If your roof has a slope of 4 in 12 or under, make sure that you reinforce the underlayment before putting down new roof surface. This is important because roofs with a low slope will have it more difficult to shed water than a roof with a steeper slope. Because of this, low-slope roofs are more likely to cause water to be backed up underneath the roof shingles.
You can use asphalt shingles on roofs with a slope down to 2 in 12 but this is only if you use two layers of 15 pound roofing felt in the underlayment. Alternatively, you can use one layer of felt of 30 pound if you so prefer. With a low sloping roof, it is best to use shingles with mastic adhesive dots or lines along the upper edges and this is to prevent the tabs to be lifted up by the wind. Here are some useful tips:
- Use a 19 inch wide piece of felt and nail it down along the eaves, allowing for an overhang by the drip edge.
- Apply a layer of plastic cement onto the felt, using a roofing brush or a trowel. Two gallons per 100 square feet should be about right.
- Cut a 36 inch wide piece of felt and press it into the cement and nail it down, using galvanised roofing nails.
- Apply a layer of plastic cement to the top 19 inches of the felt exposed and cover the cemented part with the next layer of felt and nail it down.
- Repeat until you are roughly about 24 inches inside of the interior wall line of the building.
- From now on, lay down double layers of felt until you reach the ridge. Do not use cement.
Wood Shake Roofs
You could possibly use wood shakes for roofs with a 3 in 12 slope, providing you get the go ahead from a building inspector. However, because wood does not shed water easily and it can deteriorate quickly, many roofing specialists will advice against it. If you do come across this, make sure to add a 15 pound layer of underlayment underneath the shakes. This is in addition to the 30 pound felt interlay that has to go down.
Just as with wood shake roofs, tile roofs can be approved for roofs with a 3 in 12 slope. The underlayment used for this should be of 15 pound felt, used as a double layer and cemented together with plastic cement.
Whenever you have work done to your roof, make sure to check that the gutters are in full working order as well!