You are here:

Roofing/Hip rood ventilation

Advertisement


Question
QUESTION: Hi,
I have a home that was built in 1947 with a hip roof and there are no soffits. The previous owners installed a power vent along with a couple other vents on the roof. I had a someone look at my roof the other day and he said I have hail damage and need to have it replaced. When I asked about getting good ventilation, he said that I should just go with ridge vents, but that would not address the fact that I have no air flow from the soffit area. He said that was better than not having ridge vends and that hot air would naturally rise out of the attic. I have looked around a few sites and there seems to be a few options that he did not have. What would you recommend for ventilation on a hip roof? It's not a very big house, 1329 sq ft. but of course the attic is much less. Located in Saint Louis.

Thanks for your time and help.
Bill

ANSWER: Be careful. There is a huge business in faking insurance claims/hail damage where the roofer wants to sell a new roof. If it turns into an insurance claim the insurance company will come by to check.

You are sort of on the right track for ventilation.
Ventilation does not do much without intake AND exhaust. You can't suck air from a bottle. You need to make a hole at the other end of the bottle to get air to come out through the neck of the bottle.

The intake is at the lower part of the roof where it is cooler in the attic. The exhaust is at the peak of the roof where it is hottest since hot air rises. What happens is that hot air goes out the top and the only way for that to happen is that cool air must come in the bottom to replace that air. The cool air comes in the bottom and starts to heat up. As it heats up it starts to rise and then gets funneled to the peak and it goes out taking heat with it. The intake is generally at the lowest point in the eaves. The exhaust is at the peak.

Picture your hot house. You can open a window but it doesn't do much. We all know about "cross-ventilation". We need to open a window on the other side of the house to get a good airflow. Want great airflow? Open every window in the house. You can't open too many windows.

The venting is for 2 reasons:
1. To cut down on heat in the attic.
2. To purge humid air in the attic. Humid attic air only occurs under certain circumstances. Not everyone has an issue with humid air.

I'm going to make up numbers here.
If you have an exhaust vent the size of 5 units and the eaves vent the size of 3 units then the system is the size of 3. The exhaust can't expel more air than coming in from the eaves. So anything out of equal is not a problem but it is a waste. 99 input and 7 output and you have a 7 system.

You cannot have too much ventilation. You cannot open too many windows.

Another issue. You listed square footage of the attic floor. You calculate based on the volume of air in the attic and not the square footage.
Example house is 20 feet long and 10 feet wide. Ridge goes lengthwise so on A-frame roof it creates 2 roof sides both 20' long.
Example 1: Roof slope is 10 degrees. Volume of air is 88 cu-ft
Example 2: Roof slope is 45 degrees. Volume of air is 500 cu-ft
Example 3: Roof slope is 80 degrees. Volume of air is 2,835 cu-ft.
You can see that the steeper the roof the more air there is to move yet each design has exactly the same square footage of attic floor but the 80 degree roof has 32 times the quantity of air to move so you would want more venting on the 80 degree roof than on the 10 degree roof. So you donít calculate venting by the square footage but by the volume.


---------- FOLLOW-UP ----------

QUESTION: Thanks. A lot of good information and make me wary of the guy that I spoke to.
Any thoughts on a drip edge vent or is there anything else out there to create the ventilation on the low side for a hip roof?

Thanks again,
Bill

Answer
There are many different types of vents. The important thing is what looks visually appealing to you. With a hip roof you won't get much ridge length to install vents so you'll need something on the slope if you want to increase venting. You can use power vents that can push a lot of air through small openings but you'll need to run electricity. Some work on solar but aren't very powerful. For the eaves, there are all kinds of tricks where you can add vents to the soffit, or to the lower ends of the shingles. They have special boards (like plywood) that have vents built into them. Find a good roofing supply and ask to see what they have. A lot of stuff might be too odd and they'll have to special order it.

Brad

Roofing

All Answers


Answers by Expert:


Ask Experts

Volunteer


Brad Zacharia

Expertise

All aspects of residential Roofing. This includes shingles and flat (low slope) roofs. I have knowledge in the installation as well as the design of roofs from an engineering standpoint.

Experience

I have been doing roofing for 40 years. This was my father's business and I took it over in 1980.

Publications
I have written responses to artcles that I felt needed a response to and those responses have been published in roofing trade magazines.

Education/Credentials
BSEE Drexel University
www.ZachariaRoofing.com

©2016 About.com. All rights reserved.