## Using 2x and logs for rafters

## Using 2x and logs for rafters

(OP)

Currently there are 5" dia. logs (18.5 feet long) spaced 36" o.c. being used as rafters for a building with a 12:12 pitch and a metal roof. The owner wants to add a sheetrock ceiling so he wants to sister a 2x to the log so he can run his furring strips and install sheetrock. Below are my calculations based on removing the logs and installing regular 2x rafters.

Ground snow load = 55 psf

Sloped Roof snow load = 17.3 psf

W = 17.3 (LL) + 10 (DL) x 3' spacing = 81.9 #/ft.

M = (81.9 x 18.5^2) / 8 = 3504 #-ft = 42,045 #-in.

This would require two 2x10's. However, the 5" logs will carry some of the load. What load can a 5" log handle?

Am I going about this the right way? Thanks.

Ground snow load = 55 psf

Sloped Roof snow load = 17.3 psf

W = 17.3 (LL) + 10 (DL) x 3' spacing = 81.9 #/ft.

M = (81.9 x 18.5^2) / 8 = 3504 #-ft = 42,045 #-in.

This would require two 2x10's. However, the 5" logs will carry some of the load. What load can a 5" log handle?

Am I going about this the right way? Thanks.

## RE: Using 2x and logs for rafters

## RE: Using 2x and logs for rafters

I believe you could just leave the logs alone and then design 2x rafters that are only for supporting the gypsum board, therefore you are not modifying the roof other than the ridge beam (assuming there is one).

Sistering 2xs to round logs has its complications I would think.

## RE: Using 2x and logs for rafters

How are the rafters supported at the top?

Are the existing rafters tied?

How many years have the 5" logs been supporting the roof?

Could the metal roof be considered an unobstructed, slippery slope?

BA

## RE: Using 2x and logs for rafters

I arrived at the 17.3 psf snow load by using the following

Pf=0.7 x Ce x Ct x I x Pg

Pf=0.7 x 0.9 x 1 x 1 x 55

Pf=34.65 psf

Ps=Cs x Pf

Ps=.5 x 35.65

Ps=17.3 psf

There is a collar tie at the top of the rafters.

The logs have been supporting the roof for 20+ years.

## RE: Using 2x and logs for rafters

I think I'd design a ceiling support system independent of the roof. That way you are not touching the logs, which like you said have been there for 20+ years...

However, if your calcs show the logs are way undersized, you now have some responsibility to correct this, depending on your local code requirements and the amount they are overstressed. At the bare minimum, you should tell the owner in writing your results.

## RE: Using 2x and logs for rafters

## RE: Using 2x and logs for rafters

I of 2 - 2 x 10 = 198 in^4

I of 5" log = pi*5^4/64 = 30.7 in^4

If the dimension lumber is sistered to the existing logs, the logs will carry very little because they are not anywhere near as stiff.

It is surprising that 5" diam. logs at 3'-0" centers have survived for the last 20 years, but I don't think the logs should be deemed to contribute to the strength of the roof. You could place a 2x10 each side of each log and nail them together but you will not be easily able to provide for air circulation within the ceiling space.

Perhaps the best option is to replace the existing roof with new construction.

BA

## RE: Using 2x and logs for rafters

There's lots of missing information in your post so far. I'd like to see some sketches: particularly the conditions and details of the log rafters at the ridge, collar ties and their distance below the ridge, and the rafter bearing detail at the ext. walls, all with some dimensions and member sizes. What's acting as a rafter tie at the top of the walls, the rafter bearing elev.? Are the rafters sway-backed after 20+ years? Are there 2x4 purlins at 2' o/c, or some such, on top of the rafters for the metal deck attachment? If the roof is a 12/12 pitch and the rafters are 18.5' long, then the bldg. is about 26' wide, and the rafter run is 13'. The rafter span length is the same as the run for this bending calc., and the loads should be in lbs./sq.ft. on a horiz. projection. The snow load already is assumed to be on a horiz. projection, but the DL's have to be converted from loads in the roof plane to loads on a horiz. projection. So, the DL's will be (1.414)( 10 lbs./sq.ft. or some such).

Are you going to vent this roof, or insulate it? You may want the new rafters dropped to allow for venting, insulation and a vapor barrier. I would want to study how I was going to detail the new dimensional rafters at their bearing on the walls, the ridge, around the collar ties, etc. Rafter ties, again? Do I want the log rafters and the new 2x rafters to act together, or as separate systems? You can get lots of 2x rafters at 16" o/c btwn. the 3' o/c log rafters if you lay them out right.

## RE: Using 2x and logs for rafters

Balloon framed, timber framed, or platform framed where rafters do not bear on a knee wall all properly tie the first story frame/framing with floor joists. If rafters can be fastened at their bearing point such that outward loads are resisted, they're okay. Obviously this only works at more modest spans.

My house has stood up to 70 psf pg for 75 years with 5" half-sawn log rafters , no ridge pole, no collar ties. It's hard to justify messing with something that has worked so long, especially when my homeowners (not professional liability) insurance is the only thing on the line.

## RE: Using 2x and logs for rafters

Well, that and your neck...

But I see your point, and I too have seen many older residential roofs without much in terms of collar ties or ridge beams, only the ceiling acting as a bottom chord. One of the reasons I suggested leaving the roof alone and framing out a separate ceiling as an option. But others have brought up points about insulation, ventilation, etc. that the homeowner may not have fully considered.

## RE: Using 2x and logs for rafters

## RE: Using 2x and logs for rafters

## RE: Using 2x and logs for rafters

## RE: Using 2x and logs for rafters

You asked if you were going about this the right way and I think the answer is no. The existing rafters are tied at mid length by a piece of dimension lumber. The bottom of the rafters bear on a timber beam which seems to be tied to the corner of the building and may be tied to another wall about ten feet over from the outside wall.

If the top of the wall is laterally braced, the rafters, under snow load will carry a combination of compression and bending. If the top of wall is not braced, the rafter moment is much larger.

I suggest you draw a cross section of the building complete with dimensions, then decide whether or not the wall is braced laterally at the bearing point of the rafters. Only then can you perform any meaningful calculations.

BA

## RE: Using 2x and logs for rafters

Is the dimensioned lumber doing much from a structural aspect? The owner would like to remove the dimensioned lumber so he can have a catherdral ceiling. Would it be better to just move the dimensioned lumber up higher and have his ceiling be a trapezoid shape?

The pitch is 12:12 and it is a metal roof so the snow is not going to stay on the roof.

## RE: Using 2x and logs for rafters

BA

## RE: Using 2x and logs for rafters

## RE: Using 2x and logs for rafters

## RE: Using 2x and logs for rafters

You are playing with dynamite if you add any weight to a structure like that. The best advice you can give the homeowner is to have it analyzed by a structural engineer.

BA

## RE: Using 2x and logs for rafters