From shake to tile

**Dave Hill** · 09-10-2010, 06:41 AM

Good morning!
I have an inspection later today. I haven't seen the home yet. It was built in 1977 with shake shingles. Recent upgrades include a new cement tile roof. The client wants to know if extra support was/needed to be added with this upgrade.

Any suggestions would be appreciated. For example, are extra joists needed? Thicker roofing sheathing? Etc? what would you look for?

Thanks in advance for your suggestions.

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**Markus Keller** · 09-10-2010, 07:48 AM

Don't know what type of construction was being done in AZ in the 70's this what I say may or may not apply.
Around here a house from that era typically has 2x4 truss roof framing, often times 24"OC. I wouldn't trust that type of framing for a cement tile roof. You could have collapse failure at worst. At best you could easily end up with exterior wall spread. Roof load pushes down, walls get pushed out.
Depending on attic ventilation and amount of moisture problems over the years, I seen some of the truss systems in really good shape and others that were in fairly bad shape, just depends.
I would look for ...
rafter spread, sufficient or added collar and rafter ties, rafter to wall attachment, general rafter size (anything less than 2x8 or beefy truss I would right up as insufficient automatically most likely), condition of metal plates, look at joist to rafter connection if possible see if signs of spread are visible, look along exterior walls, see if they are spreading, dip in rafters/middle of roof plane, cracked rafters, etc.
I would also try to get the make/model of the tiles. I've seen some of the cement tile products advertised as 'lightweight'. If this is one, it may be a non-issue. Try to get product info before the insp so you can check the install per Manf Specs
Whenever there are questions about a house before the HI, I always look it up on the MLS or Assessor's site to see what it looks like. This helps in knowing what to prepare for.
Good luck

**Dave Hill** · 09-10-2010, 08:08 AM

Markus,

Thanks for the speedy response. I have made some notes for my attic inspection. Excellent advice! I know most is common sense, but it helps to get some feedback from the experts.

Phil Brody · 09-10-2010, 11:31 AM

A cement tile roof has a different dead load rating than a shake and has to be designed accordingly. Note the roof pitch, truss, rafter spacing and dimensions and refer to engineer and repost for feedback.

Russel Ray · 09-11-2010, 09:37 AM

Note the sheathing, too. Most of the time around here, the original board sheathing is left in place, and plywood installed on top of it to better distribute the weight. The orignial board sheathing for wood shake around here is skip-sheathed. If the same is done over there, you'll be able to see the plywood from inside the attic.

Mike Truss Guy · 09-11-2010, 01:28 PM

Depending on the design load used on the original structure it often involves only a minimum of repairs to make a roof adequate to support tile loading. Sometimes trusses were even designed for tile when they were built with composition shingles. It depends on the manufacturer, the construction practices in place at the time of construction. Remember that back in the 70's personal computers had not been invented. Often trusses were built conservatively with standard designs pulled from a notebook. If they pulled a design that was good for up to 36' span and used it to build a 30' span it's going to be fine.

Concerning concrete tiles, there are many lightweight products on the market today. Standard tile might weigh 11 or 12 PSF while lightweight concrete tile may be 7 PSF. There are even are some made from recycled plastic which are only 3 PSF. Now before you say, but you're tripling the load...Here in the Southwest we might have designs a truss for 20 PSF live load, 10 PSF dead load, plus 10 PSF on the bottom, a total design load of 40 PSF. But that's design load. The real loads are sometimes lower. Say 20-7-8. So in theory you could possibly add another 5 PSF for tile and not go over what it was originally designed for. I've seen many jobs that add only 2 PSF beyond what it was originally designed for. Of course this is all a simplification and it all must be examined for damage, loose plates, or other issues like missing webs.

I've never seen a collapse as a result of adding a tile roof in 20+ years. This is probably for several reasons. First of all the live load is usually not present except when there is snow or workers on the roof. Tile is not typically used for snow load areas. Secondly, they usually add sheathing which adds a minimal dead load, but it also adds strength to the structure both for gravity loads as well as lateral loads.

What they need an engineer to tell for certain. Try searching Google for "Truss Engineer".

Russel Ray · 09-11-2010, 04:46 PM

I've never seen a collapse either, but I've seen lots of sagging roofs which have scared buyers off.

Mike Truss Guy · 09-12-2010, 10:53 PM

Originally Posted by Russel Ray

I've never seen a collapse either, but I've seen lots of sagging roofs which have scared buyers off.

Sagging rafters yes; sagging OSB sheathing, sure; sagging trusses due to decay, insect damage, unauthorized field modifications, foundation settling, removal of bearing walls, or improper installation absolutely, but manufactured trusses built after the mid 60's that sag only due to the recently replacing shingles with tile? IMO, not very likely. Just an opinion. Anyway, if you see sagging by all means please report it. Heck report it anyway and let an engineer sort it out.

Mitchell Toelle · 09-13-2010, 10:35 AM

Light weight concrete tiles weigh approx. what a wet shake would weigh. Remember that a wood shake is only wet for a portion of the annum. If a wood shake has been replaced with a concrete tile you will need to determine if the concrete tile is, indeed, light weight. The only way to do that is to see the original manufacturers information. If that information is not available to you you will need to recommend that a licensed Engineer, familiar with this type of structure, be called in to review the existing stucture. YOU are not licensed to do that review and you would be remiss in your duties to let your Client proceed on any other advice. They need to know that there is a possibility of sagging in the future, even if it does not end in collapse. Any structural movement is not good. Know the product installed and the needed structural support for that product. Dead weight as mentioned in a previous post.

I see this all the time in my local. Bakersfield has only about 6" rainfall per year. I have seen alott of sagging eaves, ridges, rafters and trusses (even built in the 70's) that will need additional modifications after a concrete tile has been installed. Many times we see standard weight concrete tiles installed because the cost of light weight is significantly higher. When the replacement roof covering is new and no adverse reactions are currently showing I will still recommend a review by a licensed Engineer if I lack the information noted above, ie. type concrete replacement, etc.

Do not gamble with your Client and their welfare or investment. Do not gamble with your future.

Mike Truss Guy · 09-13-2010, 11:06 AM

Originally Posted by Mitchell Toelle

Light weight concrete tiles weigh approx. what a wet shake would weigh. Remember that a wood shake is only wet for a portion of the annum. If a wood shake has been replaced with a concrete tile you will need to determine if the concrete tile is, indeed, light weight. The only way to do that is to see the original manufacturers information. If that information is not available to you you will need to recommend that a licensed Engineer, familiar with this type of structure, be called in to review the existing stucture. YOU are not licensed to do that review and you would be remiss in your duties to let your Client proceed on any other advice. They need to know that there is a possibility of sagging in the future, even if it does not end in collapse. Any structural movement is not good. Know the product installed and the needed structural support for that product. Dead weight as mentioned in a previous post.

I see this all the time in my local. Bakersfield has only about 6" rainfall per year. I have seen alott of sagging eaves, ridges, rafters and trusses (even built in the 70's) that will need additional modifications after a concrete tile has been installed. Many times we see standard weight concrete tiles installed because the cost of light weight is significantly higher. When the replacement roof covering is new and no adverse reactions are currently showing I will still recommend a review by a licensed Engineer if I lack the information noted above, ie. type concrete replacement, etc.

Do not gamble with your Client and their welfare or investment. Do not gamble with your future.

That is sound advice. I had just one comment. The weight of the tile can be determined by simply weighing a single tile, measuring the overlap used, and doing a calculation. Of course as an inspector you're not going to remove a tile, but sometimes they have a sample laying around in the back yard.