a ridge tile, if you are looking to replace the cement on a gable/apex then you need this page instead:
- Lift all ridge tiles
- Remove all cement and dust off tiles
- Replace any damaged tiles/ridges
- Prepare new cement, 3 parts sand to 1 part cement
- Lay cement around perimeter of ridge tile and bed tile onto cement
- Point-in surface of new cement
These photos explain my 6 steps more clearly:
As you can see from the photos above, the ridge tiles are securely “bedded” down onto the new cement, which is at least two inches thick. Once dry, this cement will hold the ridge tiles in place for years to come.
How *NOT* to Cement Roof Tiles
Unfortunately, lifting the ridge tiles and re-bedding them is a time consuming project that must be carried out by a competent roofing contractor. Thus it can be an expensive exercise.
Some roofers will instead follow a different method that can only be described as a bodge; the ridge tiles are left in place and a thin layer of cement is smudged or “pointed” over the existing cement.
When viewed from the ground, the finished work is indistinguishable from the correct method HOWEVER that thin layer of cement will not last long. After 1-3 years you can expect it to peel off and fall down the roof, it usually ends up in the gutter or on the floor.
Here is a photo of a bodged ridge section (click to enlarge):
Looking at the photo above, you can clearly see that are two layers of cement. The original cement has been “pointed” with a new thin layer that is now falling off.
Here are some more photos of the same roof:
How Much Does it Cost To Replace Roof Cement?
Find out here how much it typically costs to replace roof cement.
Q: Why does roof cement crumble/become flaky?
A: Usually because the original house builder used a sand/cement mix ratio that was suitable for bricklayers (5-1) but not for the roof (should be 3-1)
Q: If I “point” over the existing cement with a thin layer of new cement, how long will it last and why does it come loose?
A: Cement shrinks as it dries, so a thin layer will not adhere correctly. Expect it to last around a year before sections start to crumble (see photos above).
Q: If I completely replace the cement with a new 3-1 sand/cement mix, how long will it last?
A: 50+ years although on a newly constructed house it may crack as the roof timbers take a year or two to “settle” under the weight of the recently laid heavy tiles.
Q: Does it cost considerably more to completely replace the cement, compared to just “pointing”?
A: The tiles on a detached house can be re-pointed in half a day, it would take 2-3 days to completely lift and then re-bed the tiles, so yes it costs much more.