Your tile roof maybe weatherproof but it is not exactly entirely. To keep water out of your home and eliminate moisture between your tile and roof, you should install an underlayment. While this sounds like an extremely difficult job, with the proper tools and instruction, you will be able to complete this job in less time than you think. After determining how many rolls of underlayment is needed, you are ready for the job.
Step 1 - Make a Plan
Ensure that you measure the distance from the ridge to the eave and from gable to gable to calculate how many sheets of underlayment you will need. You will also need to know where your overlaps will be. You will not want to lap the sheets over in the joints of the roof deck. A chalk line will be needed to to keep sheets in line as much as possible.
Step 2 - Prepare Underlayment
Unroll the underlayment on a flat and smooth surface. If you are applying the underlayment in cold weather conditions, be sure that you don't damage the material by rough handling. Keep material at room temperature in the winter to prevent cracking
Step 3 - Cut Underlayment
With your utility knife, cut the sheets to in 18 foot sheets. This will allow the sheets to be easier to work with when applying them to the roof deck.
Step 4 - Lay the Roof Valley First
You will now need to cut the sheets of underlay again. Cut a strip that is 18 inches wide to cover the lower layer of the valley and cut another strip that is 36 inches wide to cover the upper layer of the valley. Once you get the sheets to the valley, fold them to fit the angles of the valley. Nail the lower layer sheet every 6 inches close to the edges.
Put a coat of roofing cement on the sheet and ensure the ends are not lapped. Add the end of the upper level sheet to the lower level sheet and lap the sheet about 6 inches. Ensure the ends are cemented, but do not nail them at the overlap point. Nail the upper sheet on the opposite edges 6 inches.
Step 5 - Lay the Main Roof
Laying the main roof will consume the most of your time. Lap the main roof sheets with the valley sheets. Put a layer of roofing cement and nail the sheets every 3 inches. After this point, you can nail the underlay every 6 inches staggered. While laying the main roof, insure that you get the underlayment securely around any fixtures on your roof. Get the material as close as you possibly can and use your utility knife to make cuts around the fixture. Place cement on the underlay and nail around the fixture. When you get to the peak of the roof, you will need to cut the underlayment even with the edges of the plywood. You want to avoid covering up the roof ventilation. Coat nail heads with the cement and ensure that all laps are cemented at the joints.